When myself and a shop were building my CAI
for my Contour, we experimented with length. This guy had done the same on a Civic and saw great gains. It involved quite a bit of math utilizing bore, stroke, TB inner diameter, etc.; but it was WELL worth it.
As an example: Most stock Zetecs w/ 5-spd. dyno at about 103 FWHP. I had chopped my stock Intake
up before meeting him, so I never had a true baseline. With my chopped up Intake
, I put down 111 FWHP. With the final version of the CAI
, I put down 122 FWHP. With the Zetec, the optimum length was a little over 30 inches.
The length of the pipe didn't have so much an effect on peak HP, but more on power under the curve (usable power throughout the rev range). By varying the length, we were able to come up with a happier power band than if we hadn't dyno-tuned it.
Hope this helps some.