View Single Post
Old 10-01-2007, 02:15 AM   #5
jigga
Scooby Guru
 
Member#: 9960
Join Date: Sep 2001
Location: in bed...
Vehicle:
2002 Impreza WRX
WRBlue Perl

Default

Quote:
Originally Posted by hotrod View Post
Scratch building an intake manifold is a bit more complicated than it looks.
Anyone can build one the question is will it perform as well as or better than the stock manifold.

Issues to consider:
Strength of the plenum -- custom manifolds for boosted applications need to give careful consideration to the pressures built up inside the plenum during high boost and on occasion you have a backfire into the manifold. If you are running high boost (say 24 psi) and have a roof area of 30 sq inches (5x6 inches) there will be 720 lbs of force trying to blow the top of the plenum off. Plus you have the constant flexing of the plenum due to pressure changes. Many of them have problems with cracking due to these two causes.

Runner length can have major effects on off boost performance due to ram and resonance effects. Longer runners move peak torque down, and shorter runners move it up the rpm band. Equal length runners give a sharper effect than runners that are slightly staggered in length. Length changes of less than 2 inches typically have very small effects, so that is all the accuracy you need regarding runner length.

The effect of resonance and ram effects under boost is widely debated so read up and make your own choice.

Runner diameter can also effect manifold behavior. Large diameter runners make for high rpm power but sluggish low speed response as it takes longer for air speeds to build up in the larger volume runners.

Last is even if it looks good the dyno and ET may not like it, so you will need to test test test to get the best out of it. Even very small changes to runner entry radius and location can have huge effects on cylinder balance etc. The difference in flow between a sharp cut off tube and a properly formed radius can be as much as 17%.

An EFI engine at least does not need to worry about wet flow like a carburated engine but if you start to use water injection then that also becomes an issue.

Good luck if you choose to try it, there are lots of things to read up on this topic so get friendly with google!!

Larry
So would you suggest that stock is better than many of the one-off jobs we're seeing floating around the forums?
* Registered users of the site do not see these ads.
jigga is offline   Reply With Quote