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NickL #847317 04/27/21 7:03 pm
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Re airboxes,i read somewhere that an effective plenum needs to be around 3 times total cylinder swept volume, out of curiosity I taped up my OIF airbox while it was off, filled it with plastic foam beads, then poured them into a measuring jug, it came to roughly 2.5 litres, so in the ball park for a 750 motor. I am convinced it helps in the low to middrange, certainly better than the previously unfiltered short bell mouths, with no perceptible change at the top end.


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NickL #847347 04/27/21 11:41 pm
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If the cam has a large overlap then at lower speeds, you loose a lot of charge out of a carb
with no bell mouth or plenum, certainly with short inlet tracks. The splatfire cam has a fair
amount of overlap for a road cam, plus with big inlet valves you increase that pumping function.
With that engine as you have it, try making a stepped key and retarding the cam by a fraction
you will find it's an improvement. Around 1/4-1/3 tooth is all you need. It fiddly to do but a small
flat file and a vice is all that's needed. Start with a drive side key.

NickL #847377 04/28/21 10:53 am
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Yes filing keys, I still have the bolt I cut them from. My main reason was piston valve clearance. Firebird is stock valve timing, it works great. Just think 600 supersports engines make well over 100hp. Maybe a 650 can get around 80 around 8,000 with very decent midrange that the 600 4s lack.


mark
NickL #847636 05/01/21 3:49 am
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I remember talking to a pair that were at a practice day with with a very nice triumph outfit years ago,
they had spent bundles on the engine with hairy cams etc, they could not get the carbs to work with
short stubbs on. I suggested trying a couple of coke cans with the tops and bottoms cut out and put on
with a couple of jubilee clips...... Voila, instant success, it would actually pull away and drive out or
corners without spluttering etc. I think they went away and bought a set of longer bellmouths for it.
If it were me i'd have left the coke cans on just to stir the scrutineers up!

NickL #847680 05/01/21 4:02 pm
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Originally Posted by NickL
I remember talking to a pair that were at a practice day with with a very nice triumph outfit years ago,
they had spent bundles on the engine with hairy cams etc, they could not get the carbs to work with
short stubbs on. I suggested trying a couple of coke cans with the tops and bottoms cut out and put on
with a couple of jubilee clips...... Voila, instant success, it would actually pull away and drive out or
corners without spluttering etc. I think they went away and bought a set of longer bellmouths for it.
If it were me i'd have left the coke cans on just to stir the scrutineers up!

This is why it's nice to get opinions and ideas from others, some people have a way of thinking outside the box...who'd have thought to strap on Coke cans?
I have a brother who is able to almost instantly see such solutions, often makes me think, 'Now why didn't I think of that?' Anyway, he tried a set of 3" velocity stacks on his stock T120 with Concentrics (1970's) and said that the bike really came alive (assuming now from midrange and up). Living in a dusty desert, I couldn't see running without air cleaners, I don't think he did for long. The question I ask now is why didn't I try to incorporate velocity stacks into a filtered airbox of some sort? My bike with cams and such may have even been better. And as a teen, I thought a cam had to be degreed exactly to spec, I had no idea I could have advanced my intake cam to close a little earlier to get back some low end that I had lost with the PM 1060's, came on strong above 4500 rpm but with 2 up not so sweet down low. Young and dumb I guess.

NickL #847863 05/03/21 3:18 am
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Plenum effect is back, if I move the box around over the thing running the vacuum moves up and down, so the effect helps when it crowds the intake a little.

So through the std 40.5mm 152cfm ,about, and 157.3 through the seat opened to the 42mm valve, both with the 34mm carb. If my base calculation is correct. So a stock valve with 34mm carb should perform really well with this port and I will do a head.

I cannot do the 42mm seat properly without the proper guide fitted and K-lined which is annoying. I cut it so I could mark it with blue, but hopefully will be close. The worst thing on these is an automotive cutter to cut 3 angles at once, as the hemi insert then gets cut in with a step to get a top cut, the deeper the worse.

