Britbike forum
Posted By: DMadigan A74 Port model - 06/25/21 11:33 pm
Printed ports to try on the flow bench.
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34mm Koso Oko, Ford Ranger valves
Posted By: Mark Parker Re: A74 Port model - 06/27/21 4:04 am
Are you planning a new head casting Dave? What size are the valves? Do you have any flow numbers on them yet? They look really good.

The std casting isn't very tall, why oval seem to work well in them. Going wide being one of the big advantages the A65 has over Triumph and Norton heads, and the triples.

These flow around 160cfm through a 34mm pwk, some read 164 or so but could be the lack of accuracy of home made gear. I cannot get them much better than that. It's probably possible but changes are hardly worth the trouble because returns are small after a certain thing and can start going backwards.

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That's with a 42mm MAP valve. Bigger valves will actually pull more air through, around 180cfm for a 44.5mm valve, but the port opens more and is a bit big for a std size engine, because the rpm would be extreme. Though I haven't tried one. Without the carb they are all about 10cfm more through a radiused entry. I could probably make a better flowing bell but it's a bit pointless.

We should have a big valve 34pwk combination on a 734 big bore pretty soon. The PWK are really nice because the throttle is pretty light. And they are physically small and don't particularly look out of place. Starting is usually one kick. I ordered jets from 145 to 180 or so to suit the Firebird. 150 seem about right but if it goes on the dyno I want some alternatives. Generally jets kits come up to 140 mains, which are small. Though a bigger displacement may be different.

From what I can read from the knowledgeable the std exhausts ports flow heaps. And high compression means the exhaust valve can be fairly small. Stock is 35.8mm and MAP has a 37mm oversize so I'm not going over that size in the future even with the biggest inlet.
Posted By: DMadigan Re: A74 Port model - 06/27/21 4:50 am
Eventually want to cast a new cylinder and head. This is an 80mm bore (XS750 pistons) which is why I need a new head (some say not just for the engine) and cylinder. The outer four studs will go down into the case.
I do not have flow numbers yet, I got the parts off the printer. I have drill the adapter for the cylinder to fit the A74 holes.
The valve are from a ford ranger with 7mm stems, do not remember the head sizes off hand.
Posted By: Mark Parker Re: A74 Port model - 06/27/21 7:01 am
The 44.5mm Jaguar valves have a reduced stem near the head. They are nice nitrided valves but I haven't been able to get them lately. To get an idea of how much flow you need this calculator is pretty good.

http://www.wallaceracing.com/calcafhp.php

I just don't know which head would be better on a 750. The 42mm is faster in the port. It varies where ever you put the probe and it's hard or impossible to hold it in the centre down in the port a bit. Both are over 400fps a little in the port. And the big valve one must be faster in the carb because more air goes through.

But how it is in the real world I don't know. It's predicting high rpm but I think it would have good power on the lower end of the predicted range.

The 42mm valve one like the ones above is like a fluke of being especially effective, with decent flow and speed. It's boosts midrange and top end somehow.
On a 750 it predicts over 80hp and I know it will be strong from virtually idle. Even on a 650 it makes the gap between 2nd and 3rd not mean much.

A friend moved the 4 outer head studs out a bit for a bigger bore.
Posted By: DMadigan Re: A74 Port model - 06/27/21 5:23 pm
Just rechecked, the exhaust valves are used in the 2000-2010 F150 16 valve 4.6L and 5.4L trucks and 4.6L cars '92-07.. I think they made a few of them. The intakes are the 3.0L V6 '99-'08.
Intake - 1.661", 42.19mm, exhaust - 1.421", 36.09mm.
Wallace calculator gives 6536 PRM.
What did your friend do about the bolt in the pushrod tunnel which is closer to the bore than the outer bolts?
Posted By: Mark Parker Re: A74 Port model - 06/28/21 12:08 am
I think the bores were extended out. He uses .5" of valve lift with offset roller rockers. On my 883 I need to use 9mm head studs because 8mm even at a depth of 35-40mm pull out. And the cylinder is heat treated to T6.

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Posted By: DMadigan Re: A74 Port model - 06/28/21 2:24 am
"8mm even at a depth of 35-40mm pull out" - which is why a new cylinder/head is needed with proper size head bolts.
Posted By: Mark Parker Re: A74 Port model - 06/28/21 5:31 am
This happened as power levels went up. Being metric thread, drilling and tapping to 9mm is easy, if done by hand and feeling the centre of the hole. Liners instead of nicasil limit the centre bolt to 8mm so that's what Ben's 750 has. The difference between 8 and 9mm is significant 10mm is slightly bigger than std. The problem with std size bolt and studs is distortion in the bore. What through bolts would eliminate, but then they have to stay in the case.

The R/h case is a bit compromised with a breather hole which I welded shut on the latest R/h case on the 883, with a reinforcing bar. Drilling holes between the crank and cylinder I would avoid, other than an oil hole to the roller which isn't were they crack. A 744 would probably be fine. The 9mm studs distort the bore slightly but it doesn't smoke, they feel totally different when torqueing.

The 750 is the nicest smoothest engine. And easiest to build. It would be good to have CNC programs for making the drive side bearing plates. They keep the primary chain in line and control crank end float.

If we consider a Spitfire engine as a race engine with 10.5-1 comp the std 68 701 flows 109cfm so 56hp is predicted between 5,606 and 7,102.

So high comp with a 42mm valve head on a 744 should be 84hp to 87 or so and a 44.5mm valve version over 90hp. But rpm is predicted over 8,000 or over 9,000. It depends if they are calculating port flow or port flow with carb.

