Do velocity stacks add, umm, velocity? I've seen long ones, short ones, fat ones, thin ones. Curved and straight. Open and screened, filtered and unfiltered. The theory is that the stack creates a ram-air effect in the carb. The length is related to the effect, but has limits. Is this correct? What about the carb underneath? Does the effective length vary with the type of carb (2-barrel, 4-barrel) and the CFM? How about where power is desired - low, mid, or high end?
The velocity stacks actually do serve a purpose, especially if you run a hot cam. Because of the intake valve stays open for a short period of time after the piston starts back up the cylinder, at low RPM some air is forced back into the intake manifold. At high RPM, the inertia of the air keeps this from happening as the time gets very small. So, at low RPM, there is a little cloud of gas/air hovering over the carb , especially when you open the throttle. The velocity stacks contain this cloud and lets it get ****ed back into the engine eliminating a lean off idle condition that can lead to detonation. Also keeps you from having to jet richer to compensate for the lost fuel without the stack. You can improve drivability and HP by putting a 6" stack between the carb and your air cleaner, but it looks funny.
That isn't what velocity stacks are for...... At WOT the intake valve is opening and closing which creates pressure waves that travel back and forth through the intake runner. Depending on the size (volume) of the runner and the density of the air/ fuel in the runner at some rpm you get a resonance of the intake runner. This leads to an amplification of the pressure waves which in turn forces more air and fuel into the cylinder. This is called the ram effect. In indy engines (current, not old) they use multiple fuel injectors to broaden this ram effect. By putting fuel injectors both at the port and in the plenum you can vary the m*** density of fuel in the intake runner (but the overall mixture stays the same). This, if properly setup, can broaden the range where resonance occurs and effectively jam more air/fuel into the cylinder. Another name for the ram effect is statically supercharged, because greater than 100% volumetric effeciencies are possible, hence you are pumping more air/fuel than the static displacement of the engine. Now to velocity stacks. They have 2 main purposes: 1: smooth the airflow going into the carb/runner which will create smoother airflow (although intake air flow is in the turbulent region.....that would be a discussion for another long post) this leads to better atomization and less pumping loss 2: Tune for the ram effect. They make them in different lengths so that you can change the length of the runner to tune for what rpm you want the ram effect to occur. Usually you want to tune it for the peak torque of your engine but this is always open to debate. The ram effect really only matters at WOT.......not part load (throttled) conditions. Hopefully , this all makes sense.......any questions?
Hmmmm... I recall seeing McLaren sports racing cars at Laguna Seca in the early seventies running big block chevy V-8's with two lengths of velocity stacks on their engines. That is to say there were four stacks of one length and four that were perhaps half as long. I don't remeber if this was a Hilborn or Lucas system but there is little doubt they were succesful. Most race engines I'm aware of seem to use the same length stacks on all cylinders, what was McLaren trying to achieve with their design?
Hey Dave, lets talk white walls skirts and kkoa shows from 20 years ago! Now your worried about velocity stacks???????? Whats this place coming to??? Alright Ill shut up!
I'm pretty sure that the length and volume of the intake runners are the same in a BBC head. I have a picture of an injection setup like that with the staggered stacks and the #3, #4, #7 and #8 cylinders have the longer stacks. My guess would be that they are trying to broaden the overall power band by having four cylinders setup with a shorter stack than the other four. Although I have only seen this injection on BBC's. You would think that someone would have made similar injection for other engines if this was the case.
I just finished putting a 1 1/4" spacer between my Rochester Quad and my dual plane EDelbrock intake , followed by a 4 " to 6" X 8" high metal velocity stack . This is on a 327 small block........now I spin the wheels in 2nd gear and it's an automatic ******. At 58 , I have been around rods most of my spare time and have never seen such a dramatic improvement in performance for such little outlay !! Don't believe the ******** youread ....try some simple "old school" tricks .....they're inexpensive and some are time -tested .Billet is not always the way to go , fo sho !! Elwood
I believe,Big Block chevys have two different length intake ports,the stacks were different lengths so the distance from the tip of the stack to the valve were the same.
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