The plenum effect gives 171 with no carb and radiused entry. I guess the more it reads whatever it actually is the better. And it reads consistently and the comparison is good. But now is pulsing a bit. And higher lift is worse than .350" it makes more noise at .388" but doesn't flow more. so maybe the air is leaving an edge and breaking up at higher speed, and its faster but blocked by some turbulence?

Through the carbs it's at least + 45% on stock. I'll take some photos when I change it over for the other side.

These little welded manifolds are round 34mm tube squashed at the head end to be 30mm high and whatever wide the threads are helicoiled to smaller dia for these screws to fit.
[Linked Image from i.postimg.cc]

They are within 2cfm and one is better further open for some reason. One side broke into a valve spring recess frown so a patch. The SRM race springs fit shorter so a washer can protect the little patch.

Last edited by Mark Parker; 05/03/21 4:59 am.

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NickL #848222 05/06/21 10:59 am
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The 42mm valve and 34mm port is up in flow by about 49% through the respective carbs. While I had a stock head in there I tested flow to see what a higher lift cam would do. I did it minus carb so might try it with it. But it went from 109cfm @ .385" to 110.2cfm @ .500". The head flatlines above .385".

Mine flatlines around .350"? So the bottom line is getting more air, so fiddling with more timing and lift will not do much.
I ran the 3 vacs flat out, the std head pulls 10" more water to flow 133cfm! But that's not available anyway as it would requite a blower.

The stock head is pretty quiet, the 34 roars and squeals with the carb on. It's 160 through the carbs, probably with plenum effect, and 170 with bell and same deal. It's measuring air going into the box which is over the whole deal, two meters, so I have to read both, but it sucks the box onto the bench and sucks the sides. I tried a couple of test pressures and they are the same within 1 or 2cfm.

So the vacs have to suck much more on the flowed head because air goes through much easier and faster, which drops the vacuum and you measure at the same pressure or vac. If the valve shuts the cleaners eat the water. And the more restrictive the port the more vac you bleed off to get the test setting. The size to flow quantity effects speed, which has energy and that energy is pushing air into the cyl. It seems to do this very well.

Maybe the valve is holding the air against that tight lower turn wall, and as it opens more the air comes away disturbing the flow a bit.

[Linked Image from i.postimg.cc]


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NickL #848239 05/06/21 4:08 pm
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Almost hate to comment since you are so far ahead of me when it comes to this topic but couldn't the increased valve opening also let the flow pull away from the top of combustion chamber that at lower lift is protecting the incoming charge from 'free air pinch'? Something I learned about reading Kevin Camerons stuff. Sure wish you were playing with Triumphs...Mark R.

NickL #848282 05/07/21 12:03 am
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I have no idea Markster. That valve being there is effecting the air coming in. The rubber plug is from the Thunderbolt head which doesn't suffer that particularly. Ports go flat after a certain lift mostly anyway. But one on this head is worse on one side. If the port floor angles up at all the faster it goes the more it pushes off that turn down. Like a ski jump. I'll have a good look at it this morning.

Not sure what a 406ci chev normally puts out, but with ported alloy dart heads a good manifold and carb can make 692hp at 7200rpm. Lots of stuff makes a difference enabling it but it would not happen without the heads and what they flow. If each inlet flows 400cfm which I doubt, that's 1600cfm per bank, equivalent to the BSA's 160cfm for each port. Dart heads say they can flow over 300cfm per port, how close to 400cfm I don't know.

I also read this; If you’re smart, you’ll begin by looking at a cylinder head’s mid-lift flow numbers instead of the peak flow numbers, since these mid-lift numbers will probably contribute more to overall flow (and power) than the peak flow number.

So through the 34mm carb .250" is giving 136cfm, .300" 155cfm and .350" 160cfm. So it;s probably good this way.

[Linked Image from i.postimg.cc]

I'm not sure how restrictive a Triumph head is with head bolts, the triples are. Basically for the valve to flow the charge needs to turn down onto it, so lift the floor, but it needs to get around that choke and height is limited so wide is what's left, and can be done if there is space for it and metal.