Posted By: DMadigan Re: A74 Port model - 06/28/21 6:38 pm
Like the sound.
Instead of the outboard outrigger bearing you could use a modern generator and skim the stator support a little to put the outrigger bearing just outboard of the sprocket. This is a GSXR1000 rotor on the crank.
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There is approximately 10mm of clearance between the back side of the rotor and the sprocket with the rotor sitting on the stator mounting surface. If the earlier narrow hub sprocket were used there would be another 7mm of clearance. You could fit a 6005 25 x 47 x 12mm bearing in there, possibly a 6205 25 x 52 x 15mm.
The rotor fits on a taper so the adapter would be the taper and inner bearing race surface.
The stator would mount to the primary cover. This is the wrong one (0.5mm too large in diameter) but gives you the idea.
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Just about ready to test. I need to make collars to hold the valves in position.
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Posted By: DMadigan Re: A74 Port model - 06/29/21 2:23 am
Off track a bit but the GSXR1000 stator/rotor with outrigger bearing will fit in the case.
[Linked Image from live.staticflickr.com]
The Lucas stator mount has to be recessed 0.41" for the 6005 outrigger bearing support which also holds the GSXR stator. The crank shims inside/outside the distance piece will set the exact position of the crank.
The lump on the left stator cross section is the wire harness. This would be rotated so it exits at the rear post and tied off.
Posted By: Mark Parker Re: A74 Port model - 06/29/21 5:13 am
I just use a 16005 bearing in both places.

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Posted By: Allan G Re: A74 Port model - 06/29/21 5:22 pm
Originally Posted by Mark Parker
I just use a 16005 bearing in both places.

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Same on mine.
Posted By: DMadigan Re: A74 Port model - 06/29/21 6:15 pm
What do you mean by "both places"? You have another16005 bearing somewhere?
Here is a modified drawing with a 16005 bearing outboard of the Lucas rotor.
[Linked Image from live.staticflickr.com]
6005 bearing inboard of the GSXR stator on the left and a 16005 bearing outboard of the Lucas rotor on the right.
Is that essentially what both of you have?
Posted By: Allan G Re: A74 Port model - 06/29/21 8:05 pm
Originally Posted by DMadigan
What do you mean by "both places"? You have another16005 bearing somewhere?
Here is a modified drawing with a 16005 bearing outboard of the Lucas rotor.
[Linked Image from live.staticflickr.com]
6005 bearing inboard of the GSXR stator on the left and a 16005 bearing outboard of the Lucas rotor on the right.
Is that essentially what both of you have?


Essentially I copied Mark, the design is slightly Different but it is as good as the same on paper.

Because my belt drive is wider than the standard setup I have my primary case spaced out. But with a better design it need not be.

I believe marks other 16005 is on the mainshaft outrigger.

Your setup looks very neat Dave and possible adds improved charging over a standard RM21? Does it require the crank case machining to house the GSXR setup?
Posted By: DMadigan Re: A74 Port model - 06/29/21 9:18 pm
You could say a bit advanced charging. The GSXR1000 generator is rated at 400W at 5000 RPM by sources that I could find.
The white outline on the left is the 6005 bearing and GSXR stator mount. The stator face of the A65 case has to be lowered by 0.41". Obviously easier on the earlier models with the removable stator mount or the studs alone mount. Easy enough to reverse with a spacer.
Yes, one 16005 in the outrigger but where is the other one?
Posted By: Allan G Re: A74 Port model - 06/29/21 10:19 pm
Originally Posted by DMadigan
You could say a bit advanced charging. The GSXR1000 generator is rated at 400W at 5000 RPM by sources that I could find.
The white outline on the left is the 6005 bearing and GSXR stator mount. The stator face of the A65 case has to be lowered by 0.41". Obviously easier on the earlier models with the removable stator mount or the studs alone mount. Easy enough to reverse with a spacer.
Yes, one 16005 in the outrigger but where is the other one?


All interesting stuff. Mark has two outriggers. One on the crank and the other on the gearbox mainshaft. (Sorry it’s a short and sweet reply, bedtime here). Hth
Posted By: Mark Parker Re: A74 Port model - 06/30/21 9:51 am
The head on Ben's bike above has 44.5mm inlets with 38mm TMs, looking at the flow figures on that big carb head the smaller 34mm port may not particularly flow less with that valve, but it will be higher in speed. Which should have it drive harder making more power across the range. It may or may not be better than the 42mm valve version. Which I think could have been the ultimate production head as the more efficient port easily fits into the std 68-701 casting with no break throughs. And a 744 version would have been a rocket. Easily done and super effective.

I've done 3 of these heads which all flow about the same. I should have valves for them all soon. The Thunderbolt head on the Firebird read 168 compared to 107 stock for each port. These Lightning ones read around 170-174 and 164-5 through the carb. A stock 68-701 109 and around 105 through the carb. The numbers are just numbers and show at least a substantial improvement.

I ordered some MAP rods for the Firebird and when I fit them I need to decide between a 90degree crank I have or the 360 re-balanced to 56% which BSA testers said was best. Both would be fun to experiment with, though I already know how smooth the 90 can be. I'm hoping the 360 would be as smooth at 4,000 then get smoother. And lumpiness below that isn't too bad, it doesn't need to run under 3,000 really though I often do as it's smooth and flexible but being dead smooth at 2,000 isn't as useful as higher up.

This is the head for Nick, part done, that's been winning so well for them on the 750. It has 42mm inlets and std exhausts. On it are different possible inlet valves though this early model head has small valve seats the 42 being the maximum. It takes luck to fit the 44.5 valve on std seats, but they must be the later heads that came with 40.5mm inlets.

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Posted By: Allan G Re: A74 Port model - 06/30/21 11:08 am
Originally Posted by Mark Parker
Are you planning a new head casting Dave? What size are the valves? Do you have any flow numbers on them yet? They look really good.

The std casting isn't very tall, why oval seem to work well in them. Going wide being one of the big advantages the A65 has over Triumph and Norton heads, and the triples.

These flow around 160cfm through a 34mm pwk, some read 164 or so but could be the lack of accuracy of home made gear. I cannot get them much better than that. It's probably possible but changes are hardly worth the trouble because returns are small after a certain thing and can start going backwards.

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That's with a 42mm MAP valve. Bigger valves will actually pull more air through, around 180cfm for a 44.5mm valve, but the port opens more and is a bit big for a std size engine, because the rpm would be extreme. Though I haven't tried one. Without the carb they are all about 10cfm more through a radiused entry. I could probably make a better flowing bell but it's a bit pointless.

