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Old 02-02-2009, 04:36 PM
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Hilborn Stacks - Right Length?

Hey there, engine gurus;

I'm looking for opinions regarding the optimum length for a Hilborn EFI fuel injector velocity stack.

My engine builder said that when the previous stacks were cut-down from their stock length to fit under the hood, we threw away 40 ft-lbs of torque.

Now, I'm making custom curved velocity stacks, and I'm wondering if I'd gain anything by making them a little longer.

It's a streetrod. The engine is limited to 6250 RPM, and will rarely if ever see more than 4500 RPM. It's a 327 Chev in a '32 Ford. It has a CTS-V T-56, 4.11's, and huge 265/70R17's in the rear. Not sure on the cam specs, or if any of this info is relevant.

I'm guessing that the optimum velocity stack length varies by application, and the only way to know what's best is by trial and error on the dyno. My engine guy says to make them the same length as the ones that came from Hilborn. But I'm curious to know what you all think.

If any of you have input on this, I would appreciate it greatly.


Joe G.

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Old 02-03-2009, 01:37 PM
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Yes there is a science behind the tubes. Read these old threads.

Velocity Stacks and their effects....

small exhaust diameter up front and big out back??

Here is the math;

Assuming the (exhaust) collector size (or carb plenum. Injector stacks have an infinite 'plenum' - the atmosphere!) is reasonably larger than the primary tubes, each tube acts independently from its neighbors. The main advantage gained in equal length, independent primary header tubes is from the strong negative pressure pulse that is reflected from the tube end when the strong positive pressure pulse from the exhaust valve reaches the collector. Obviously, the opposite is true for intake manifolds - the opening intake valve causes a negative pressure pulse to travel up the intake runner and a positive pulse is reflected back down the tube. You want this positive pressure pulse to reach the intake valve the instant it closes to supercharge the cylinder. Other pulses from other header tubes are of much smaller magnitude in the tube of interest and can be ignored. Thus tuning length is very easy to determine once you have an estimate of the speed of sound in the hot gasses (or cold gasses for intakes). A useful equation is

L = 120V/rpm

L = pipe length, less port length in head, in inches

V = velocity of sound in hot gasses. Values of 1300ft/sec to 1700 ft/sec are common. For cool intake air it is about 1100ft/sec

Using V = 1100ft/sec the equations simplifies to

L = 132,000/rpm.

4000rpm peak boost requires 33" tubes!! That length includes the ports in the heads. Remember the old late 50s Chrysler cross ram manifolds? They had 30" long runners so the peak torque occurred down in street driving speed range. The ones below were obviously designed using the above formula (works for intake runner length design under carbs just as well as it does for open injector tubes).

383 in 1960 Dodge Polara

Injected racing engines have those short stacks 'cause their peak power RPM is way up there. SBCs can rev @ 8000rpm with ease so their stacks can be 1' tall and do the deed.

Last edited by willys36@aol.com; 02-03-2009 at 01:43 PM.
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Old 02-03-2009, 07:29 PM
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Thanks for the info and the links. That is exactly what I've been looking for.

Joe G.
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