Last edited by Mark Parker; 05/07/21 12:07 am.

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NickL #848284 05/07/21 12:32 am
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I wish you could have ridden my outfit before and after i fitted the cam i ended up with.
I started with a spitfire and tried about 4 different cams over time. The spitfire always
ran out of breath at around 6k with the big engine. That was with a big valve head and
reasonable porting, 33mm carb venturis. With the cam it would rev well past anything
i ever used it to and would pull a house down in mid range. Max HP jumped with no loss
of tractability.
Whilst i respect your work on these heads i cannot agree about the performance gains
with a better camshaft. My only experience was on the track and lap times were what
counted, i never had the head tested for flow etc. I do know that at my last meeting in
the UK at Brands, we had a bloody good battle with a 900 weslake and the guy who rode
it even commented on how well my old beezer pulled out of corners. The engine was
only 820 at that time running t140 pistons.

NickL #848299 05/07/21 3:19 am
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More lift on my 883 made it better, moved the power up. As an 810 it would pull well to 7500 on the data logger about 55hp as 883 those 38mm ports and 44.5mm valves ran out around 6000-6500, and it died, though it had lots better power while it worked 65hp. Offset rocker buttons added about .020 lift. And it was better. And I was able to get 74hp. It pulled to 7,000. But changing the head made 40cfm difference with the port the same size and it's still pulling at near 8,000 and the guy wanted to give it 1000rpm more to see where the graph went over. I wasn't that curious.

It would be interesting to run it again and see what richening it, and better mufflers did, but I don't want to blow it up doing rpm it will never see. A better cam may make it better, if it was easy to swap and cheap.

It's just I measured the std head and extra lift doesn't do much, but different heads can act differently how they respond. And duration makes a difference.

From what I've seen the A65 stock motor isn't restricted by the cam so much as the head. It's not a direct relation ship between cam spec and rpm levels when cylinder head flow is changed appreciably. More air allows higher rpm. If the head is at a good place, then testing cams can show what spec works best.

Why I'm interested in the difference between how the current rig goes with the two different cams. Is one noticeably better? And it's not particularly peak hp but drive.

I'm thinking I should give the Firebird a short dyno and hope the mixture is close and it stays together. It's not like I don't test it on the road but it makes me nervous. Like going home, Deb where's the bike? I need the trailer and degreaser. Should get Ben to take it up.

The head on the 883 didn't make much difference in the mid range, the Firebird feels different from when you take off. And seems to totally transform it in a 650cc kind of way. And it's hard to quantify without a stock one to compare, so I need some sort of test. The SV is out of rego so I cannot do rollons with that which would be easy.

Last edited by Mark Parker; 05/07/21 4:52 am.

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NickL #848316 05/07/21 8:16 am
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With an 89mm stroke i was never game to use 8k rpm. I would have visions
of tops coming off pistons etc. Plus lower rpm gearchanges seemed to give better lap times.
When the last motor i had was on the dyno with 80 x 89 it was getting 80hp+ at 6800-7000
it didn't get much better at 7.2k which was my maximum. It had B44 slipper pistons in it then.
It's not an easy thing to get a sidecar on a dyno here i've found, the shops don't want to know.
To much trouble to fit it on etc.
Using those offset rocker arm things is not a great idea i was told, they have a habit of breaking off.

Like so many of these things, testing the bike is a problem up here, there are no practice days
anymore, so if i try something it must work first time....... Or the weekend racing is stuffed.
Back in the uk, Mallory Park was open every Wednesday PM for bikes, we got 10mins in each hour
for sidecars. Plus most clubs had practice days prior to meetings at the circuit. It's a bloody long
trip to go down south to test the old crate as all our circuits (3 of 'em!) are shut or booked solid.

Last edited by NickL; 05/07/21 8:17 am.
NickL #848321 05/07/21 9:42 am
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I have JE forged pistons and good ring pack, they are not the worry, it's everything else.