We should have a big valve 34pwk combination on a 734 big bore pretty soon. The PWK are really nice because the throttle is pretty light. And they are physically small and don't particularly look out of place. Starting is usually one kick. I ordered jets from 145 to 180 or so to suit the Firebird. 150 seem about right but if it goes on the dyno I want some alternatives. Generally jets kits come up to 140 mains, which are small. Though a bigger displacement may be different.

From what I can read from the knowledgeable the std exhausts ports flow heaps. And high compression means the exhaust valve can be fairly small. Stock is 35.8mm and MAP has a 37mm oversize so I'm not going over that size in the future even with the biggest inlet.


Hi Mark, have you done any moulds of the stock ports (thunderbolt or Lightning heads)?
Posted By: Mark Parker Re: A74 Port model - 06/30/21 11:57 pm
The better '71 stock port.

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On the left in this one.

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Posted By: DMadigan Re: A74 Port model - 07/01/21 6:04 am
It looks as though most of the modification is to widen the port around the guide restriction, yes?
On a completely different note, after looking at the "SlickShift" post on the Triumph board, there likely is a way to add this to an A65. The shift shaft and clutch lifter are fairly close to each other. The main difficulty is the shift shaft mounts to the inner timing case and the lifter mounts to the outer timing cover.
Posted By: Mark Parker Re: A74 Port model - 07/01/21 8:44 am
Yes Dave and lift the floor. But this is a '71 with the deeper and wider bowl.

I have a stock '71 head in like new condition but the front rocker middle mount has been broken off by the stud going too far down making it pretty unusable, but good to measure against the others for a calibration head, once I establish exactly what its got.

I took the inlet guides out of the stock 68-701 I have left and will port it to the 42mm valve size and 32mm round std carb mount. The one above is done to 30mm but there is no reason to have them so small. I can do it like this with it going oval inside the head. It was around 138cfm for just the port and 144.4 with a radiused entry. I can get it fairly decent at 32mm, possibly over 150 but the carb will restrict that, the better 34mm moves the carb back. So if you had a stock Spitfire it could look totally stock and kick butt all the same because it could be close.

I used to think it required a big carb to get decent flow but the 34 is so close to a 38mm without lifting the guide 4mm and having break throughs and patching. I was thinking a casting could be altered a bit, and it could, but probably not to make a 744 or smaller motor better. A very peaky high rpm 744 might make more hp right in the top end, but though you can get more cfm it's not that much more and it may not make more power even if you buzz it. Personally I would not make a head, even a 4valve, getting it smooth and reliable at high rpm for what the std casting can do I think would be better.

From the side.
[Linked Image from i.postimg.cc]

You can get it pretty good without raising the floor, but if you go wider still and a bit higher the floor can be filled which even if you don't get more flow at that stage the port's smaller and faster and can be better flowing in the end.

This is the bearing housing on the front, the rear one I just bolted a piece to with bearing captured and seal but it's a bit wide and wouldn't fit, I've narrowed it a couple of mm and should put it on the Firebird which needs one. But one piece are better. The hole in the front one is for timing, just put it on full advance and put a drill in the hole and mark it.

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Posted By: Allan G Re: A74 Port model - 07/01/21 1:55 pm
Quote: Personally I would not make a head,


I dont know Mark, I think if you were to redesign it enough so that you could use a flatter chamber (weslake style) I think it would be a good thing. You'd have a much better turn into the bowl for a start. you could then get away with flat top pistons also which should be a weight saving on the reciprocating mass, and I am sure a better combustion profile.

The first head I did anything to was a late OIF head, I did nothing more than widen the port around the sides of the guide, this had a good response and helped high RPM.

The best one I ever did was a small port head, keeping the 38mm inlet valves and just raised the floor only and 928 carbs, still with filters and standard exhaust. This pulled like a train and rev'ed really well. I came up with some fag paper maths at the time and wasn't looking for high revs, I was trying to target my max power for around 6000 revs, what I wanted was something very tractable with good pull, the SRM race cam I helped also with top end also and was a good combination and I got more than what I was looking for.

Aside of that the early small port heads are better than the later heads anyway, some have put 40mm valves in them (my currently used head has them) but I don't think its worth it, they respond better all round with the smaller valves on this head IMO.


I have David Vizzard book which I think I saw a screen shot from on another thread, its an interesting book but I still haven't a decent quiet place where I can really study the book properly enough to make the information stick. Its not light reading by any stretch if you intend to learn from it, although an interesting read regardless.
If you don't mind me asking Mark, are you adjusting the strength of the vac from your flow bench to replicate real life comparision to how much a piston can draw at a realistic rpm? Im not trying to call you out, I am curious if your able to measure comparable results at slower vacuum depression.

AIUI, the book displays 2 manometers, the U tube manometer which will be for the total vacuum, as taken before the vac system. The other being an incline manometer which which a more sensitive gauge and will display around 10x the scale of the U tube manometer. This will (I think) take its actual flow readings from the head.

I need to get my head around the "test piece" which it goes on to mention, AIUI this is used for calibration of your flow aparatus.

Either way your yielding great results from your bikes. I'm just a curious sod who enjoys learning.
Posted By: Mark Parker Re: A74 Port model - 07/01/21 11:27 pm
Allan there is a page that shows Harley heads in various stages and the hotter ones, or more advanced have the hemisphere closed with squish areas. That could be good. But I'm just thinking for practical application of what we have. I could not make a head and I've welded one up before to use flatter pistons and squish areas but without having the ports the best. Ones with better ports were better.

There is a danger with high dome high compression pistons that more compression can lose power because it blocks flow. 650cc engines probably more susceptible needing smaller chambers.

On my 883 the guide is lifted and I've done all sorts of things to make a 38mm port flow but it's probably more effort than it's worth because it wants to make power over 8,000 where it will never run. It's power is excellent in the midrange but that head on a 750 would probably have you waiting for power to come in
and using rpm where that was working best you probably would not do unless you were drag racing and a bit smaller with less flow may be better anyway.

A 750 that responds like the 34mm carbs on the 650 would be great. I didn't know that head would do that. It's no bottomless pit but it's very nice if you can hang onto the vibrating bars. It's probably capable of high rpm, but even if it was smooth you don't really want or need to use more than 7500 really. Nice to known it's there but not used much.
Posted By: Mark Parker Re: A74 Port model - 07/02/21 5:20 am
Just arrived from MAP in the states. Delivered to the door Friday today after ordering last weekend. So apart from some other nice stuff a few valves.

