Can someone explain velocity stacks

[daepp]

I think I understand taller is better - why?

Our MFI systems use them - but surely they don't produce any more flow.

Some MFI move the butterflies higher in the stacks if I understand this correctly - why does that help?

And those that have tennis balls etc in them - once they remove these, do they run them with no air filter? How do they manage that?

Thanks in advance for any info.

[Dantilla]

The answer is in the name- "Velocity" stacks.

Similar to exhaust design, there is a black art along with the science to maximizing airflow into, through, and out of an internal combustion engine. Just like calculating proper tube length for headers, somebody smarter than I figured this out.

[daepp]

But isn't the speed of the incoming air simply dependent on how fast the pistons are sucking it in? And why is taller faster?

[signature65]

Faster air comming in creates denser air in the cylinders. Denser Air means bigger bang! Its all physics and the Bernoulli's Principle. Google the principle and it will all make sense. Its also the principle that allows planes to fly.

[jpnovak]

Intake (and exhaust) length lend themselves to acoustical tuning of the air flow. The idea is to setup the length tuned to a specific frequency. This beat frequency can be correlated to a particular rpm of the motor to help cylinder filling and exhaust gas scavenging. The acoustical tuning is dependent on runner length, diameter and taper.

The length is independent of the volumetric airflow through the tube. The volumetric airflow is dependent on the engine displacement and the corresponding rpm. The intake velocity is dependent on the cross-sectional area (diameter) of the intake runner.

In general, longer runner length is for lower rpm and shorter runner length is for higher rpm.

The velocity stack is the end of the tube that is flared or rolled to facilitate and smooth air flow into the open end of the tube.

[JFairman]

The displacement of the cylinder has alot to do with the optimum volume and height of the intake runner and velocity stack. It's a math equation to figure out the optimum size and length for top end horsepower or midrange torque or a balance between the two without variable valve timing.

A taller and longer intake stack and runner below the butterfly will have more volume of air moving through it at full throttle and the moving air has alot of inertia at high rpms so as the intake valve is closing the air doesn't want to slow down and stop moving through the intake runner and velocity stack right away and a slight positive pressure builds up at the closing intake valve so more air is forced through it at that point.

At really high rpms some of that slight positive air pressure in the intake caused by the intake valve having closed against the inertia of the high speed air flow through the intake runner is still there when the intake valve begins to open again, and that causes a little more air to go into the combustion chamber during the intake stroke, and the entire intake event which continues into the beginning of the compression stroke after BDC especially when long duration cams are in use.

hope i said that sorta clearly...

[bdisco]

Velocity stack - Wikipedia, the free encyclopedia

hmmm.

[boba]

If you want to see how far this was taken in the old CanAM, look at the V8's with different length velocity stacks. The idea was to balance low end and high end between cylinders.

I will look for photos.

[tcar]

Second question, yes, they run without air filters usually.

Talking about the cars w/ tennis balls in the stacks.

[boba]

Originally Posted by tcar

Second question, yes, they run without air filters usually.

This is true for race motors which will be refreshed on a Hr. schedule.

Many are run with screens to keep large bit from being ingested.

But, if you look at stock MFI cars you will find velocity stacks inside the air cleaner box.

[daepp]

Originally Posted by boba

But, if you look at stock MFI cars you will find velocity stacks inside the air cleaner box.

This I was aware of, just wanted to know the "why". Thanks for educating me!

[psalt]

Hello David,

If you are interested in this subject , Philip Smith's "Scientific Design of Exhaust and Intake Systems" is a good place to start. The first thing to understand is that flow in the intake and exhaust manifolds is not smooth, it is a pulsing mess of waves caused by the opening and closing of the valves. There is always a mixture of exhaust and fuel in the air. For any given intake length, there is a rpm point when the returning wave pulse will result in a mild supercharging effect on the next intake charge and VE will be boosted, sometimes above 100%. Some of the seminal work on the intake tuning side was done by Jaguar in the 1950's with their slide valve injected XK Lemans engine. They produced a famous chart showing the effect of intake "trumpets" from none to 32" on VE ( which corresponds to torque peak) that has been reprinted ad nausem. Longer tubes peak torque earlier and lower high speed output. MB also experimented with mechanically variable stacks on the W196 in the same era. The variable length intake tube evolved into a rotating drum, which BMW has used on throttle less V-8 engines to widen the torque curve.

[Wavey]

Here's EVERYTHING I know about velocity stacks:

Short for high RPM/horsepower (think F1 engine).
Long for torque (think classic fuel injected drag race engine)
Mixed lengths for wide powerband (think CanAm engine)

Nice wide rolled-over radius at the inlet works best for all lengths (per David Vizard's tests):

[RWebb]

search on 'wave superposition'

the basics will be in a Fresh. or Soph. phsics text - from there you can go to intake design, exhaust design, audio speakers/room analysis, blood flow/hearts and a huge array of natural phenomena

Smith's book is wonderful but quite old (1950s?) -- doubt if it is in print but get your library to use inter-library loan

there must be some more recent book on all this but have not seen one that is minimally technical.

BTW - no one has yet mentioned the variable length intake runners used in some modern engine designs...

P AG uses Vario-Ram as their name.

[doug_porsche]

I read once that the best intake designers in F1 actually are acoustic engineers. Would love to see a very slow motion CFD animation of this.

[Wavey]

Originally Posted by RWebb

BTW - no one has yet mentioned the variable length intake runners used in some modern engine designs... PAG uses Vario-Ram as their name.

Excellent point Mr. Webb!

[psalt]

BTW - no one has yet mentioned the variable length intake runners used in some modern engine designs...


A throttle less V-8 BMW with a sealed rotating drum in the intake manifold varying the runner length IS a modern engine. This could not be done without variable valve timing.

All the more modern text just quote Smith's book from 1962, it has been reprinted 12 times. Do you know of any errors in it ? Many of the innovations from the 1950's, like moveable aerodynamic surfaces, were banned after the big accidents and no longer pursued in racing.

[daepp]

Wow - so much more here than I expected. Thanks again.

psalt - sorry, but what is VE?

How about Porsche raising the butterflys on the MFI - same flow improvement reasons?

[vincer77]

[QUOTE=DAEpperson;4979147]
psalt - sorry, but what is VE?

QUOTE]

Volumetric Efficiency = volume of air captured in the cylinder (at standard pressure, temperature)/displacement cylinder

[m110]

There was an interesting Pano article about intake tuning and the compromise of velocity stacks at different RPM (specifically the Vario-ram system back when it came out). Chris Powell in Bellevue (no affiliation) put a 993 tuned system onto a 2.7. I asked him about it but it was way over my head!
FS: Ultimate 2.7L Hot Rod Motor (Price Reduced)