Trivia for tonight. I started doing a head to run std valves. I will call it head X. So I made manifolds first so I could line them up. I want the floor to run in one sweep over onto the valve, being straight is ok, but if going up the the air wants to continue, if it starts to curve down it might follow the sharpening turn once it has started. So I traced through the manifold with a scribe to hack out the oval, then hacked out the sides of the guide area and bowl then joined them up checking where the manifold aimed and where the floor would be.

So put the manifold on, a valve and loose guide in, and tested with calibration from the stock head. Of interest full vac on the stock head was pulling 6" more vac than X.
Stock 109cfm, and through the X, 34 carb is 142cfm, 148 with a bell on test pressure. So on track and the floor is now setting.

Interesting trivia, I reduced the test vac on X without carb or bell by 4" and it's still flowing 126cfm (instead of 141), which means that at lower rpm making less vac the head is still flowing more air and the motor will make more power, 17cfm is quite noticeable. When the port is done, I'll check that again with the finished port size, which is smaller. Smaller effects speed which effects how it pushes in. It wants speed and air volume not one or the other.

Should get club rego Nick. Just need some lights. Only a pink slip here. 60 days for testing, just fill out a log and go. Or go on club runs. You might have to turn back if you're run is to Cairns. Just say it was getting late or something, so going home. They can check if there was a run and you are fine, if you are not on slicks. And you have always got, slicks were allowed on the isle of Man. Don't wear patches. Probably get busted for wearing 'Sons of Arthritis' up there.

This is the starting point and I think it's the better '71:

[Linked Image from i.postimg.cc]

And on the left, it's wider and deeper in the bowl than earlier heads and about as good as the better of the Commando stock heads. See how it's narrow at the guide where the guide obstructs flow. The one on the right is the exact same size with 30mm hole but flows 143 or so. A 650 doesn't need them that little.

[Linked Image from i.postimg.cc]

Last edited by Mark Parker; 05/07/21 10:29 am.

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NickL #848363 05/07/21 11:18 pm
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No club rego here and the 'sons of arthritis' mob are way too hard for me.

Who did your nicasil coating Mark? and are they any good?

NickL #848373 05/08/21 1:12 am
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Place in Melbourne done once and no problem. Place in Qld is much cheaper, however it came off twice and the second time destroyed the piston, so cheaper to pay more the first time in Melbourne than twice plus piston.

John Hill in the UK does alloy cyls and Nicasil bores to your piston size. It's more expensive to machine and have bores done here. A JE forged piston can run .002" mine are .0025". I've not tried the 76mm A65-70 pistons EdV does from JE, Just 79.5 and 80mm B44. They work great with the long stroke because the skirts are short. They really need longer rods. I had a taller alloy block cast but never used it.

[Linked Image from i.postimg.cc]

This is a sketch of the XR750 port. So I'm trying to improve the bottom turn and make everything smooth. And stop break aways and turbulence, the turbulence I think it needs is entering the cylinder and its what it is.

[Linked Image from i.postimg.cc]


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Electrosil?

Humming and harring about doing a big motor with the
Norton crank i have here. Couple of options really, make it
90deg (never tried 90, used 68 +76+180) have the crank
ground down to A65 size then he can use his map rods with it,
or it means corrillo's. Big cost these days. Seems a shame as
the crank is standard and would dress up at +10.
Just mulling it all over at the moment.
If he uses his rods i can have the crank ground to give 90mm stroke.
Another option.

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Ideally 80x74. Or 80X89. And 90degree. Map do long rods for Nortons and for those cranks in Triumphs. But they have a smaller pin. But bigger bolts I'd expect. JE will do custom piston sets in 4s I think. You could use B44 spec modified. You could try the pin higher in the piston, or longer cylinder. They need long rods to cut down the side thrust at rpm and vibration.