These are the nitrided stainless swirl pattern inlets 42mm and std nitrided exhaust valves. Why they are good is because with a K-linered guide they can be fitted fairly tight reducing wear. And when they do eventually wear out they can be lined again without removing.

(pictured are actually the big exhaust valves. I have them in the Firebird but std exhaust valves in Matt's winning sidecar. So the only reason to require bigger exhausts is worn seats or using a blower, according to Vizard.)

[Linked Image from i.postimg.cc]

The beauty of this set up is it fits straight on a stock 650 bottom end. The calculator predicts over 70hp but that remains to be seen. Seems no problem with piston to valve clearance, just needs good springs set to the correct fitted length. Nice on a stock 650 and enough flow for possibly a predicted 82-87engine hp on a 750 race motor.

I may be able to fit the steel rods before I can get it on a dyno. And hopefully have it balanced smoother at higher rpm. But it's a bit hard to get motivated to pull something that runs nice apart.

This shows how the 34mm id tube is squashed and lined up, Then the tags just welded on. The oval is right out to the 1/4"unc screws. I just fit helicoils in the holes.

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Posted By: DMadigan Re: A74 Port model - 07/02/21 5:48 pm
A quick first measurement, at 0.40" lift, 10" H2O, 90F, 34mm Oko wide open no venturi, the flow through the as printed port is 82 CFM. The print surface is much rougher than a stock port so next is to smooth it off.
From the calculator, as a street engine it gives 65 HP, as a typical race engine it gives 71 HP.
Too bad they do not make clear print wire. That would be interesting to see. Maybe plugs could be printed and cast in clear resin.
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Posted By: DMadigan Re: A74 Port model - 07/02/21 7:18 pm
Here is the area distribution of the port.
[Linked Image from live.staticflickr.com]
I will try widening the port elliptically at the narrowest point to reduce the restriction. widening the port to 1.20 inch^2 changes the dimension from a 1.1812" circle to a 1.1812 x 1.2935 ellipse.
Posted By: Mark Parker Re: A74 Port model - 07/02/21 11:07 pm
Fun calculator isn't it Dave. I started hacking a std port yesterday afternoon. And have it roughed up to put some JBweld in. It's way too rough to flow decently but I might see what the stock size port does with the stock valve. I've done it before but it will be interesting. The bigger valve gives an engine more for only plusses so probably false economy using the std valve. And 34mm seems the best size really.

It's interesting people equate hp with cams and compression and carb and valve size when the stock head is this big restriction. It serves us though because BSA made big efforts in other areas.

That 65 or 71hp possibility Dave can you imagine what that would have done in 1970? Honda claiming 67hp from a heavy 750.
That's like your 400ci V8 kicking out 650 or 710hp. And BSA were virtually there with a 650 if they worked on the head. Oil supply, lasting T/side bearing and vibration and bullet proofing then so worth doing. May have added a little cost but not near the cost of producing the threes. And the hp and reputation would have ensured demand.

That's a great diagram, so that's the guide valve restriction, it seems to only restrict not serve any good purpose. The reduced stem on the valve doesn't make a difference really, but it may in a restricted area.

This port is tiny, not the best on an engine but should end up much better than stock, while looking stock.

[Linked Image from i.postimg.cc]

Just to add, I made a couple of old guides a bit under size so I can just slide them in and out, then re-fit the good ones and have them k-lined when finished, and cut the seat.
Posted By: DMadigan Re: A74 Port model - 07/03/21 12:13 am
The port area plot is just the port itself without the guide and valve stem cross sections subtracted out. Your port is about as rough as the printed port.
One thing I will have to consider is how thick to make the walls of the port as someone like you will probably want to start carving.
Posted By: Mark Parker Re: A74 Port model - 07/03/21 1:02 am
This is a XR750 port it's wider than I can fit, but like I was trying to say even the 883cc engine would be pushing the rest of it to use the flow. I ran it on the dyno as it was which was over 14.5-1 lean at max rpm and leaner before that, it needed to be 12.5 for max hp. The operator was surprised at 85rwhp first run but we ran a few more runs and got graphs to 83 but he said take no notice of these because it's two points too lean. And pointless moving the timing. What it may give with a bit of fiddling could easily be more which would take it over 100hp at the engine, he also wanted to run 1000rpm more to see when it would stop making power. But we were just about at 8,000 already.

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Longer rods in that engine would make it better, stronger, but it's my road bike and the crank is .050" under and I don't know what is the next weakest bit.

The bike is also lighter than the firebird and is barely ticking over going pretty fast. Ben's 750 is really nice because the motor needs no rubber mounts is easier to kick and you rev the engine more. The head on it is similar to the 883 but flows a bit less. For enjoyment of riding it's really nice. But if the head we are going to swap on with 34s makes for a stronger midrange it's pretty pointless going bigger cc. The 883 has heaps of grunt and if the 750 can be closer, it can also rev more. And fits the cases and space much easier.
Posted By: Mark Parker Re: A74 Port model - 07/03/21 4:25 am
Dave if the patterns for this head still existed simply more metal in the pushrod area a bit more between the fins on the outside of the port and make the valve seat 4mm higher. That would give lots of potential and more margin for hand working on them. But if you had a program to hack out the best shape rather than drill the port or do it by hand because it's time consuming and a bit hard.

As it is it can work good just modifying the stock head, it's already machined, even if when you start you are thinking I wish it could just do it itself. It would be nice having the floor metal but there is no room to weld. And a new casting could have that.

