Velocity stacks

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hemicat
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Velocity stacks

Post by hemicat » Mon Jun 16, 2014 5:44 pm

Does anybody know if velocity stacks are a necessity on throttle bodys? Think I am not getting enough air because of the distance between the top of the stack and the air filter top. Seems to flatten out above 3500 and have changed everything but that. Thinking about taking them off and running it to see if it band aids the issue. If they are essential then I could resort to getting taller filters. Tried leaning and richening the A/F, messing with timing, changing heads, exhaust, cam, cam timing, went from dual carb to dual TB EFI, electronic dizzy to crank trigger, different valve lashes and nothing has seemed to really help the problem.

gearheadgreg
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Re: Velocity stacks

Post by gearheadgreg » Mon Jun 16, 2014 7:00 pm

If you have less than 1/4" clearance between the air filter lid and your stacks, it is certainly restricting flow. It's super easy to take them off - try it and see if there are any changes. Stacks are always better than not, in terms of airflow, so if they are too close to the lid, get shorter ones!
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raul arrese
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Re: Velocity stacks

Post by raul arrese » Mon Jun 16, 2014 8:14 pm

Run it with no air filter and see if it makes a difference .

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Tony Z
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Re: Velocity stacks

Post by Tony Z » Mon Jun 30, 2014 12:56 am

If I remember correctly, you want to have the same height above the stacks as the stacks are wide.
On my 48IDFs I have 100mm air filters giving me approx 45mm between stack and lid. I am not 100% sure this is high enough, but it works for me for now
stacks increase the efficiency of the throttle body

hemicat
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Re: Velocity stacks

Post by hemicat » Sat Jul 19, 2014 11:39 pm

Finally got around to getting my trailing arms clearanced for my shocks and getting it out on the road again. Took the stacks off and hit the rev limiter in third and was still pulling hard. Guess now I will need to look into taller filters.

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RHough
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Re: Velocity stacks

Post by RHough » Tue Aug 05, 2014 12:21 am

Tony Z wrote:If I remember correctly, you want to have the same height above the stacks as the stacks are wide.
On my 48IDFs I have 100mm air filters giving me approx 45mm between stack and lid. I am not 100% sure this is high enough, but it works for me for now
stacks increase the efficiency of the throttle body
Height = stack diameter is more than you need. Can't hurt though. Height = 50% is plenty.

To avoid restricting the intake you need to space the top of the filter far enough away from the intake so the area is not reduced. The intake area is Pi (approx 3.14) times the radius squared: Pi r^2 = Area

The intake area between the intake and the top of the filter can be figured as a cylinder with the same diameter as the intake and height equal to the distance between the intake and the filter top. The surface area of a cylinder is Circumference times Height or Pi D x H = Area

If you have a 50mm diameter velocity stack entry and the filter top is 50mm away the numbers look like this:

Intake Area is 3.14 x 25^2 or about 2000 sq mm
Available area for intake is 3.14 x 50 x 50 or about 7850 sq mm

The available area is almost 4 times what the intake would have if the filter top was not there ... no restriction due to area.

It works out that about 25% above the diameter of the intake is all you need to feed the intake. No real point in building a filter taller than 50% of intake diameter.

Cheers,
Randy

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