More power with more intake length

jseng1 said:

Someone was having a little mid range stutter with his hot cam so I suggested he experiment by adding velo stacks to increase the intake track length. So he attached a pair of velos and "95%" of the stutter was gone to be replaced by a significant increase in power. Approx 10" from the head to the end of the carb or velo stack is what's considered to be ideal by myself, Ron Wood and Leo Goff (Peter Williams used 12"). You can't always achieve that with an aircleaner and stock oil tank but any increase in length is an improvement because the longer air column gains momentum and creates a ram effect. All the best racers who have tried it know its value. The owner of the bike in the photos is working on an aircleaner now and is incorperating the velo stacks into the aircleaner box. Longer stacks are also being ordered for more experimentation.

Special order velo stacks are available from Japan as shown below. But I don't know how to order them as the website is written in Japanese. Extending the manifolds or boots should have the same effect but its best to keep most of the weight closer to the head.

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In 1978 Peter Williams told me he used 10.5" head to bellmouth end! Which is what I was using at the time! Now a little more.

SteveA said:

In 1978 Peter Williams told me he used 10.5" head to bellmouth end! Which is what I was using at the time! Now a little more.

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It's getting late here, I am now struggling to remember if the 10.5" was valve head to bell mouth or head face to bell mouth end!

It helps that I have a Rickman frame, I guess.....

I do recommend you go long on the intake, however. Either way, whatever Peter said it wasn't 12"

SteveA said:

It's getting late here, I am now struggling to remember if the 10.5" was valve head to bell mouth or head face to bell mouth end!

It helps that I have a Rickman frame, I guess.....

I do recommend you go long on the intake, however. Either way, whatever Peter said it wasn't 12"

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Simple - if you are going to do serious tuning, do not use a Commando frame.
When I got my Mk3 Seeley frame, it had been raced with a Laverda 750 motor. I tried to get the whole bike, after I had tracked it for two years. The reason I wanted it - my mate rode it around the outside of my Triton in a corner, popped in front of me, braked and caused me to crash. There was not another motorcycle with which anyone could do that. The Laverda motor was slow because the exhaust system was wrong. A Seeley frame is serious business, it is a major advantage. The problem with it in Australia is it is post - 1962. So you race against bikes with very different engines, in the class which finishes at 1972, An 1100cc methanol-fuelled CB750 is pretty quick - but still not enough. They do not develop 140 BHP, and that is what they need - fast down the straights and too slow in rhe corners is silly stuff.
I estimate that I am at least 10 MPH faster in corners than the Hondas so when I enter the straights the others have make that up, when you are already 10 MPH faster. Even it they get to the next corner ahead, they need to stay there.
I am no longer allowed to race, but I need to go there again. I had them cold with all the tight parts of Winton Raceway ahead of me. They were done, and they knew it.

Can you get mid-range stutter with a 2 into 1 exhaust system which stops the cam behaving like a race cam ? If you get a jump in power mid-corner, you will probably crash. If you compare the lap times when you have separate pipes with megaphones with those when you have the 2 into 1 pipe, it is rare that you will be quicker with separate pipes. With a Commando engine, it must always be about cornering, accelerating faster down the straights does not matter. With a Mk3 Seeley frame, the pipe goes under the frame - the header pipes are of equal length.
If I entered a 700 metre straight at the same speed as one of Rex's bikes, it would be one third of the straight ahead of me at the next corner. As it is, I can stay with them comfortably. It takes a lot of power to make-up a 10 MPH speed differential at the start of the straight.

I probably have weird outlook on life. I started racing with a bike which was nasty and well behind the 8-ball. It taught me a lot. I usually geared it high to stay out of trouble. When it had separate pipes and megaphones, it was impossible to ride. - Slow everywhere and very nerve wracking. It was like my dog - it would bite me.

I remember an article of Peter Williams Monocoque and a photo showing extremely long intake manifold hoses. I can't find the publication but I remember him trying up to 12".

I got an email from someone who tried 16" from the bell mouth to the head. The intake track was straight and consistent and created a "supercharger" effect at 7500 RPM which overheated the motor on the dyno. But it had no power at lower RPM which made it impractical unless velo stack could slide (telescope) and change length with the RPM. If someone could actually make it work and verify it on a dyno (and it didn't break the rules) it could do wonders for a Norton land speed racer.

jseng1 said:

I remember an article of Peter Williams Monocoque and a photo showing extremely long intake manifold hoses. I can't find the publication but I remember him trying up to 12".

I got an email from someone who tried 16" from the bell mouth to the head. The intake track was straight and consistent and created a "supercharger" effect at 7500 RPM which overheated the motor on the dyno. But it had no power at lower RPM which made it impractical unless velo stack could slide (telescope) and change length with the RPM. If someone could actually make it work and verify it on a dyno (and it didn't break the rules) it could do wonders for a Norton land speed racer.

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Now that's a different statement, Jim. I have better recall today, after I measured what I actually have now, it is the 10.5" head to manifold interface to end of bellmouth.

But Peter told me they found 10.5" valve head to end of bellmouth to be best, which is what I had then. So I am running a longer set up than the one I used in '78. But I am sure Thruxton tested longer lengths before deciding on the best set up. They tested and used a lot of stuff. Apparently Peter came up with a revised cam profile every couple of weeks. If you look at Croxford's Barcelona endurance bike you will see an exhaust using straight pipes right back to the wheel spindle, slash cut. I can tell you from my own testing around '78 this gave tremendous mid-range but killed any possibility of revving over 6000. Great solution for an endurance event, useless for many other events.

And what you see isn't always what you get. It was nearly 40 years of me thinking the ideal primary exhaust pipe length was 28" to the megaphone, which was measured from the Thruxton bikes. It wasn't until Norman White told me there was another 2" of pipe inside the megaphone that I knew different!

