True 'velocity stacks' that have been used on port injectors for decades take advantage of the little pressure boost available from the 'organ pipe effect'. That is a big subject and if you are really interested, look at these old treads discussing it, here and here (this one is specifically about exhaust tuning but the principles are the same).

Actually, every intake and exhaust design must take these dynamic air movements into consideration or they could actually hurt performance unintentionally.

Your suggestion of building a plenum under the carb is in effect a tunnel ram manifold that will hurt bottom end performance but improve top end. A velocity stack above a a carb will have little affect on port pressures since the carb internals will kill the pressure pulses. If you are really interested in delving into the theory a little deeper, send me your email address and I will send you a scan of my senior project which was to design and build improved intake and exhaust manifolds for a Briggs & Stratton 5hp one-lunger.

 

velocity stacks

Velocity stacks mount on top of your carburetor, not under it.
Velocity stacks act to straighten out your air flow prior to entering the carburetor. There are several versions on the market. Some look like someone cut the end off of a trumpet, others are straight with just a flare at the very top. Usually they are about 4-8 inches tall and made from a variety of materials. Check out the ones on my avitar. As you can see they can be pretty simple, and are easy to make.
The tobacco tins which I have on my 3 dueces, have the bottoms cut out and the top ring that the lid attatches to is in tact, so I will have a place to mount the air filters.
The ones I have are old tech from way back. Tobacco tins are hard to find now days. You might just want to measure the diameter of regular tin cans and see if you can find one that has the right diameter, go from there. Yes they do help increase horsepower as the air going into your carburetor isnt turbulent but flowing straight and orderly, allowing a fuller charge of air.

There are spacer plates made to go under carburetors that can also enhanse power. You can find them as well in any speed equipment catalogue. Spacers usually run from $15-16 on up to $40 or so, depending on usage and materials used in the manufacture. They are a bolt in item. You have to remove the carburetor, bolt the spacer plate to the intake then put the carb on top of it. Spacers can help in a variety of ways in tuning an engine. For instance if you have a small displacement engine with an open plenum intake, killing your bottom end, a 1-2 inch spacer can help in restoring the lost power, with very little or no effect on your mid and upper end power.

The aim with my stacks isnt to increase velocity of air flow into the carburetor as a trumpet type supposedly does, but to just get the air to stop "tumbling" before it hits the carb, so I get a good consistant signal.

 

Velocity stacks

Mike,
WE use tuning the velocity stacks to help get the power curve to where we want it to be on a race engine..this goes along with the length and diameter of the individual header pipes..

The length of the pipes will help put the power curve where we want because of setting up a resonance in the pipes at certain RPM..

To get it right requires a number of pulls on a dyno to determine the best length for the velocity stacks and header tubes to get the results we want..There are some formulas to help get close for a start..

In order to have a tuned manifold with a carb we use a plenum with tubes of an equal length between the plenum and the intake ports of the head..

It can be done and does work..just takes a lot of work and time and money to get an engine package to work really well..

I see a bunch of the other guys weighed in on this one while I was typing..

OMT

 

tubing tuning

One More Time pretty well covered the super technical end of it.
One other thing I havent noticed is on tube diameter on primary exhaust. The different diameters as well as length can make differences. From an article I read several years ago put out by Hedman, most street headers with equal length tubes will be between 34-48 inches in length. The longer the tube, the more it helps bottom end, from what I got from it. Also a smaller diameter tube increases the hot exhaust velocities for more lower end. Shorter and larger works more for top end.
There is also the matter of going with Tri Y headers on a V-8 which have influence on the lower end, conventional 90 degree headers, which due to the design help top end, and then the 180 degree header, which greatly helps the mid range.
In the article, they asserted that for street engines its of no benefit to use primary tubes that are larger than the diameter of the exhaust valve. That being said, for most all small block engines, 1 5/8ths primaries are as big as you need to go, that going larger will hurt your bottom end more than the smaller tube will hurt your top end.
Of course this data is only about 30 yrs old and the thinking could have changed on it by now. Although I do notice that vertually all street headers for small block engines are either 1 1/2 or 1 5/8ths diameter primaries.
when you get into the big blocks, engines with 1.73 ehxaust valves etc, then of course the primaries have to be large enough to handle the volumn of exhaust being put out, so the most common primary tubes would be 1 3/4 inch.
I was once told by a guy that built headers that its a waste of time on a street engine to use primary tubes larger than the throat of the valve seat. That being the case, 1 1/2 inch primaries should do the job in all but the most unusual of situations. The guy did caviat that with the point that you would want to run shorter primaries in correspondence to the higher the RPM you want to tune for.
I fully admit to not being the most literate on headers, but these are the guidelines Ive always gone with, except when I was racing.

 

All those issues that Max mentions are covered in great detail in the links I posted above. Suggest you read them for the details.

 

velocity stacks

BTW, Willys36, that is one sharp looking coupe you have there.

 

Wow, so many replies Thanks a lot guys. Willys I have a 3hp Briggs and Stratton that I plan to put on a minibike chopper, do you think your plans would work on that one? Thanks for the links to the threads as well. And I looked at your project journal and saw that you made your own tunnel ram

Is it hard to weld cast iron? do you need a special welder?

Max Keith, now that's hot-rodding right there:thumbup: tobacco cans as velocity stacks, I LOVE IT! Thanks for the explanations as well.

Now you all said that it takes away the turbulance of the air, how about the K&N velocity stack? It's cone shaped and someone told me that the air picks up speed as it goes down because there's a vacuum in there or something. Then it goes into the carb with a lotta speed and smooth flowing, true false?


Thanks,




Mike

 

velocity stacks

That is the purpose of trumpet velocity stacks, is to increase the speed of the air going into the carb,and with high enough velocities, can even "compress" it a lb or two, with the right setup.
Thanks for the observe on my velocity stacks. As I mentioned, thats old school low tech stuff. But then I cut my eye teeth on a 46 Merc coupe with a 53 Merc flatty with 3 3/8ths pistons and a 4 inch stroke, running what are now a set of ultra rare Ardun Hemi heads, and Stromberg 98's. The guy that designed those heads was none other than Arkus Duntov, the father of the Small block Chevy. Long before he began wearing a Bow Tie.

Awesome intake manifold.

It looks like that intake was made from CRS and steel tubing; brazed together.

 

Yes it is cold rolled. It was initially built as a tunnel ram. See my journal and the link to the article about the history of the car. The first picture is me @ ~18 in 1969 building it! I designed it in physics class and it came on like a bear on the top end but really struggled on the bottom. The photo above shows how I cut it way down to a single plane manifold trying to get it to perform on the bottom end. It sorked great but had it powder coated but the guy screwed up the job so I threw it out. I modified a 354 cast iron quad manifold to accept my Holley 95 3-bbl. That is also shown later in the journal. Have been running it ever since. Incidentally, that was before there were aluminum 4bbl manifolds for hemis. You could get 2-4s, 3-2s, 4-2s, 6-2s, blowers, injectors but no single 4s.

 

velocity stacks

You mention the multi duece setups. Does the name Mani-Free ring any bells? they used to make 4 and 6 duece units, using heavy hose between the carb towers on some of their intakes.

 

Yes those, but Weiand and others made conventional multi-carb manifolds too.