Velocity Stacks

M

Mike Crehan

Member
I heard that 97s need stacks to run properly on an A so this morning I made these - and a sack full of swarf.
 

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Pete said:
Pretty short for an A. They should be good at around 7000 rpm.
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I don't know a lot about these things but if I remember correctly Velocity stacks are tuned for a certain RPM by length and diameter and even flare. Incorrect size and you will see little to no difference in performance.

Someone please correct me if I'm wrong but, the air going into the engine is not a smooth flow but rather "pulses". If you can "tune" the input air going into the engine at the same pulsing it will "push" a little more air then static into the engine.

This resonance is set by the size. How to determine the size? I have not a clue!
 
Looks like the engine with the blue carbs is very similar to mine externally - same headers, similar looking head, I've got a Malory distributor. This is my first go at them. It will be easy enough to change the length when it's all up and running.
 
Mike V. Florida said:
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This resonance is set by the size. How to determine the size? I have not a clue!
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There are a lot of formulas including some online calculators. Eventually you have to make some assumptions to get the answer which then means the answer might be wrong. With other types of carbs and engines the calculations have got me close and fine tuning on the rolling road has got me home.

Essentially a change in section ( which also applies to the open end of the velocity stack) in the inlet tract creates a pulse of energy which operates just like a sound wave. The object of the excel use is to get this to help with cylinder filling at a point in the rev range most helpful to improving the cars performance.
 
The pictures in post #5 show the needed radius at the point of entry / top or the stack. A tube only type of stack can reduce the flow potential at WOT.
 
Mike i beleive a guy by the name of Jerry Jobe , (no sure) produced mine , 1st pic is on a pair of 81.s . next is on pair of 97.s, both engines would not run sweet till we used them . no 2 pulled 5900 rpm, Its is the same set up we will run again this year at Bonneville, Sure wish i could buy some more of those stacks, Anyone Have some to spare ???
 
These do taper inside and there is enough material to reshape the tops to a full flare like Quickchanges but when we were messing about on a synonym with some sided raft carbs we found they worked better with the fully radiussed 180 degree ends.
 
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Some modern motorcycles have 2 sets of stacks, one nestled into the other. A stepper motor moves the top stack up out of the way depending on RPM. But this is an "A"! Who needs a computer and a stepper motor when we have a simple lever on the steering column?
 
For a 1st go they look really nice
Velocity stacks like lots of other things like porting and header lengths only on a flow bench or a dyno will you find which is best
 
Brendan Correct BUT They are of larger cubic inch, Now on an A with two V8 carbs or more & being of small cubic inch ,there is not enough suck to pull in the required amount of air , & its my understanding instead of putting in smaller ventruies by using stacks it pickups air speed , I stand to be corrected on this , But before i fitted them on two carbs it ran like a bag full, once fitted the engine ran completly differant, On a sinlgle 81---97 not needed at sea level.
 
quickchange said:
Brendan Correct BUT They are of larger cubic inch, Now on an A with two V8 carbs or more & being of small cubic inch ,there is not enough suck to pull in the required amount of air , & its my understanding instead of putting in smaller ventruies by using stacks it pickups air speed , I stand to be corrected on this , But before i fitted them on two carbs it ran like a bag full, once fitted the engine ran completly differant, On a sinlgle 81---97 not needed at sea level.
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Like stated before, you tune the diameter and length of the ram pipe for the rpm range you want a boost. This range will be fairly narrow, maybe 200 rpm. The ram pipes can be above or below the carbs or injectors. Here is a pic of the manifold I had on my B engine when it was in my circle track car.
This was designed for around 4000 rpm. As it was the carb sat above the hood.
This setup was good for .2 sec. better lap time over a pair of Stromberg 48's on a street type manifold on a 1/3 mile paved track. Carb was 650 Holley.
 

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