SC'd runner length

Jody Shapiro jshapiro at bit-net.com
Tue May 13 15:42:43 GMT 1997


On Tue, 13 May 1997, Mike Turner wrote:

> A really dumb question: Does increasing the cubic inches,  from say 350 to 400 
> on a small block chevy, shorten the effective runner length when using the 
> same intake? Since the effective velocity increases (more air per revolution 
> thus per second), does this not move the resonant length of the runner up in 
> frequency thus shortening the effective length?

I would think no...

The reason the air column starts oscillating is because air is rushing
down the intake runner, then the valve closes and it bounces back off it.
Then it bounces back off the interior of the plenum and heads down the
runner again.

The frequency at which the valve opens and closes is directly related to
the RPM of the engine.  I don't see how engine displacement affects
that...

Tuning the runner length is designed to get the air column to be heading
downward toward the intake valve when it's open to help cram some more air
in the cylinder.  With a constant runner length and size, you can only
tune for a given frequency (== given RPM), so you pretty much pick the RPM
you'd like to get the extra help and set up the runner for that frequency
resonance.

-Jody

--
97 Blue Z28 M6 (Memphis #1997)
  Eibach's, !CAGS, !DRL, Crane HI-6, T-Rex, Kenny Brown SFC's,
  MBA plate, 1LE shaft, Vortech 7lb S-Trim
  To be installed: AutoMeter FP & Boost Gauges
Best ET (stock): 13.941/101.18   Best MPH: 13.999/101.61
http://www.bit-net.com/~jshapiro/z28/  




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