The Misunderstood Velocity Stack

[klrskies]

Intake runner shape and length and Velocity Stack length and shape as well as intake tract taper are part of intake momentum and reflected pulse tuneings' adjustable parameters. Consider how often we see the distance from the intake valve to to velocity stack entrance about 12" long. It's no accident, as it generates a strong 2nd wave reflection at the rpm range many street engines operate at. In cross section, an involute curve shape is ideal, but a 1/4 radius is pretty good in general.

Download and play with the free Lotus engine simulator for an excellent intake system modeler. All the resonate tuneing benefits are only available over a narrow rpm range however...600~800 rpm's of usefulness at a specific reflection. That perhaps makes them a bit less intresting for street engines perhaps, and more in vogue on narrow rpm range, track engines, but the results are available to those who impliment them into their tuneing program.
regards,
Ken

[FJCamper]

Hi-

Having an inch of clearance inside a Weber air filter between the velocity stack and top of the filter is okay, and about all we can get for a Ghia.

I don't run filters on our racing engines, but filters are mandatory for street cars.

And Klrskies comment on the fairly narrow RPM range in which intake runner or velocity stack lengths affect power is very true. Racing engines are very narrow in their power bands (one reason we use close ratio gearing) and one skill of a good driver is keeping the engine in the power band while doing all the dodging and weaving stuff as well.

FJC

[eugene]

This is from a book on tuning A-series engines(Minis & Co). As you can see from the pics and the changes in airflow the ideal shape is not as obvious as someone will think. The last shape will help with those applications where space is a problem but dosn't cure the air-fuel mist reversion in such a degree like a longer VS will.

[helowrench]

that is a wonderful chart to have eugene.
It makes me wonder if those improvement percentages cfm dependent, or across the board?
Rob

[klrskies]

They are improvement (or loss) in flow over a carb with no pipe.

[TomandBunny]

I have a weber 48mm dcoe side draft carb. the velocity stacks are about an inch or less from the top of my air cleaner, I can not change to the larger air cleaner because of part of my roll cage blocking a larger filter, so I had to go with a K&N shorter filter. my question is, I have a 2276 with a t4 turbo, I had a holly carb but because of a mis calculated mod I could no longer use the holly and had to go to the side draft. so here is the question, with the engine size and turbo, if I am ripping through the dunes is the engine going to starve for air? I would love to go to a shorter pair but I am not having any luck finding any. the local vw shop looks at me and says DUH! which drives me crazy, I have talked to another shop and they say it will work fine but talking to another shop they say I am going to limit my air flow. which is fact and which is fiction? also does anyone know where I can actually purchase a shorter pair like 2 1/2 inch so I can feel comfortable knowing there is plenty of room from my top air filter plate and the stacks?

Thank you

Hi-

Having an inch of clearance inside a Weber air filter between the velocity stack and top of the filter is okay, and about all we can get for a Ghia.

I don't run filters on our racing engines, but filters are mandatory for street cars.

And Klrskies comment on the fairly narrow RPM range in which intake runner or velocity stack lengths affect power is very true. Racing engines are very narrow in their power bands (one reason we use close ratio gearing) and one skill of a good driver is keeping the engine in the power band while doing all the dodging and weaving stuff as well.

FJC

[Piledriver]

http://www.carburetion.com/sitesearch.a ... r%2045DCOE

Be warned: I think the prices are EACH, but a Google search found that in .00324 seconds, there were many more, that was just the one on top...

[FJCamper]

Hi Gerico,

You need an inch or more (24mm to 36mm) of space between the top of the velocity stack and the top of the air cleaner.

Actually, some dyno time has been devoted to this very subject. For instance, Kadrons can pick up a bit of power if used with an enclosed air cleaner.

Webers, not so much. But factory air cleaners for Weber 44 IDF's allow between 24mm to 36mm "headspace" above the commonly provided 2.5 to 3-inch velocity stack. Most of us have some aftermarket air cleaner housings, so don't take the 24/36 headpace measurement as gospel, just a general rule.

FJC

[FJCamper]

Gentlemen!

Jeff Lain at the Kaddie Shack has made an exciting discovery concerning Kadrons and velocity stacks.

See: http://kaddieshack.com/aircleanerdyno.html

Now, Jeff's news does not make all that came before in this old thread wrong, it just goes to show us all again that what is "true" is true under certain conditions. And he has found the right conditions for more power.

FJC

[Steve Arndt]

Keep in mind the draw through turbo side draft may experience less fuel stand off since there is no reversion wave going back through the carb. You still need plenty of space for easy airflow to enter, but you aren't going to be as concerned with the standoff vs normal webers per throat.

You can cut the velocity stacks shorter and weld back together.

[jhoefer]

That dyno run also shows why you are better off running a standard style filter box around the velocity stack (with appropriate stand-off distance and large enough filter area) than tiny filters that go right on the stack itself. Having the filter element that close to the VS disrupts the surface airflow making it less efficient and also just can't pass as much air as the stack wants. Assuming you can fit the larger filters and don't want those tiny filters for looks.

[neil68]

For Weber IDA's, JPM manufactures some new stacks (and venturies). The stacks "appear" to be similar to the stock IDA stack, but apparently provide a slight improvement in cfm. The venturies provide 19 cfm improvement versus stock vents in their testing. They have spent a lot of time on computer and dyno testing these parts:



I just purchased their 44 mm IDA venturies, but haven't bought the stacks (yet ).

[Roflhat]

Gentlemen!

Jeff Lain at the Kaddie Shack has made an exciting discovery concerning Kadrons and velocity stacks.

See: http://kaddieshack.com/aircleanerdyno.html

Now, Jeff's news does not make all that came before in this old thread wrong, it just goes to show us all again that what is "true" is true under certain conditions. And he has found the right conditions for more power.

FJC

very interesting that!

Is there not a formula somewhere for finding the best size of stack for an engine?

[neil68]

I'm trying the Weber IDA aftermarket venturies from JPM. These are their Raptor venturies that replace both the stock Weber vent and the auxiliary stack:

[neil68]

My stock IDA stacks were vibrating loosely, so I decided to buy some Raptor stacks. Just installed them tonight, and they are a very precise fit...well machined: