Custom intake manifold idea: Good or bad?
 

well, I'm pretty sure that my stock intake manifold is FAR from ideal on my current car, and I basically have no need at all to have ANY of the stock equipment on there anywhere, since I'm running a full standalone. I've decided to build my own intake manifold and I have a couple different ideas. The car is a turbo halfbridge, and has the lumpy ported idle. Redline is whenever it stops making power, probably somewhere around 8000-9000RPM.

We were talking in the cofee shop last night and I made a design for this though I don't know if it's pointless, just overly complex, or perfect if it's done right, so I want some input.

Idea 1: Short runners (I have a friend that took some engineering in school and is confidant that he can come up with a "tuned" length for ideal high RPM power. Coupled to 4 ITB's (2 per rotor, one per port) - 2 ITB's for 0-50% throttle and all of the idle air on the primaries (which are just streetported) to maybe have a stockport idle? and all 4 for high RPM airflow... Velocity stacks on all 4 and a custom plenum/whatever to optimize airflow and keep all that boost pressure in.

I really like this and don't think it's too complex to pull off, but I'm not sure what size TB's to use or if this is even any benefit over a single large TB and conventional custom manifold, which is option number 2.

The stock TII manifold is fine. It's a straightforward plenum design with separation between the primary and secondary throttles. It's the NA manifold that is funky with aggressive ports.

Keep in mind that short runners will make very good high RPM power but will kill the low end. Unless you make variable length runners all you can do is shift the torque curve up or down.

To go with 4 ITBs you will need a new lower intake but I assume they are easily available from places like Tweakit. I've never looked though...

Idling off the primary ports only is still going to give you a bridgeport idle.

My car is seriously suffering in the topend and I can't figure out why, I thought it MUST be the stock intake manifold which looks pretty puny, and has LONG runners compared to some other cars I've looked at (h22 and DSM)

It's possible the 25G you got on there is simply running out of headroom... A bridgeport will put a serious hurting on a small framed turbo... Just a thought... :dunno:

tweakit has a 2 ITB manifold, ITBs, and Turbo plenum that is right up your alley. Unless you have some significant building experience it would probably be cheaper and easier for you to order everything from them. You already have the standalone, so that is the biggest hurdle with a ITB setup. You can get the 55mm ITB, lower intake manifold, and turbo plenum for abotu $1200 Aus... or whatever that translate to in american.

A dyno graph with a datalog would really help diagnose the top end.

yeah, the only dyno's around here don't do load. :(