When you widen it you cannot help smiling seeing the test numbers suddenly jump. That doesn't last though because you are then fighting for more, fighting for 1 or 2cfm and it can be getting colder where you are testing. Where I am I need to calibrate right when testing because it can change so much with temperature in the winter. Tee shirt weather outside now people at the beach, but it's snowing a couple of hours away in the mountains where the kids are. And will get cold later.
Posted By: DMadigan Re: A74 Port model - 07/03/21 5:16 am
There is still the problem of the studs being too close to the bore for 80mm so it needs a new cylinder which means a new head. I will print a modified port with it growing elliptically to reduce the restriction.
Posted By: Mark Parker Re: A74 Port model - 07/03/21 7:30 am
With an unmachined casting those holes would not be drilled. 9mm studs seem trouble free. I put bearing blue on the head and bolt it down minus gasket then sand the high spots. Did the same with the unmatched cases measured to the bearing holes to get them square with a file. No leaks plenty of compression frown

I can run 12.3 or 11-1 depending on one or two head gaskets. Too chicken to run just one with this head. Never had such good response. And it did spit me off once though not being careful with a cold tyre. Being dumb would be another way to describe it. Sort of fixed the old fairing and got a better one.

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The thing is actually idling in this photo.

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Posted By: Mark Parker Re: A74 Port model - 07/05/21 8:48 am
Just fixed this tiny little port so it's smooth. This is with std valve calibrating off the std port which is around [email protected]". This head must be earlyish because these ports measure 29.5mm not 30mm. @ 28" the std port needs 3 vaccs the modified one needs 4.

So through a stock 30mm Concentric it's about 143cfm @ .385" and about the same @.350". The stock side is probably around 105cfm through that carb. Just the port is 146.66 and with an old bell-mouth 150. So again the carb takes a bit off. But this might be good with a pair of std GPs on a Spitfire. Though a good bell-mouth with no gauze on the 30mm Amal might do the same and be easier to live with.

My thought with this head was to make it a better breathing head that can use the stock GP carbs on a Spitfire, and make it perform noticeably better. I'm just thinking of selling one and advertise it on E-bay as a ported Spitfire head to suit GPs. Because it sounds a bit more special than Lightning head.

I just want to make sure it's a noticeable improvement if someone buys it so they are happy with it. I guess I should measure speed but I can guess its similar to the 34mm port.

[Linked Image from i.postimg.cc]

The reason to test with a bell or radiused entry is to take the effect of the sharp entry away. Otherwise you can change flow quite a bit by accidently making that sharp edge a bit rounded. Having the carb on takes that edge out of the calculation and works better as well. On these figures it should give a good hp boost, but you don't know what people will do setting stuff up. If it gets near 70 engine hp it would be good.

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Last time I tried this size port it was 137.5cfm just the port so 146 is pretty cool.
Posted By: Semper Gumby Re: A74 Port model - 07/06/21 2:58 pm
Interesting...

(following)
Posted By: S-NJ-W Re: A74 Port model - 07/06/21 8:06 pm
Originally Posted by Mark Parker
So through a stock 30mm Concentric it's about 143cfm @ .385" and about the same @.350".

Mark and Dave - How do you match up this flow rate to the actual needs of an engine and at what point do you determine that a larger carburetor is needed? I apologize if it’s an ‘idiot’ question, I am just trying to learn here.

If you consider a Lightning 650cc twin engine, running at 6000 rpm. That is 2x 325cc cylinders at 3000 intake cycles per minute but as each cylinder has a carb, you only need to consider one cylinder.

A 325cc cylinder is 19.83 cubic inches and at 3000 intake strokes per minute it needs (19.83*3000)= 59490 cubic inches of fuel/air mix per minute. There are 1728 cubic inches in a cubic foot, so that cylinder needs 34.43 cubic feet per minute.

What’s the relationship between the 34.43 cubic feet per minute the cylinder needs and the flow rate of 143 cfm at 28” of mercury you mention?

If that cylinder is breathing through a 30mm Concentric and the bore of the Concentric works out to 1.18 square inches, then at 6000 rpm the 19.83 cubic inches is inducted in (60/(6000x2)) seconds, or 1/200 of a second.

The speed through the choke is then (19.83/1.18)x(200/1)= 3361 inches per second or (/12) 280 feet per second.

Granted, that’s the average speed. Because the intake stroke is one half of a revolution, the peak speed is likely to be a pulse, lasting 1/400th of a second at (2x280)= 560 feet per second.

Given that air/fuel mix has mass, the energy needed to accelerate the mix into the cylinder (via the vacuum created by the descending piston on the intake stoke) is energy robbed from the output of the engine. However, because of that mass, there must be momentum at play here causing the mix to 'pressurize' around the back of the valve as it closes.

I am not well read on this, but the only place I have seen a discussion about the maximum air flow through a carburetor is in Phil Irving’s book ‘Tuning for Speed’. I have the 6th edition and this is discussed on page 14. Phil is calculating the intake speed of a 350cc single at 7200 rpm breathing through a 1 3/8” carb. I guess it’s an AJS 7R. Phil calculates that to be about 290 cfm (I stole his math for above). Phil thinks that about 300 cfm through the carb is about optimum for max power but goes on to say that there is no hard or fast rule. This is an old book, so might be way off now.

That said, if this was a 750cc Lightning, then the flow through the 30mm Concentrics isn’t much more at the same rpm. The A70 used the same 30mm units that the 650 did.

When and why would you consider fitting bigger carburetors to your custom engines?

Given that a 650 Thunderbolt is breathing both cylinders through a smaller carb (28mm) and so at least doubling the air speed through that one carb, does carb size really make much of a difference?

Thanks for getting this far..

Steve.
Posted By: Mark Parker Re: A74 Port model - 07/06/21 10:43 pm
The port and to some extent the engine size determines what the carb flows. There is a point where the carb size is a restriction. But as you note it's complex because of gas speed. But for power it needs speed and volume. The highest speed is caused by valve overlap that initiates the speed and higher compression faster exhaust that pulls it through. A std 30mm carb flows around 105cfm on a std head but 143 on this one at the same vacuum 28" of water. I'll check the speed next time I test but it's quite possible to fill the cylinder with more air than it's displacement or less depending on its speed.

And this can be very apparent at lower rpm. The race 745 is only using 6800rpm max to conserve the A10 crank when it should pull hard to 7500 at least, and you have to wonder what power an 840Tri or 920Norton or 1200Vin it's beating can make on methanol. 8 races from 10. It's very fascinating.

