I have a 45 dcoe on a street port 12a. I'm playing with my carb to find the "tune" I prefer. What I want to know is how do longer or shorter velocity stacks affect the performance? Does the cab see it at a part of the intake? Meaning if I make them longer does it improve the low end, and shorter improve the top end etc.? I've noticed that some have a "belled" lip too, what's the benefit there?

 

Correct me if I'm wrong anyone but IIRC this is how it goes:
Longer velocity stacks = better low end
Shorter velocity stacks = better top end
None = not really great at anything.

It serves to smooth intake flow and increase intake speed.

Thanks to wiki for the pic.

 

+1. Pics are worth a thousand words! Cant realy Simplify that ne More. Always loved the look of Giant bell mouth Velocity Stacks theyre badd ***!

 

Those things are so damn sexy, I want to make some for my Nikki.

Btw if the bells are contained inside an airbox, I've read that it's important to have the lid of the box sitting higher than the lip by at least the inner diameter of your bores. The same article also said that it's very important to have those smooth, shaped lips to reduce turbulence.

 

Thanks Guys! I thought this was how it worked, but didn't want to spend a **** pot on stack to find out for sure.

 

Tell you what bro, you helped me out with that carb info you gave me a few weeks ago. I have a set of stacks that I got for a IDA. Your welcome to them just cover shipping and there yours...

 

Awsome! pme'd ya

 

This can be affected by the ports (or cam timing for pistons) but the way to calculate velocity stack length is by measuring from the combustion chamber, to the end of the intake track (tip of the stack in this case).

The equation is (RPM x Intake length)=84,000 for mild timing, closer to 80,000 for radical (Bridge ports, high lift roller cams)

So for instance, if you want your velocity stacks to provide max torque benefit at 7000RPMs, 7000(L)=84000....... L=12inch intake track, pick velocity stacks that get you close to this. However, say you like your top end but want a boost to your midrange (using 5500rpm as an arbitrary midrange RPM): 5500(L)=84000..... L=15.2 inches, or a stack (or total intake length) 3 inches longer than the one used in the first calc.

Different stacks will give a boost at different point in the rev range, using this math you can get close to your desired point.

 

Hrm that's really useful info.. anyone have measurements on the stock port runners / manifold save me pulling mine apart?

Btw how did you get that equation? Also wouldn't it be affected by the ID of the intake, whether you have a 2 or 4 barrel carb, etc..

Am very curious about this as I'd like to make some effective stacks for a nikki, with longer stacks on the primaries and shorter on secondaries. Cheers

 

I've got a couple laying around, stock intakes w/nikkis. I'll run a tape measure up the intake runner this afternoon and post the length for ya tonight.

 

Found a great article linked from the wikipedia velocity stack page (at the bottom) on bell mouth design. I'll quote the good bits and add a few pics to summarise.

Different designs

Air velocity (red is higher)

Maximum "spit back" or return pulse

 

Oh awesome thanks for that!

 

That information is from an article popular hot-rodding did on "ram tuning" which is basically optimizing the intake length to take advantage of pressure waves that travel back and forth for a natural supercharging effect.

They dyno'd a car back to back with nothing but velocity stack changes and noticed how the blip in the torque graph moved. The equations are from Chrysler's lab notes on the very subject when they got serious about the subject back sometime in like the 70's.

As for your question, runner diameter will effect the powerband, but changes to the length will have their own effect, separate from the runner diameter. However a radially cammed engine (and with larger diameter runners) would probably benefit from doing their calculations with the 80,000 figure as opposed to the 84000 as stated.

 

With a typical side draft carb mounted on top of the engine, the intake manifold runners are so long that you are not going to be able to make much difference by playing with velocity stack length. In other words, the optimum system length is shorter than the length you already have, which is the sum of (side port + long 2 barrel intake manifold + carb + velocity stack + air filter clearance) lengths. You are already too long to tune for 8000 rpm power.

On the other hand, if you have an old Lake Cities manifold that points straight out at the right inner fender, THEN you can see results by playing with velocity stack lengths.

The distance from the end of the stack to the air filter must be added into the above length calcs, because the filter will be the thing that stops the pressure wave on the inlet end. Oneiros is correct that you want a lot of clearance between the end of the stack and the filter. If you are running a filter with a flat metal plate over the stack, and filter material around the side of the stack, then your horsepower will definitely be hurt because air cannot flow straight into the stack.

In 1988, my over-the-engine manifold with a 2 barrel had torque power to start coming on about 4000 rpm with a street port.
My current street downdraft Weber IDA style has power coming on about 4500 rpm with a street port.
My current race car's downdraft hogged out IDA with a PP has decent torque at 5500 rpm, and great torque above 6500 rpm.

 

Are there any self-adjusting sliding stacks that extend and retract based on RPM? Maybe electronically or vacuum controlled? That way you could have good torque throughout the entire RPM range, innit? Kind of like how VRIS works...

 

Yes, the car that won Le Mans. But I doubt any of us could afford the engineering it would take to make a functional set for a street car.

 

Picture is worth a 1000 words.
The baddest Velocity Stacks on the planet!
1991 LeMans R26B 4 rotor. Was banned after it was the first and only Japanese car/engine to win the LeMan 24hr.
http://www.japanesesportcars.com/spo...mazda_787b.php

 

I'm bringing this back to life. Have a question. Instead of the canister and air filter, I have filters like this that I got for the track car...

Good or bad idea? The car gets towed to the track, its not a daily. I just don't have much experience with Webers and wanted to know your thoughts.

 

I had some like that. Seemed pretty restrictive, but I don't have any numbers to back that up.

 

where can i find those stacks for my IDA??

 

From what I understand, those "screen" type tube tops work fine and are used alot on the track. They aren't going to filter as well as the "paper" filters, but as long as you driving on asphalt and not through alot of dirt and dust situations, you should be fine with those. Alot of people run with no filters at all. (though I wouldn't recommend it) You can also run with the "sock" style slip over filters.

 

Thanks Legokcen! This brought back some memories, I sat here LMAO remembering my old mans racing days as a kid. One of the hotrods he had back then was an El Camino he had put a big block in with dual webers. (maybe triples, I'm not sure) But I do remember how pissed mom would get on the weekends at him, I used to wonder why till I saw him taking her nylons and sticking them in the glove box! LOL He'd cut them up, wrap em over the stacks and secure them with rubber bands. Worked great! Mom was always griping about dad ruining her legs, which had never made sense till I realised it was Leg's panty hose! LMAO!

 

I'm not a genius, i'm just a seventeen year old kid. But i run a weber, and have for many years. And just recently stopped running stacks about 6 months ago and notices alot more torque at the bottom end, and only slightly less pull around 7-8k rpm.

 

I have actually gone through the embarrasment of buying nylons and trying this. They were so restrictive my car would hardly run! I wouldn't do it. Might work on a big block but not on a rotary

 

On a side note, has anyone run, or know of someone, that is running a terminator II carb with the stacks that come with it - and is it possible to get a correctly fitting K&N style filter to sit over the stacks on this carb. Not worried about coming out of the bonnet.