80x74 lets it rev. In theory the 34mm head may see 90-100hp and pull hard in the midrange with 744cc but it may need biasing a bit for top end with bigger dia headers. It depends what that size head supports, and what it does. The idea that the opposition use big bore engines might mean you can as well. If they say Devimead didn't produce them till later, I made mine, made a pattern and had it cast. Anyone with a back yard could have done that.

An 80X74 could be 360 if its balanced and is ok till 8000 or a bit more. The bigger bore will breath better and you could go bigger valve and carb size. That XR750 port is huge but they pullover 9,000 and wear out. I couldn't fit all the XR port in the BSA head. It's 49mm wide, I could only get 43mm but I'm sure the big motor could get 100hp if it would stay together. I had 8mm head bolts now 9mm and the back has 10mm. Because it was pulling the threads out. It ran fine on 12-1 on 98 but piston to valve was pretty close. It was 11-1 when I dynoed it. 12-1 and correct mixture might have meant degreasing the dyno room.

12-1 gave unreal response from the big thing, I thought it was about 10-1 till the platting failed and I measured the good side.

A 12-1 744 might do the job. I know bigger can have more torque in theory, conventional theory, which I'm starting to think is questionable. Have you seen that video Matt has on his page of Asians in sandshoes and jeans drag racing little strokers with almost pushbike tyres doing 10 second quarters? I think one did 10.6 and nearly 200kmh. Like a pushbike with lawn mower motor and chamber almost.

I did this head for Chris in the US. I don't think he's run it yet. His bike had an A10 crank, high comp and had a stock 71 head except for 34mm TMs, if I remember right, a LSR and very fast. It made 60hp on the dyno, around 68 at the crank. It would be interesting to see what it dynos with this head. It looked really nice.

[Linked Image from i.postimg.cc]

This was experimenting with a round 32mm entry. The X head is off this scale .300" is over 165 and .350" over 172 what ever that actually is. And I expect it's wrong, but under the same conditions and comparable.

Putting hp graphs overlaid like this would be interesting. I put the calculators figures on the right. But it's power curve that's so important.

So overlaid dyno graphs would tell us a lot.

[Linked Image from i.postimg.cc]

The calculator has settings of how hot the motor is to get 68hp. With 110cfm it needs the super hot 'pro-stock setting,' which predicts 65hp, a bit less than the reality. But I think the rpms are right around 6,000, no more after that. So with 160cfm that same setting gets 94hp? It's rpm range predicted is above 7,800 though. On the milder race engine setting if it actually had 122cfm, which I checked a 71 head close to lately, then at 160 it would be 84hp. Still at higher rpm. Why using an 80X74 might be better. That's allowing a bit for the head to be over reading. But the 34mm head on a 750 could be right in the ball park, as it seems to be on the 650 at the rpm you can hold onto the bars. Even with lower comp and stock pipes.

Last edited by Mark Parker; 05/08/21 12:11 pm.

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NickL #848456 05/08/21 8:50 pm
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We are hoping to see Chris this year. I did the dyno work with him and was really amazed at the numbers we saw at only 6k. With your head it should make some serious record breaking horsepower.. PRT
PS, His bike is beautiful!

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Going to go with 80x74, looking at the 90deg stuff at the moment.
It'll be a while away though. May run as a 360 to begin with. I just
feel happier not using that A10 crank.

He can always have the crank built up a couple of mm if he wants to take it further later on.
Lots of blokes used the srm 750 barrels with 85mm cranks, just made a spacer and modified
the tappet area.

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I hope it goes good for him. If he dynoes it, it would be most interesting to compare graphs. That bike is very photogenic.


[Linked Image from i.postimg.cc]


I think this is Chris's graph I drew on tests done by a US magazine around 1969 or1970. RWHP to convert to crank hp as used by factories times by 1.1343.

You can see the advantage or not of marketing by the factories. Lots of people loved to be able to say their bike was the most powerful. But manufacture created variation as did preparation, though these would have been to factory settings. The Trident was a good one and maybe the Honda wasn't.