As for telescopic velocity stacks I believe that has been done on race cars, not too difficult mechanically, just needs a power source like two stroke power valves sometimes use.

10.5" Is what I got from Ron Wood (Axtell dyno tuned), and Leo Goff. That's from the bell mouth to the head. Hard to beat the power they produced. Ron Wood told me that the folks at Norton didn't believe his HP claims so they flew him and his 750 short stroke out to the UK so he could prove it.

But the 16" from the bell mouth at 7500 RPM is breaking new ground. Requires telescoping V stacks. More experiments need to be made but if it really works that well then.......

jseng1 said:

10.5" Is what I got from Ron Wood (Axtell dyno tuned), and Leo Goff. That's from the bell mouth to the head. Hard to beat the power they produced. Ron Wood told me that the folks at Norton didn't believe his HP claims so they flew him and his 750 short stroke out to the UK so he could prove it.

But the 16" from the bell mouth at 7500 RPM is breaking new ground. Requires telescoping V stacks. More experiments need to be made but if it really works that well then.......

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FWIW and kinda different but on topic : guy I worked with raced 2 stroke scooters and had all kind of speed records using a stepper motor to vary exhaust length. Principal worked for him so why not on our intakes.
would take a bit of working out but so many clever dudes on this forum…

If you really want to have fun make some motorized intakes that vary the length based on rpm. A more practical approach would be dual intakes that switch at a certain rpm from long to short. Not a lot of room regardless of the frame for something that complicated.

swooshdave said:

A more practical approach would be dual intakes that switch at a certain rpm from long to short. Not a lot of room regardless of the frame for something that complicated.

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I think the Yamaha V-Max had something like that.

Under most circustances the mods only have effect over a limited rev range, so the gearbox is very important. I always thought I would not like the heavy crankshaft ,however I now love it. The only thing which slows the 850 motor is probably the crank balance factor. However the standard gearbox ratios are also too wide to allow better performance. I bought a 4 speed close cluster and raced with that. In the old days races were push start, so the high first gear did not matter. A four speed close ratio gear set with a Commando first gear should be adequate for any fast riding situation. In a race, you never usually go below 2nd gear.
I will probavbly never get to race with my 6 speed TTI box. But I think my Seeley 850 would now be extremely difficult to beat with any of the bikes currently in 70s historic races.
The last time I raced, I expected to spread the 4 speed gearbox. It did not happen -so at turn one, I was just behind the leaders. At turn 2, I rode under and past the three of them and was well ahead when the fuel line came adrift. I had them cold with all the tight parts of Winton ahead of me. Usually I would have come upon corners to find there was no safe way through.
I suggest an unmodified 850 motor is extremely fast - the gearing slows it down. It just took me too long to come to grips with it. I had to learn how to use it effectively.
When you lower the overall gearing with the Commando motor, you do not usually get faster acceleration. because of the poor throttle response at low revs. The crank needs to be kept spinning high. With wide ratios, the revs usually drop too much when you upchange, You cannot sustain high revs with a low balance factor. With a road bike, it usually does not matter.

Rex Wolfenden's 1100cc Honda CB750s are not slow,

We measure from the valve to atmosphere on our A65. There are formulas to work out best length if you know engine specs, for both torque and hp peaks. The longer runners give a stronger signal because the high pressure wave does less laps. They have about 6 or 7 variations of answers. 12" is good on an A65.

mark parker said:

We measure from the valve to atmosphere on our A65. There are formulas to work out best length if you know engine specs, for both torque and hp peaks. The longer runners give a stronger signal because the high pressure wave does less laps. They have about 6 or 7 variations of answers. 12" is good on an A65.

Intake Runner and Peak Torque Calculator

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Maybe that's what 12" meant in reference to Peter Williams bike - to the valve. Not exactly the same as 10.5" to the head but close enough.

From memory it was 13 inches from valve stem to bellmouth on my Thruxton Velocete and nothing in the way. At WOT it was louder than the fishtail. Easy 100mph + for as long as the road suited. Best fun.

Wonder if this would produce similar results (for Mikuni)

TomU said:

Wonder if this would produce similar results (for Mikuni)

Quad Flow Torque Wing - Thunder Products Carburetion Performance

Quad Flow Torque Wing increase air flow, air speed and fuel atomization and performance. For carburetor or EFI on Motorcycles, ATV, Dirt Bikes

thunderproducts.com

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Nothing here for Amals, so guess I will have to spend my mad money on something else. Anyone remember those propellers you could buy to put under your car’s carburetors after the last oil embargo of the 70s? They guaranteed better atomization, hence more power and better fuel economy. I tried to find an ad for them on the internet, but no luck.

TomU said:

Wonder if this would produce similar results (for Mikuni)

Quad Flow Torque Wing - Thunder Products Carburetion Performance

Quad Flow Torque Wing increase air flow, air speed and fuel atomization and performance. For carburetor or EFI on Motorcycles, ATV, Dirt Bikes

thunderproducts.com

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The burb sounds good I wonder if they work anybody tried them

jamesp said:

Anyone remember those propellers you could buy to put under your car’s carburetors after the last oil embargo of the 70s? They guaranteed better atomization, hence more power and better fuel economy. I tried to find an ad for them on the internet, but no luck.

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While doing that, you could also try filling the fuel tank with water, adding the magical additive that was promoted a few decades ago. (just kidding)

There are a couple intake runner length calculators online (probably more than these two I found). These are a bit crude and the terminology is a bit off when it comes to area of the port. However, they do lend some food for thought. The goal with the calculators is to determine intake runner length to produce peak torque at a specific RPM.