We VW-Porsche people have such a short runner length that swirling has only a small effect. Some very smart designers make swirled intake manifolds, or will cut your manifolds. The Navy has used this technology in A-Sub plumbing for years. Both are subject to traps and eddys on the way from Point A to Point B in a curving tube. Some designers have attempted swirl-effect velocity stacks on the theory that swirling air might mix with the fuel better, but swirling doesn't help until you get below the throttle plate.įluid dynamics and air flow share some principles. The super-short (but flared) stacks are really just for air guide, and have little to no effect on fuel standoff. But most stack offerings are straight, with a slightly flared top to act as something of an air guide and avoid an abrupt transition.
Savvy tuners made VS out of tin cans and hose-clamped them to the carb, and mounted the air cleaner to the top of the can.īruce2 is on target with the hyperbolic shape for best air flow. This caused engines to go lean at 4000+ RPM's.Ī few of the aftermarket air cleaners had a short tapered stack on the lower half, but not many. Tuners that swapped the oil bath for a racy-looking flat air cleaner got reduced intake restriction, but lost the VS effect. The old single barrel Solexes had too-small a small plenum under the carb, and at "high" (4500) RPM's fuel standoff was a real problem. The oil bath air cleaner stack acted to contain fuel fog stand-off at high RPM's. It was approx four inches tall, and it wasn't there just to keep the oil out of the carb. The VW oil bath air cleaner had one inside. There is a trick velocity stack question that many VW guys never think about, even though they know the answer.ĭid the VW Bug come equipped with a velocity stack as standard equipment? Plenum carbs don't need the same size venturis as non-plenums to give equal performance.įirst, yes, the airhorn design of a velocity stack smooths airflow due to its tapering.
And, a last oddity between plenum and non-plenum carbs is there is no direct comparison between performance and venturi sizes. Shorter runners lengths give more power at high RPMS, longer lengths give more power at lower RPM's.īy the way, the Solex carb design sold by Kadron is a plenum type and needs no VS, even though you can buy them. For variable RPM road racing, a 1.6 engine redlined at 6000 needs about a 16" runner length, meaning the total measurement from carb mouth to intake valve. The short Weber manifolds most kits have work just fine with the appox 3" high VS in the same kit. For racing, you can change this by just changing VS!įor the street, you have to compromise. Race tuners know there is a specific length from the carb mouth to the intake valve that produces the best power at a specific RPM. VS stack height is related to manifold lengtht. and this is why many old time Weber tuners think of air filters as fire safety devices. Just let a high-rev valve float on an intake valve allow a manifold backfire. If there is no VS, the air/fuel mix gets messed up, and a carb fire is a good possibility with aerated gas now being ejected into the engine compartment. The velocity stack (sort of a misnomer) is just an extension of the carb to contain that air/fuel revulsion. Webers are forced to have velocity stacks, because at certain RPM ranges, the return shock wave from the intake valve literally forces air/fuel out of the top of the carb. The disadvantage of the plenum system is it is not as tunable as the Weber type system.
Plenum designs are much easier to tune than nonplenum systems, which covers the Weber, Dellorto, and similar designs. The force of this "reversion" can be made worse by some cams.Ī plenum design gives that shock wave a place to dissipate. The intake valve slams shut as the air is flowing, and a shock wave created by the sudden halting of flow shoots straight back up the manifold into the carb. The air coming into a carb and intake manifold is not just on a one-way trip. The plenum space allows a common area from which the cylinders draw an air fuel charge, and the plenum acts as a cushion to dampen the violent reversals of air velocity that occur between the carb throat and the intake valve. V8 builders have a plenum area under their big 4bbl carb manifolds. They correct an airflow problem non-plenum carbs have at certain engine speeds.Ī "plenum" is just a space or area.
All the average tuner sees is ads that sell them.įirst, VS are not for looks. That's probably because good info on them is lacking. I find the velocity stack to be one of the most misunderstood pieces of tuning equipment.
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