Intake manifold and port

A normal vertical twin race bike is usually horrible at low revs - 1 because of the cam and the exhaust, 2 because of the high balance factor of the crank. The cam and the exhaust usually start to work at 4000 RPM, and the crank is balanced to run smooth over about 5000 RPM. With a megaphone exhaust 5000 RPM can come with a very big bump. With my 500cc Triton, I was always petrified by the thought of being balked in a corner, At 5000 RPM the bike is usually doing about 60 MPH. If the power falls through the bottom, you still need it to get around the corner. Slip the clutch and everything happens - sideways and often down. You are left with a dilemma at 60 MPH. I can get around corners faster than most because of that bike. Most guys never have to deal with that sort of shit.
 
acotrel said:
A normal vertical twin race bike is usually horrible at low revs - 1 because of the cam and the exhaust, 2 because of the high balance factor of the crank. The cam and the exhaust usually start to work at 4000 RPM, and the crank is balanced to run smooth over about 5000 RPM. With a megaphone exhaust 5000 RPM can come with a very big bump. With my 500cc Triton, I was always petrified by the thought of being balked in a corner, At 5000 RPM the bike is usually doing about 60 MPH. If the power falls through the bottom, you still need it to get around the corner. Slip the clutch and everything happens - sideways and often down. You are left with a dilemma at 60 MPH. I can get around corners faster than most because of that bike. Most guys never have to deal with that sort of shit.
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When I rode that bike in a race, I became a guided missile, I always needed a clear run in front of me in corners, and never lose speed. I have never hit anyone, but I used to live with horrible anxiety.
 
Schwany said:
I would imagine a lot of ideas that ended up working out extremely well in the olden days started on a whim and a drawing on a napkin in a bar or a scratch pad on the drawing board or work bench. The continued engineering, testing and finally technical specifications came after the implemented idea (whim) showed some level of improvement. One man's whim is another man's success story.
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You've got it right. 31 is about the max you want for a 750. 30 is OK. C.R. Axtell did a ton of dyno work and found that 30 was good enough. 32mm didn't really get you anything more unless adding bigger valves and displacement etc. I personally found that 28.5 gave poor performance compared to 30mm. The difference was obvious. Going about 2mm wider (staying high) around the guide also helps similar to what Mark has been doing. The benifits of going higher and wider around the guide didn't become popular until later XR 750 and Nascar development. The ports in my race manual are close to the Axtell ports that I measured. Comstock did some flow bench work with those Axtell ports and found them to be excellent. That flow bench graph was posted somewhere on this forum years ago. See it below.

Intake manifold and port
 
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jseng1 said:
You've got it right. 31 is about the max you want for a 750. 30 is OK. C.R. Axtell did a ton of dyno work and found that 30 was good enough. 32mm didn't really get you anything more unless adding bigger valves and displacement etc. I personally found that 28.5 gave poor performance compared to 30mm. The difference was obvious. Going about 2mm wider (staying high) around the guide also helps similar to what Mark has been doing. The benifits of going higher and wider around the guide didn't become popular until later XR 750 and Nascar development. The ports in my race manual are close to the Axtell ports that I measured. Comstock did some flow bench work with those Axtell ports and found them to be excellent. That flow bench graph was posted somewhere on this forum years ago. See it below.

Intake manifold and port
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What are the numbers on the left side Jim, they are hard to read? I only measured speed at .385" valve lift, I tried at part throttle and wide open which isn't very effective because the less throttle the more the tube takes up space. Didn't think to do it at lower lifts.
 
I don't have better photos with clearer numbers. I think the numbers on the CFM chart are 200 400 600 etc. The numbers on the velocity chart are harder to read but you may be able to read the 1st digit of each.
 
I suggest there is a fundamental problem when we start modifying. When I tune for racing, I usually set up a couple of things then optimise everything else to suit it. If you are optimising port shape, when you make a change you are probably required to re-optimise the rest to suit it. And even then the result might only work at it's best on only one type of race circuit. The motorcycle set-up is not 'one-size fits all'. Post Y2K, in have only raced on two circuits - Winton and Mount Gambier - the straights and corners are similar. Phillip Island is entrirely different, and just changing the gearing might be nowhere near enough to be competitive. Quite often when a change is made the bike will become faster on some parts of the circuit and slower on others.
The size of the inlet port and jetting as well as crank weight and gearing affect throttle response. - Probably, it is not until you ride the bike on a circuit, you discover what needs to be changed. A horsepower and torque figure might not mean much.
As far as port shape is concerned, I simply do not know what gives the best results under any circumstances. I know modifyied valve seats give better performance when the 3 cutters are used. But that bowl in front of the inlet valve could do anything. Joe Craig was involved in a lot of racing. One thing which has always puzzled me was how did they get the featherbed frame so right - it could not be a fluke.
 
This discussion encouraged me to dig out my old "blue prints" of the Norton cylinder head drawings, namely the 1966 Domi 88SS and the Commando performance head with the larger, re-angled, inlet valves which I suspect was derived from the Dunstall big valve head. Fundamentally, the Commando head is an 88/650SS head with minor modifications to accommodate the larger cylinder bore and inlet valves.
I am not going to publish pictures of the drawings here because I have them for my personal use only, sorry.

All exhaust ports/valve size and shape remain unchanged from the original Doug Hele design.

The inlet ports have changed slightly over the years. I cannot say for certain what the first SS head port size was, but was probably 1 1/8" as measured on my 88 and identical to the first Commando. Doug Hele fitted 1" sleeves to the 650SS because he found that gave better performance.
The 1966 88SS drawing has 1 1\4" ports. I cannot imagine why they made them so big. At the bottom the port has a 1" radius blending into a 0.44" radius to the valve seat. at the top a 2.5" radius blends into the 0.44" radius to the valve seat. There is no widening around the valve guide. Does anyone out there have one of these heads?

