Long velocity stacks

jseng1

jseng1

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Made up some extra long velocity stacks for my little 32mm PWK carbs to find out how they perform on the street. Suprisingly there is no change in low or midrange that I can tell. I get the same surge around 4000 RPM and good torque from the bottom. Top end may be better but I'm only going by roll on wheelie seat of the pants thing. Without the stacks it was approx 5" from the head to end of carb. The stacks added another 5" for 10" total which is what works out best for racing. The ID of the alum tube is 33mm and its important to keep it the same ID (or nearly so) as the carb so you get velocity and momentum. The OD taper at the end of the tube inserts and butts up inside the carb bell mouth for smooth flow. A lot of trouble to go through for a steet bike but I had to check. Would probably have more effect with a hotter cam. No room for aircleaners unless I fabricate a new low slung oil tank so they will probably only be temporary.

Long velocity stacks


Long velocity stacks


The unfinished low slung oil tank that would allow aircleaners all the way back to the rear fender.

Long velocity stacks
 
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A surge at any revs is not good for racing. Smooth power delivery brings no unwelcome surprises when you arre powering through corners. A 2 into 1 exhaust system can dictate what the cam and inlet tract achieve. When you are at high speed on full lean you rely on grip at the rear wheet contact patch. Smooth power and less lean are better than peaky power and lots of lean.
When I ride my Seeley 850 - because of the steering geometry and power characterists - it requires almost zero thought. The bike is not powerful but it does everything right, so it is faster than anyone would expect. I learned to race by crashing.
 
Dan1950 said:
Longer port runners will have more effect.
Click to expand...
Yes but the carbs run into things whereas the velos have more room. I can make further tests with Keihin CR smoothbores and there are longer velos available to suit. The Keihin vents are located outside the velos so extending them shouldn't be a problem.
 
I cannot see how the length of the inlet tract makes much difference. If you only race on one circuit, you might be able to optimise the length. But under normal circumstances, you usually adjust the gearing to suit the circuit on which you are using the bike. Wherever the torque peak is, you usually adjust your riding to suit it. A number on a dyno probably does not mean much. You can get big horsepower and be unable to use it effectively because of the way it is delivered. With every improvement, there is usually a down side.
With my first race bike, I was always struggling to get more torque. It had tons of top end, but if the revs dropped because I was balked in a corner, when it came back on song it would usually go sideways. It ended up that I really needed more gears.
 
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Dan1950 said:
Longer port runners will have more effect.
Click to expand...

add to that correcting for dead flow zones in the intake, like raising the floor of the lengthened intake tracks, and then opening up the bottleneck that typically occurs at the valve by removing material... oh, and fitting OS valves to actually take advantage of all this work, which will just be guesswork unless you have a dyno handy.

Ranting a bit, maybe, but I had a great talk with Herb Becker this past weekend at Doug's funeral, he shared lots of insight about his experimentation over the years and the tuners he admires. Interesting are his thoughts on how well designed the Norton twin head was/is (compared to period offerings). Point of the chat was that I'll be moving on to a race bike build this winter and give it a go for one season in memory of Doug
 
acotrel said:
I cannot see how the length of the inlet tract makes much difference. If you only race on one circuit, you might be able to optimise the length. But under normal circumstances, you usually adjust the gearing to suit the circuit on which you are using the bike. Wherever the torque peak is, you usually adjust your riding to suit it. A number on a dyno probably does not mean much. You can get big horsepower and be unable to use it effectively because of the way it is delivered. With every improvement, there is usually a down side.
With my first race bike, I was always struggling to get more torque. It had tons of top end, but if the revs dropped because I was balked in a corner, when it came back on song it would usually go sideways. It ended up that I really needed more gears.
Click to expand...
Al - if you read Phil Irving's Tuning for Speed then you will see how inlet tract length will make a difference. Also for exhaust pipe length - especially with megaphone.
Cheers
 
acadian said:
add to that correcting for dead flow zones in the intake, like raising the floor of the lengthened intake tracks, and then opening up the bottleneck that typically occurs at the valve by removing material... oh, and fitting OS valves to actually take advantage of all this work, which will just be guesswork unless you have a dyno handy.

Ranting a bit, maybe, but I had a great talk with Herb Becker this past weekend at Doug's funeral, he shared lots of insight about his experimentation over the years and the tuners he admires. Interesting are his thoughts on how well designed the Norton twin head was/is (compared to period offerings). Point of the chat was that I'll be moving on to a race bike build this winter and give it a go for one season in memory of Doug
Click to expand...
They sure worked well on my race bike with Axtell port copies and larger valves.
Long velocity stacks
 
robs ss said:
Al - if you read Phil Irving's Tuning for Speed then you will see how inlet tract length will make a difference. Also for exhaust pipe length - especially with megaphone.
Cheers
Click to expand...
I have always known about tuned lengths, but suck it and see might be better. The tuned length of a pipe is the speed of sound divided by the frequency. The tuned length is only ever right at one rev level. With a four stroke engine, both valves are open at TDC - reverberation occurrs through inlet tract, combustion chamber and exhaust, It is the sum total which probably matters not individual lengths. Longer inlet tract might give more ram. but it is joined to the exhaust system. You can try changing the length of the inlet tract, but what you pick up on the hurdy-gurdy, you would probably lose on the merry-go-round.
Manx Nortons were designed for the IOM. On short circuits, they handle better than other bikes, but are not the fastest.
Phil Irving is usually correct in what he says, but sometimes he is misinterpretted. When he was writing, two-stroke technology was not very far advanced - modern exhaust systems are very different. A 2 into 1 exhaust on a Commando makes a big difference to the way the motor performs. I think it dictates what happens in the inlet tract.
 
I suggest anyone would be crazy to race a Commando if it had separate pipes with megaphones. You would not need to get balked anywhere, or you would have a problem. Especially with a wide ratio gearbox.
 
acotrel said:
I suggest anyone would be crazy to race a Commando if it had separate pipes with megaphones. You would not need to get balked anywhere, or you would have a problem. Especially with a wide ratio gearbox.
Click to expand...
Damn, looks like me and Peter Williams and a good few others were crazy!

img032.jpg
 

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BTW, the bike pictured had 10.5" inlets and 36mm Mk1 Amals on an 850 engine with 34mm ports and the mid range was phenomenal, it pulled like a train from 4000 to 6800!

If you found yourself in the wrong gear it really didn't matter, 4 speed with Manx close ratios!
 
jseng1 said:
Peter Williams Monocouqe setup.
Long velocity stacks
Click to expand...
My inlet tracts would be a good inch longer than that. But I would say one thing - when I began racing, I suspected that most British circuits were not as tight as Australian circuits. There was a race in the Trans Atlantic Series when PW won ahead of the Yamaha 750s. To do that on a big circuit, he must have been getting a lot of top end.
Being in the wrong gear is one thing, but losing revs midcorner and dropping out the bottom of the power band is something else. Mostly when using megaphones, there is a surge of power at about 4000 revs, as the revs increase. On a short circuit, you need to be able to ride around that. Phillip Island is different, the bike is on full blast most of the time, so a bump in the power delivery does not matter so much.
 

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