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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
Last time I tried this size port it was 137.5cfm just the port so 146 is pretty cool.
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?
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.
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.
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.
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.
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.
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".
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.
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.
Here is a comparison of the first port with the constant area port -
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.
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.
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.
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.
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.
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.
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.
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.
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.
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+
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.