The Commando head with big valves has 32mm ports. At the bottom the port has a 0.5" radius blending to the valve seat. at the top a 1.2" radius blends into the 0.5" radius to the valve seat. There is no widening around the valve guide.

It does seem that changes to the Commando inlet port size and blend from the parallel section to the the valve seat were made over the years.

I hope this is understandable without diagrams.
 
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This is a BSA starting with a 30mm port. I do not know how much metal is in a Norton head, so don't copy this without looking carefully, and this has a 75mm bore that is usually on an oversize 76 or 76.5mm. Because this isn't holding back with reshaping what started out as a very poor port. The combustion chamber and valves were good. But going from std 40.5mm to 42mm jumps flow quite a bit. And width at the guide has gone from stupidly narrow to around 38.5mm and a finished height of about 22.5mm at the guide, with the JBweld on the floor. that's to make the top and bottom runs straight with a single curve over to the valve, on a Thunderbolt head they twist a bit but still flow the same. Both are oval where the manifold meets, and this is scribed to cut out easy. I'm an artist and do it free hand with not as much measuring as before.

Intake manifold and port

This is part done rough with a cutter and the undersize guide is just pushed in and out to work on it. It shows the height and the width as it's being hacked out. The bowl is deepened, you can also see the floor where it's still the old shape as it was no doubt done with a drill and that area will be filled and lifted to be flat and blend with the new shape, and turn down to the valve. If you only look at a std port and see this it looks weird, but having done this shape so many times std now looks weird. Last one I pretty much had the target flow straight up, and on this around 180cfm @ 28" and just over 400fps at 28". According to my primitive cheap gear.
And that speed measured on a tall manometer is high for a 34mm port. The RH4 port seems quite slow so for its size is choked further in and the 32 runner is probably having it flow no more than a 30, except it has a larger carb.

Intake manifold and port

 
lcrken said:
Most of us are happy to share our technical information here to help out our fellow listers. I'm sorry you don't feel that way.

Ken
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Schwany said:
I post about all kinds of things I've done for my personal use. Unfortunately, I have no technical information to go with it. 🤣
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I have had jobs for which I have signed the official secrets act and been vetted for national security. I have done work on projects that I won't tell anyone about. I respect other peoples intellectual property so will not give away drawings which do not belong to me. However, where there is information such as radii in inlet ports, or dimensions of camshaft lobes that I could measure from hardware, then I am happy to release that. Alas, having shared my office with Doug Hele, much of what I learned from him about the Domiracer I have forgotten as I foolishly did not write anything down. Some I have mentioned on these pages. For example - the actual cam lift diagram for the 1962 domiracer and that Doug NEVER called the domiracer frame the "lowboy"
 
hybridracer said:
I have had jobs for which I have signed the official secrets act and been vetted for national security. I have done work on projects that I won't tell anyone about. I respect other peoples intellectual property so will not give away drawings which do not belong to me. However, where there is information such as radii in inlet ports, or dimensions of camshaft lobes that I could measure from hardware, then I am happy to release that. Alas, having shared my office with Doug Hele, much of what I learned from him about the Domiracer I have forgotten as I foolishly did not write anything down. Some I have mentioned on these pages. For example - the actual cam lift diagram for the 1962 domiracer and that Doug NEVER called the domiracer frame the "lowboy"
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I'm the worst about the technical side of motorcycling. I think of something I want to try and just do it without drawings or anything to refer to later other than the finished product. Shameless
 
I know a bit about racing which I have learned from other people, and a lot I have learned from experience. As a scientist, Ihave been trained to observe. Each time I have raced, I have noticed what has changed, and I usually know the reason for any changes. Some people believe racing is about having balls. When I go to a race track, it usually takes me about 5 laps to get near racing speed. During that time my brain speeds-up, so the corners become slower, and it is easier to notice what the bike is doing. Everything after that, becomes smooth and calculated. I do not worry about the competition. If there is somebody there on a similar bike, who really has a go at me, I love it. And if there is a problem with their bike, I help them.
I do not need to spread information on this forum. However I would love to see all the old racing Commandos speed up, and start winning more classic races.
In Australia, I am also subject to the Crimes Act - the government could get me to tell the Russians all I know and it would set them back 30 years.
 
Schwany said:
I'm the worst about the technical side of motorcycling. I think of something I want to try and just do it without drawings or anything to refer to later other than the finished product. Shameless
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I am just as bad. I never record jetting, timing, gearing - it is all in my head - remembered. The only time I usually stuff-up is in remembering frame rakes. But when I really think about it in context, I remember.
My primary chain cover is a work of art. It is a 40mm strip of steel around the sprockets which has 4 tags for dzus fasteners, and there is an aluminium plate which clips to it. It looks like an aircraft part. The steel strip is zinc plated and chromate passivated. The only problem is, a drop of battery acid made it fizz. The electroplaters who did the finish really liked it. And so did the guys who stuck the tags on with the MIG. When I was in the Ordnance Factory, I did a lot of work with the platers. In my last job, I was a contractor. My boss was a 28 year old kid. When I showed him the Seeley 850, all he said was 'I am impressed' - I had to laugh. One of his guys - an engineer, spent about 3 months fiddling with the design of a small plastic box. I managed the grenade project. Motorcycles teach you a lot. I am not an engineer, I am an industrial chemist. I have spent most of my life in engineering factories.
 
Some silly things sometimes get said on this forum, however I am always impressed with the skills of the guys who play with old motorcycles, I have worked with labourers, tradies, engineers, scientists and university professors.
 

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