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Old November 21st, 2008, 06:16 PM   #11
TheDuck
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Name: Duckman
Location: Pensacola, FL
Join Date: Nov 2008

Motorcycle(s): '08 250R, '06 Chopper, '80 Puch Magnum II, '01 SV650, '99 CR250/CR500AF Sportbike. '02 CRF450/CR500AF Dirt

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Doing a little math with an engine tune friend today on this topic, I think the optimal intake boot length for our engines, would be about 7 inches long from the head of the intake valve back to the output of the carb.. but same diameter as factory. From what I was told, the exact diameter doesn't matter much unless you wind up restricting the flow to a lot smaller than what your carbs can flow or make them so big that the turbulence inside of them is weakened. Keep them about the same size.

That should tune it for the maximum harmonic at 12,000RPM. Where AreaP says the peak power is with their exhaust.

If you're looking for power a little lower down the RPM scale, then try these, all of which are for the same diameter as factory.

8.4inches long for 10,000 RPM (factory peak HP)

10.5inches long for 8,000 RPM

14in long for 6,000 RPM

If you do this, you will probably need different throttle cables, yours will wind up too short!

Now don't commit to this without talking to someone else on the topic, we could be waaaaaaaaaaay off but he's a rather bright engine tuner. He might be correct!

The formula is stupid simple though:

84000 / RPM = length in inches

Anyone else's input?
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