[Linked Image from i.postimg.cc]

This is my 883 on the same graph. Different dynos but sort of comparable. The BSA would have been better at lower rpm without being on full throttle. The first run read 85 on the screen but the rpm trace was messing up. Which doesn't effect the reading at the drum but was meant to control rpm which it didn't. I was told not to worry about these readings it was way too lean. But this is breathing influencing where the graphs go. Lots of components influence that to make the best use of the breathing you have.

[Linked Image from i.postimg.cc]

Last edited by Mark Parker; 05/09/21 8:56 am.

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Mark P., when starting as a line boy (1970) at a Honda/Triumph dealership, if someone wanted to take a test ride on the CB750 we would go along on the Trident as we knew we could always catch them.. just my experience so maybe your graph is fairly representative of a typical Trident or Honda of the day. Mark R.

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Looking at those a 6500 rev limit is about right.
Little point in going beyond that, all those revs for just a few horses.

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Except Chris's graph is from dynoing with the std '71 head with 34mm TM carbs. Not the ported one. Which I'd like to see when and if he tests it.

BSA tested a race engine in 1970 with A10 crank and 34mm carbs at 78hp @ 7000, power was falling by 7500 to 77hp. Really nice power curve. They put F750 3 tests on the same graph revving past 9500 trying to match it, which they almost did. A tuned 654 was kicking the 3s butt till about 6500. Why dynoing a 654 with this head might be interesting.

So where that head does its best I don't know. But I know the A10 crank problem. Maybe an outrigger bearing may allow more safe rpm? Or better safety for what rpm you run.

I emailed Gary from SRM. He was saying a customer dynoed his road 750 80X74 with 68rwhp using their race cam and ported head. That's around 77hp at the crank.

He is very enthusiastic about their race cam, saying they tested many cams in their outfit to arrive at it. He's also enthusiastic about a 90degree and the extra torque they produce. And loves what you are doing. With the most underrated British twin ever. He left SRM recently, but said they have a couple of their race cams for 90degree engines.

Comparing the flow of a stock head to one I'm doing with stock valve, I can turn the vac down by 7"w and it still flows 16cfm better than stock. Which means bottom end and midrange should be better, as with response as it turns more air on whatever rpm you open the throttle I expect. So it's a bit weird. A plate near the air intake flows a little better till it sucks it on. You have about 1/4 of a second before the vacs eat water. I extended the hose toward the roof before it loops over.

Pretty much finished that port. Std 40.5mm valve flows really well. Still has a 34mm manifold at the other end. The difference between stock and ported is about 9"w if that means anything to anyone, That's with 3 vacs flat out pulling max vacuum on one then the other, the stock one is that much more restricted or choked, through the exact same valve. If my measuring gear is a bit sus this difference isn't. Its pretty unconventional wisdom that you can get so much more air through a std valve on a port that isn't much bigger. It make you wonder if it can work.
.

Last edited by Mark Parker; 05/10/21 8:40 am.

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NickL #848612 05/10/21 11:06 pm
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Yes, the SRM cam is not a typically radical cam, in fact it's milder than
many 'street' type cams around. The quietening ramps are very effective
too. With that in my old 730 thing (79x74) i achieved 64hp @ around 7500
That was the best i could get with that engine back then. It had a pair of
those nucleas valves, standard size in it with the weber at 33mm and long
trumpets. The short large radius type may have improved it a bit. That was
the sweetest and most reliable of my racing engines, revved to 8k no problem,
started easily, was quiet, relatively smooth, just 20hp light!

Gary must have retired by now surely? i used to speak to him back in the 90's
i doubt if he remembers me though as we were never quick like Stuart D.
He dug out some CR gears for me back then when they were hard to get.
Over the years i bought a lot of stuff from SRM, 750 kit and two 850 kits
the last 850 kit was very good the casting and material was really nice
that was in about 2000. They were always a good outfit to deal with as the
blokes had 'been there and done it'.

