The Misunderstood Velocity Stack

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The Misunderstood Velocity Stack

Post by FJCamper » Sat Nov 08, 2008 11:56 am

Image

I find the velocity stack to be one of the most misunderstood pieces of tuning equipment. That's probably because good info on them is lacking. All the average tuner sees is ads that sell them.

First, VS are not for looks. They correct an airflow problem non-plenum carbs have at certain engine speeds.

A "plenum" is just a space or area. V8 builders have a plenum area under their big 4bbl carb manifolds. 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.

Yep. The air coming into a carb and intake manifold is not just on a one-way trip. 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 force of this "reversion" can be made worse by some cams.

A plenum design gives that shock wave a place to dissipate.

Plenum designs are much easier to tune than nonplenum systems, which covers the Weber, Dellorto, and similar designs.

The disadvantage of the plenum system is it is not as tunable as the Weber type system.

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 velocity stack (sort of a misnomer) is just an extension of the carb to contain that air/fuel revulsion.

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. Just let a high-rev valve float on an intake valve allow a manifold backfire ... and this is why many old time Weber tuners think of air filters as fire safety devices ... flame arrestors.

VS stack height is related to manifold lengtht.

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. For racing, you can change this by just changing VS!

For the street, you have to compromise. The short Weber manifolds most kits have work just fine with the appox 3" high VS in the same kit. 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.

Shorter runners lengths give more power at high RPMS, longer lengths give more power at lower RPM's.

By the way, the Solex carb design sold by Kadron is a plenum type and needs no VS, even though you can buy them. And, a last oddity between plenum and non-plenum carbs is there is no direct comparison between performance and venturi sizes.

Plenum carbs don't need the same size venturis as non-plenums to give equal performance.

FJC
Last edited by FJCamper on Wed Sep 25, 2013 9:10 am, edited 3 times in total.

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Post by doc » Sat Nov 08, 2008 12:14 pm

Very, very interesting! As usual, I was completely in the dark. Thought VS were for speeding up airflow. Sort of a hot rod "Tornado". Wrong again. :oops:

Thanks for the great lesson. 8)

doc

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Post by Piledriver » Sat Nov 08, 2008 1:07 pm

The bell mouth can improve flow significantly.

The bell mouth on stacks also provides one more feature--- They can reduce the height of fuel stand off 50% or more.
(The stand off gets wider and more uniform)

If you don't already have stacks on your Webers, you should consider getting a set for the latter reason alone.
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Post by Bugfuel » Sat Nov 08, 2008 3:01 pm

great topic and good info!

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Post by Bruce2 » Sat Nov 08, 2008 3:41 pm

doc wrote:.... Thought VS were for speeding up airflow.
You aren't completely wrong.
With no stack, there's a sharp corner at the top of the carb. Air doesn't like to turn sharp corners, so the flow isn't as high as it could be. Put a VS on it, you will take out that sharp corner, and reduce the inlet restriction. The end result is you will gain more flow through the carb.

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VW Oil Bath Air Cleaner Vel Stack?

Post by FJCamper » Sat Nov 08, 2008 4:51 pm

Followup-

First, yes, the airhorn design of a velocity stack smooths airflow due to its tapering. Should have said that.

There is a trick velocity stack question that many VW guys never think about, even though they know the answer.

Did the VW Bug come equipped with a velocity stack as standard equipment?

Well, yes. The VW oil bath air cleaner had one inside. It was approx four inches tall, and it wasn't there just to keep the oil out of the carb. It was a real velocity stack.

The oil bath air cleaner stack acted to contain fuel fog stand-off at high RPM's. 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.

Tuners that swapped the oil bath for a racy-looking flat air cleaner got reduced intake restriction, but lost the VS effect. This caused engines to go lean at 4000+ RPM's.

A few of the aftermarket air cleaners had a short tapered stack on the lower half, but not many.

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.

FJC

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Post by brookester » Sat Nov 08, 2008 6:53 pm

Do velocity stacks have any effect on draw through turbo setups?

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Post by Piledriver » Sat Nov 08, 2008 8:18 pm

brookester wrote:Do velocity stacks have any effect on draw through turbo setups?
Other than perhaps improving the carbs performance by providing smoother airflow, probably not. They could not HURT.
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Post by sideshow » Sat Nov 08, 2008 9:13 pm

Any opinion on the factory stacks for the IDF? I have seen quite a few in the ¼ inch tall range, but also have one set that is non belled and about ~1.5-2 inches tall? Straight maybe slightly smaller id that the after market shiney ones.
Yeah some may call it overkill, but you can't have too much overkill.

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Post by Bruce2 » Sat Nov 08, 2008 10:19 pm

The short ¼" tall ones are almost worthless. The taller ones are better, but far from the right shape. The aftermarket ones are better, but still not ideal.
The ideal shape is a hyperbolic curve. Next time you are passing by a nuclear power plant with those large cooling towers, study the shape. That is the ideal shape, from the min dia to the top.

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Shape matters

Post by FJCamper » Sat Nov 08, 2008 11:26 pm

Gentlemen,

Bruce2 is on target with the hyperbolic shape for best air flow. But most stack offerings are straight, with a slightly flared top to act as something of an air guide and avoid an abrupt transition.

The super-short (but flared) stacks are really just for air guide, and have little to no effect on fuel standoff.

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.

Fluid dynamics and air flow share some principles. Both are subject to traps and eddys on the way from Point A to Point B in a curving tube. Swirling fixes most of that. The Navy has used this technology in A-Sub plumbing for years.

Some very smart designers make swirled intake manifolds, or will cut your manifolds. We VW-Porsche people have such a short runner length that swirling has only a small effect.

The bottom line is, the velocity stack is first a fuel-air standoff container, and secondly an airflow guide.

Of course, they look great and purposeful doing it.

FJC

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Post by Scott Novak » Mon Nov 10, 2008 4:34 am

The next question is the how severe is the effect of mismatched velocity stacks. Most of the stacks being sold are one size fits 48 IDF and too big for everything else.

Scott Novak

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Post by Bruce2 » Mon Nov 10, 2008 3:50 pm

Big stacks on small carbs will make that sharp corner at the inlet of the carb that I refered to above. WRT to air flow, it might as well not be there.

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Mismatches

Post by FJCamper » Mon Nov 10, 2008 7:51 pm

Hi-

My old original Weber IDF's came with stacks that fit, and maybe by good luck the set of EMPI-HPMX IDF copies I bought last year came with the proper stacks.

I just walked back to my shop and looked at some old 1970's era 48IDAs and my 44IDF's.

I think people buy the wrong size because a vendor just advertises "velocity stacks" and shows a picture, and the buyer has no idea of the difference between the 48IDA and the 44IDF.

My guess is they harm airflow because of the edge the too-big stack creates.

FJC

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Post by gerico » Mon Nov 10, 2008 11:09 pm

I run air cleaners on the street for obvious reasons. In the Ghia the height of the air cleaner is somewhat limited by the hood spring so the top of 3" stacks only has an inch or less clearance from the top of the air cleaner.

Any idea how much space is required between the top of the velocity stack and the top of the air cleaner? Is the airflow being disrupted by the relatively abrupy 90 degree angle change?

Thanks FJ for an informative thread.

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