I guess the bottom line is how much air an engine can burn that determines its power so all the cam selection is doing is getting the most charge in for most of the rpm range you are using. Same with everything else exhaust configuration etc, but a basic effecting it all is that intake port. I've watched dyno testing on youtube They swap to better heads and get big increases with mild or hot cams, the cams made a difference but the hot one with worse head got nowhere near the mild with good head. They make shorter lower engine blocks just to fit inlet runners under bonnets to allow 1000hp from around 400ci normally asperated V8s.
Posted By: NickL Re: A74 Port model - 07/06/21 11:17 pm
The Weber books by Passini i have show revs/main venturi diameter/cc guidelines
for both single and multi cylinder setups fed by a single venturi. I know they are
probably old school now but he says in the text that they are guidelines not gospel.
Actual choke size is shown as being related to RPM more than anything.
I knew lots of blokes who fitted large carbs onto bikes and removed them later to regain
lost performance, both at the track and on the road.
The ideal carb for everyday type use tended to be a twin choke progressive type,
giving cost effective overall performance.

Posted By: DMadigan Re: A74 Port model - 07/06/21 11:40 pm
It is very much a dynamic model but at low RPM the flow and piston movement is more in sync. You could measure the flow at many valve lifts and then with the cam profile integrate it to get roughly the total volume that would flow disregarding initial cylinder pressure and port wave effects.
Charles Taylor (MIT) wrote two volumes on internal combustion engine theory and practice with empirical data.
Without a dynamic model, testing and comparison with other engines is really all you can do. The maximum speed in the port should be less than 0.6 Mach.
I made some geometry calculations of the port that I modelled. Turns out what looked reasonable was actually not. I was trying to have an acceleration from the carb to just before the turn into the valve and leave room for the valve spring. Plotting the area versus the distance along the port it was had more reduction than I thought.
[Linked Image from live.staticflickr.com]
The circular area dips down to 1.08 in^2 at the minimum. I thought that I could fix this by making the port wider. I started with the mean line and reduced the constriction but when I plotted the new cross section the radius of the port floor got too small. I decided to grow the port wider linearly using elliptical sections then back to the original circle at the top of the seat. Leaving the vertical height constant I tried several variations on width contours. When plotting the area though they looked terrible, moving width points (yellow) to get a smooth area was not working. What I finally decided to do was curve fit the height and area shapes then use those splines to calculate a width, then spline that shape with the original height to get a final area. It takes a bit of manipulating in Excel to do this but it is the easiest way to get a decent port shape that matches the carb and seat diameters and generate a smooth port volume. The higher the order of curve fit, the more inflections there will be in the curve so several orders of fit have to be tried.
I am printing this new shape to compare against the original circular port.
Posted By: Mark Parker Re: A74 Port model - 07/07/21 11:42 am
This is very interesting Dave. I do art, and this is like reverse sculpture because you are creating a hollow space and it is to direct and deflect air that can be traveling 600fps through that passage, so anything straight is free hand and not particularly straight, ports sort of match but are not exactly the same but can flow within 1 or 2cfm of each other. Curves are complex and you get used to what looks right. The right hand white plug I put on earlier represents the best at that size I think. It surprises me how good it works, it's so simple. And it's based on that excellent XR750 port developed from a Gold Star 350 BSA originally.



There are also pictures of plugs from Nascar ports and a new casting could have height for them, though width may be better because the top and bottom turns are closer. It will be interesting to see the next print. There is interesting detail in that

I wish you could put one of my heads on your flow bench.
Posted By: DMadigan Re: A74 Port model - 07/10/21 10:04 pm
Had to wait for more print material. New port printed and measured 88 CFM with the as printed port. So the port widening was almost a 10% improvement but was it enough or can improvements be found elsewhere? One thing to try is to put taps on the lower surface to find out if the flow is separating.
When we did wind tunnel testing at Douglas (ancient days) we had scanivalves - solenoid operated valves that changed which port connected to the pressure sensor. That way we could measure the taps all around the airfoil on a single run. For this even a manual scanivalve would help.
Next I will print a port with no area change.
Remember what the door mouse said, "feed your head".
Posted By: Mark Parker Re: A74 Port model - 07/11/21 2:50 am
88 @ 10" is equivalent to about 147cfm @ 28" that amount through a 42mm valve and 34mm PWK is great compared to the stock head. You multiply by 1.67.

Mine are reading around 160 or more through my vaccs which I could not guarantee are accurate to that set measure, through a 42 valve and 34mm PWK. Can you shape the ports you have made? Make them smooth and alter that shape to see what to plot and print?

Having said that, guys on a car site flowing heads with 42mm valves are reporting over 162cfm I expect with a radiused entry and no manifold. Mine are showing over 170 with a short manifold and radius, what ever that actually is on something that cost no more than $150.

The speed then determines how effective that flow is. But the more it's maxed out for the size seems the better it is. And low lift flow is important.

How hard is it to plot out bearing carriers. Because if you have that you can shop for somewhere that can cut them out cheapest.

If the port is anywhere near as rough as mine when just hacked out it needs smoothing, and will vastly improve. High polish flows just air better but needs to be a smooth shape more than hi gloss. Making them shiny makes irregularities easier to see. But I think it's maybe 400 on a stick makes a nice smooth, less than polished finish.
Posted By: DMadigan Re: A74 Port model - 07/11/21 3:40 am
Most modification would be build-up. The wall layer is not thick enough to allow carving. I could put a sanding roll on an extension to smooth the walls before installing the guide.
I could try fairing in the guide. Putting pressure taps along the bottom and top would be interesting.
No point in measuring without the carb as it is needed in real life. When I first measured the flow it was 81 CFM but them noticed the slide dropped about 5mm.
I have the constant area cross section intake printing now. It will need another guide. Probably should print another head so I can make changes between the two and compare. I will print the exhaust valve with the head because there is no need for it now. With two heads I can compare the flow with lift.
What bearing carriers did you have in mind? I can make them on the CNC lathe or mill.
Posted By: DMadigan Re: A74 Port model - 07/11/21 8:50 pm
Here is a comparison of the first port with the constant area port -
[Linked Image from live.staticflickr.com]
The wall thickness is constant so you can see the difference quite clearly.
The area was linearly increased from the carb to the valve which might not be the best having the discontinuity in the area curve. It does look a bit like Mark's port.
Here are the height, width and area plots for the second port where I reduced the amount of constriction, the linear area port and a new port with a quadratic increase in area.
[Linked Image from live.staticflickr.com]
The blue height line is the same for all the ports. The second port which reduced the constriction resulted in a not very smooth width but better flow than the circular port. The linear increasing area port shown in the above picture reaches a maximum width over height ratio of 33%. Using a quadratic increase in area with zero slope at the carb gives a peak width over height ratio of 29%.
The quadratic area port will be the next to try.
Waiting for another head to print.
Posted By: Mark Parker Re: A74 Port model - 07/11/21 10:49 pm
Outside alternator and behind clutch which needs the seal. Just with 16005 bearing. Both use spacers to fit. The front one is captured by a circlip.