At the moment we are looking at the gearbox problem and sorting out speeding
it up and then i can address the crank outrigger, i'll have to make up some sort
of structure to support the bearing as with a bigger sprocket i can't easily access
the existing tapped holes for the alternator supports. I'm working on it, as they say.
Putting a small needle race on the change shaft has made the horrible early
mechanism around 250% better, so i'm pleased with that. It'll probably wear the
shaft a bit in time but we'll see. I put a small o-ring in there as well to seal it.

BTW, we have another A10 crank, maybe a 90 deg version of that eh?
Another option.

Last edited by NickL; 05/10/21 11:29 pm.
NickL #848639 05/11/21 4:28 am
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I have a flywheel centre machined up. I don't think is has bolt holes. It was a spare. And I have the last crank I had cut together because I might put it in the Firebird. I don't know exactly how strong they are, other than they are extremely tough to cut. There are lots of nuts and bolts and locktite. I tell myself the flange is bigger diameter than a Norton with more bolts plus the big ones up the pins. I advanced the left cyl, SRM the right, so it needs doing their way to suit their cam.

I think Gary only just left. They port heads and use nice machines and have a flow bench, but have never used it? And he doubts anyone there will use it.

I have a map we gave the machinest to cut the crank and bolt it up, I'll see if I can find it.

Bits

[Linked Image from i.postimg.cc]

This may be interesting, This is fit the cylinders easily on your own. Advantages, the block sits on a piece of wood and gravity stops the followers falling out. The pistons are already fitted in the bores and hanging out the bottom with inner clips in, fit pins at your leisure and clips. Check about 7 times each. 90 degree doesn't matter, put sealer on the joint, check followers, slide it on.

This is the first cyl I made with thin liners. I flogged it for years. The pistons were cast and had low skirts that came out the bottom a bit, It cracked a liner. So I then used Nicasil and pistons that fitted properly. I regarded this cyl scrap. I practiced tapping the threads oversize. Drilling the old holes bigger using a very quarely set up press was no good. It was not accurate. Holding between feet and drilling straight down meant you could feel the hole and get them exact, It didn't matter, it was practice and just scrap. Have another set machined almost ready to Nikasil$$$ for Bens 750. But I had a liner and it could be replaced and bored pretty cheap. And we had Pauls old 79.5 pistons done a million miles but sort of ok.

There are reasons not to but it was cheap the holes had threads and were straight enough to get bolts in. What's not good is liners can turn, doesn't matter on a short stroke because there is no cut out for the rods.

A hot rod 750 would need 9mm bolts, 8mm would pull out, even in Iron they may be a problem. Though if you used an iron block you could wait and see. They are reduced. Std are just under 10mm in pommy thread and distort the bore. The 9mm is rare but used in some VWs I think.

[Linked Image from i.postimg.cc]


This graph is for reference, it's a hot XS650. Stock is around 43rwhp like A65s, this lists what it has with the 750 big bore kit. Fairly close to the A65 750 Gary was talking about. But the aim would be getting as near to 80rwhp as possible with a very strong midrange.

Did the second port on the std size valve head. Before putting filler in, I compared it to the stock one just running 3 vacs on full, stock pulls 9" more water in comparison. It's a fair bit of extra air but I think it's better hitting the valve from the back and being smaller and faster.

Goodness knows what a stock A65 would go like with this head. It shouldn't be much behind the heads with 42mm valves. If BSA made them like this the stock clutch would not be good enough.


[Linked Image from i.postimg.cc]

This is Ben's head, the valves are 44.5mm like in mine. Mine though I lifted the valve guides and stuff. But really for that size valve it could probably flow 230cfm, not 200, and 200 is more than the 883 can use, Ben's is around 190. Better than an 8V modern Triumph head. But the thing is, the port and valve is probably bigger than required. I'm not sure what a 42mm can actually flow, if I make the port a bit wider. On mine it was out between the fins and higher. Numbers at speed mean more than just numbers. Yamaha spent millions on an XS head just for flat track, it made their 750 engines around 100hp.

[Linked Image from i.postimg.cc]

Last edited by Mark Parker; 05/11/21 12:11 pm.

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