I decided to modifiy my own OE plenum for the following reasons:
-certain modifications have already been proven to show HP/TQ gains.
-it's cheaper and more fun to do it myself
-it's completely custom and how I want it (I'm okay with a looseing a bit of low end power to gain it up top b/c I'm not worried about everyday driving, but autoXing)
-I can take the extra time to match all mating surafces and smooth out any casting lines
parts:
-Upper plenum (connects to TB housing and spacer/lower plenum)
-AAM angled plenum spacer (adds extra volume to plenum biased towards the front)
-Lower plenum (connects with spacer and upper plenum; distibutes air into 6 runnes and attaches to lower collector)
-Lower collector (takes air from lower collector and brings it to heads)
Here's what I did:
1) removed all the casting line and #'s from all of the peices (smoother surfaces = less restictions = more HP)
2) matched all of the mating surfaces and removed all material around the surafces so that every thing mated perfectly (lots of times at the mating surfcae of 2 components, one will be slightly bigger/smaller - by removing mateiral off the smaller one, flow will be less restricted [inconsistancies in surfaces can cause turbulence, eddies, ect for the air flow...all of which increase the resistance)
3) Lower plenum: removed material from top of runners and smoothed out edges; more mateiral was removed from the front runners. It was found that the 2 front runners (which supply the 2 front cylinders) had more restrictions than the others...greater restriction and non-balanced flow take power away - by opening up the volume around the front runners, this alleviates the problem.
4) Upper plenum: removed interior posts, welded holes and smoothed out. Matched mating surface to TB.
5) Coated Lower/Upper plenums and spacer with Hight Temp ceramic spray paint (flat black). I did this b/c ceramic is a great heat transfer barrier; heat gets transfered from the cylinder heads to the lower collector to lower plenum to upper plenum...all of this gets fairly hot and in the end, the heat is tranfer to the air flow through it. By reducing the amount of heat being transfered to the air, the air temps will be lower, which increases power. (colder air is more dense, so the colder the air, the more air that will get into your cylinders ...assuming A/F stays the same, you ECU will compensate and add more fuel --> more POWER)
**other cars have used "Phenolic Spacers" to help with reducing heat transfer. It's a non-metallic mateiral that's a good insurlator. For my purposes, I'll had the spacer for a different reason, so i just coated it to also help with heat.
Here's a few pics of the process to coat the plenum spacer:
step 1: tape the flat surfaces...I don't want to paint the surfaces to ensure that it's super smooth and will have a proper seal w/ the gaskets.
step 2: cut off excess tape
step 3: paint
step 4:remove tape and BAKE! (I did it @ 300 F...not too hot, but enough that it sets the paint well - the spacer shouldn't see any temps over this, so I didn't go higher) The pic doesn't show it the best b/c of the lighting, but onyl the exterior/interior surfaces are painted, not the sealing surafces.