Smoothing the walls would be a good idea. Different carbs flow differently and you usually do not get sent he carbs. So it's not a bad idea measuring with a radiused entry as well. The 71 head at least this one with numbers blank, is better with it but not the 68-701.

You could try more exactly copying my port lowering it. Matching curves. It looks quite different to stock till you get used to it then it looks normal and the stock looks odd.

I'll measure one.

Std 29.5mm entry. width @ guide 18mm Height just before guide 27mm Smallest width port 27mm 109cfm, less through carb.

Ported 29.5 (40.5mm valve) 34.5mm 25mm 29mm 143cfm bare 154 through radiused entry.

34mm carb oval entry 38mm 22.5mm 34mm 42mm valve 160cfm + through carb. Measures over 170
but likely mid to high 160s with radiused entry. Seems fantastic on a std Firebird, great response from low rpm, midrange and vibration area where it goes nuts.

Very nice to ride. Except for vibes.
The small one I might try wider and then with a 42mm valve and see if it can be better. It could always go to 32mm if necessary. As the more it flows the more hp and midrange and response. It just needs to stay reasonably small. But you do not know until on a motor exactly what it does. I don't think there is any reason for running less than 34mm really, but this can be very stock looking.

But here in is the problem. And some interesting stuff.

If the std 68-701 port is the usual 109 the std '71 I have with numbers blanked out is 138 and 142.5 through a bell. Better than any stock Commando head.

It's std but I'm not sure if all '71s are like that. Because I'd measured them before and should have noticed. The work on that is in the width and depth of the bowl area. I wonder if A70s got something special?

The 29.5 68-701 ported is 145.86 bare, 154-156 with bell. The problem is the 30mm Concentic. When fitted it's around 140 only. Restricting a stock motor to around 66hp @ 7,500+ with a bit of luck. Why the latter Spitfire heads had 32mm Concentics I expect. And why the GP at that small size may flow a little better than 30mm Concentrics on this head.

[Linked Image from i.postimg.cc]
Posted By: DMadigan Re: A74 Port model - 07/12/21 7:11 pm
Measured the linear area duct, 96 CFM or 160 CFM by your measure.I smoothed the walls a bit but not polished like yours, more like the as cast.
One problem with a bell mouth, unless the shape is the same it can have quite a difference on measurements. I have a comparison somewhere of shapes, I think a 3:1 ellipse is optimal.
Next would be to make the quadratic area duct.
When it comes time to put the bearing on the mainshaft I would make a complete new door, same with the crank. The GSXR rotor is much lighter than the Lucas and the stator/bearing mount is best made in one piece. The cases that I machined for the 90 degree XS crank use the bolt-in stator mount so it will be easy to modify it for the GSXR.
Posted By: Mark Parker Re: A74 Port model - 07/12/21 11:19 pm
I have a couple of radiused pieces and they do about the same even when they don't fit exactly. I tried polishing this little port but then adjusted it a bit and used a 340 red paper taped to a stick to finish it. It may look polished but it like a sheen. I might polish the exhaust ports a bit as it looks better than rust.

[Linked Image from i.postimg.cc]

It seems to be the bowl and guide/valve area that is crucial, and the carb. Lectrons give very little disturbance because they are so smooth bore. Just holding the 34pwk against the little port increases flow a bit, the 30mm Amal chokes it a bit. The step with mismatch sizes like 34 carb 30mm port isn't as bad as it seems because the port is sucking the air through the hole and has a bigger area to get it from.
Posted By: DMadigan Re: A74 Port model - 07/13/21 11:06 pm
The quadratic area curve intake as printed gave 94.5 CFM and smoothed a bit gave 96 CFM so no improvement there. The open port with no carb, just the step in diameter for the carb nose gives 98.5 CFM.
It might be worthwhile widening the area where the guide and valve stem obstruct the port. The final area will be smaller due to the stem.
Posted By: Allan G Re: A74 Port model - 07/14/21 7:44 am
Don’t forget that most modern vehicles (more so performance ones) are using a rough port and not a smooth one. When a smooth port will flow well for air will will also allow fuel molecules to stick (shiny port) where a rougher port will not (or not as much).

I was looking at the drawings for the Mk4 spitfire heads yesterday in Peter Crawford’s book “Thunderbolts and Lightnings”. The images are clear enough but the print not so much. But it gives something interesting to compare against my small port heads. Which in their stock form perform far better than the 29.5mm port heads.
Posted By: Mark Parker Re: A74 Port model - 07/14/21 9:35 am
I did the other little port, it looked the same but wasn't, it's very particular. Now it's within a couple of cfm. 143 through a 30 Amal. But the front of the carb could be profiled for a little more like was done in the day. But a good bell mouth covers that anyway.

That's enough to enable 68hp from a stock engine in theory. It's 154+ through a bell like the other side. It's not easy to cut them out and get the flow through such a small port. Testing with a probe in the centre of the port it reads 48" on both. Vizard's chart only goes to 38" and 400fps. For comparison the 34mm port with 42mm valve is 46" with 44.5mm valve it's 42". The 34mm carb and 46" 400fps+ port works splendidly.

I will try and keep this little port to std valves if I can. It would be fun on a Stock Spitfire esp with GP2s and high comp. Especially if a mate had a triple or a Bonneville.

[Linked Image from i.postimg.cc]

Allan you can polish the port to look like chrome almost, it used to be done. In Vizard's book it shows cast high end ports and ones ported by machine and by hand. They are not polished but more like what I'm trying to get, smooth and almost a sheen. Sometimes they have ridges to help wet flow. The latest photo is rougher finished but I've seen Ducati guys porting years ago that must have read about rough and they really were, I hope they were not the finished thing.

XR750 port. For a two valve race bike these are pretty up there. Based on a 350BSA Gold Star. But developed into an oval port, it's very efficient but requires rpm 9,000+

[Linked Image from i.postimg.cc]
Posted By: Mark Parker Re: A74 Port model - 07/19/21 10:51 pm
I bought some soda blasting stuff that cleans the alloy nicely. If I run it in the ports it dulls them off a bit. It's not as aggressive as sand and there is no sand.
It does not promote corrosion like some chemicals and it cleans better.

[Linked Image from i.postimg.cc]
Posted By: DMadigan Re: A74 Port model - 07/20/21 12:19 am
Tried another variation taking into account the area of the guide and valve stem on the width. To make the area follow the quadratic curve the width has to make a sharp jump essentially the opposite of the area curve. I put a fourth order curve fit through the points and made a port with ellipses of height and width at each station (blue line). The width has to come back to the valve seat diameter somewhat quickly. This port measured 95 CFM as printed.
Accounting for the guide and stem obstruction does not appear to make a big difference over the quadratic area port.
To eliminate the diverging walls of the quadratic port where it meets the seat I calculated a cubic area function with zero slope at the carb and seat.
[Linked Image from live.staticflickr.com]
This results in a wider port at the start of the guide (~3") with a shallower convergence angle of the width at the seat. This will be the next print.
Posted By: Mark Parker Re: A74 Port model - 08/14/21 11:52 pm
I started another big valve head the other day. And made a new curved manifold to get the angle better. I tried a little different shape but both cfm and speed are quite down so far.

So I put the new manifold on the previous 44.5mm valve head. These two heads had big valves and ports from years ago. These had less than 160cfm trough 38mm ports. Because the ports needed filling I could move the port down and line it up better.

This compares gaskets, The port with the 30mm Amal fitted flows a little less than 109. And the big valve port is probably giving more than a 650cc could use without turning very high rpm.

[Linked Image from i.postimg.cc]

What I did to try another way to measure was run the 4 vacs flat out. On the plate with a 162.4cfm hole and on the ports. The 162.4cfm plate pulled 34"w the good head through the carb 30.5"w and with no carb and radius 29"w.
Posted By: DMadigan Re: A74 Port model - 08/15/21 6:07 am
Finally got the print done for the cubic area transition intake and attached it to the head. Same settings as the others read 97.5 CFM as printed, a slight improvement. The sides appear to angle more inward toward the stem than the previous port. I will put the quadratic port inside the cubic and difference them to check the transition is correct.
From the top the port looks like a cobra head seen from behind.
[Linked Image from live.staticflickr.com]
Posted By: Mark Parker Re: A74 Port model - 08/15/21 7:49 am
That's really good flow with that valve size Dave. The question would be port size and speed in the port, but the carb size is limiting it down. There is so much suppeltey to the bowl shape, I do not know how you can print them. If it isn't smooth you also loose flow. I looked up some port molds and they often have approaches to and from the guide, though they can disturb stuff depending on where the air goes.

The difference between the good and bad port is width at the guide, good is 39.5mm other is 37mm. The port height is also less 23.25mm compared to 25mm. 39mm width at the guide is getting dicey on the std casting, going through between the cooling fins is patched easy but into the pushrod tunnel more severe and complicated.

The 34mm carb takes around 10cfm 191 drops to 180 and looking at this beautiful short stroke Norton 750 running against V4 Hondas and stuff from the '80s you can see how top end on a fast track would be important. Doug uses 36mm carbs and 8500rpm. It's great to watch as he makes top videos.

Posted By: Mark Parker Re: A74 Port model - 08/18/21 4:56 am
I'm on the second big valve port and waiting for JB to cure. I fixed the first one. Width is the thing, it may be different if it's higher but there isn't much point when there is nowhere to go and it isn't necessary. It's dicey getting them to 39mm width or more but it gets the flow.

The ports with 42mm valves may work better wider, but maybe when I do another one. These with 42mm valves are 37mm wide approximately, at the guide, compared to 28mm with a std 1970 head. I think improvements on the 42mm valve heads might come from width. If they put a little more metal in the pushrod tunnel and between the fins on the other side it may be better, but compared to Triumphs and Nortons and triples it's miles ahead.

The 38mm XR port is 49mm wide, my 883 port is 43mm, but it goes through both sides. The same width ratio as the Harley, the 34mm port would need 43.84mm width. But going through means a less than smooth surface.
Posted By: DMadigan Re: A74 Port model - 08/18/21 2:05 pm
The last port has a maximum width of 41.54mm at 77.6mm from the front of the carb spigot, 31.4mm from the upstream side of the seat insert (along the port centre line). 34mm carb I.D., 36.5mm I.D. seat insert. The valve head is 42.16mm diameter.
A trend line through the cubic port area including guide peaks at 46.5mm width at 82.4mm from the carb spigot, 26.5mm from the upstream side of the valve seat.
Posted By: Mark Parker Re: A74 Port model - 08/19/21 8:38 am
I have the r/h port done ready to test, it's 39mm wide, if it's not measuring 178 through the carb I'll go wider on the outside as it's easy to patch between the fins. I'll have to watch just where that stud is though. This is another old head that had 44.5valves and 38mm ports. Ben's 750 has that size but is better shaped and flows much more. I can swap his head with the 34pwks and it should pull better, and I don't think maximum flow is much different.

I'd like to know what the Westlake/Rickman 8valve heads flow, the modern Triumph 4 valves are 181cfm.

On a triple, though I no longer have one, I'm pretty sure those studs are very close. The triple heads are definitely not that cheap or easy to get.

[Linked Image from i.postimg.cc]

This is that 38mm carb head, it would be interesting if I can get it better as it's going to be near the valve that is the limitation. If this flows more the speed will go up.

[Linked Image from i.postimg.cc]
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