Upper Plenum / Intake Manifold
I purchased the following manifold already extrude honed and chromed from Joe (aka Cabojoe on 3SI). I was very impressed with the extrude hone and quality of the chrome job.
I quickly proceeded to close off the EGR port (the non-reversible way). BTW - that is not me in the picture, but instead Del Glasser from Custom Welding & Fabrication.
I then ported it near net shape and sanded it down until I was satisfied with the finished product. I compare it to a stock upper plenum below, right.
Then I realized I needed to open it up to match the throttle body (bored out to ~64.5 mm). First I opened up the gasket (stock size gasket laying underneath it). Then I carefully hogged out the upper plenum. It takes awhile to remove ~2 mm of aluminum with a die grinder, but it had to be done. Now, for the first time since the car had an over-sized throttle body, the air won't have to contend with a nasty 2 mm step.
A few more pics showing the larger mouth. The stock opening measures in at ~60.8 mm. I opened mine up to ~ 65.0 mm. The throttle body is now a hair smaller than the gasket and upper plenum opening (just what the doctor ordered). After finishing it off by hand with 180 grit sand paper, it transitions smoothly into the rest of the plenum.
You can really spend a lot of time porting and wonder what, if anything, you'll gain by it. I spent many hours working on removing the restrictions seen below. Excuse the picture quality, but you can almost make out all 12 areas that can benefit from a little porting. There are two in each runner located near each bolt hole. It appears Mitsubishi wanted ample wall thickness around each bolt and in the process intruded in on the runners. If you decide to do this yourself, be careful not to take off too much, especially around the two outer-most bolt holes (wall thickness will get dangerously thin if you port it straight across).
Here is a closer look at some of them. Note that grinding it flush didn't take much time, it was the sanding by hand afterwards that took forever and a day. Funny thing is, sanding it smooth by hand probably did little to help performance. By the time the air hits the injectors (slightly downstream), you actually want turbulent flow, so I probably should have just left them alone after die grinding (as shown below).
At this point, there isn't much that can be done to improve flow through the upper plenum unless you start from scratch (as in a custom plenum). These runners don't need anymore material removed to match the gasket. Either it was gasket matched before it was extrude honed or the extrude hone itself removed enough material to make it match. I would recommend that anyone who plans to port & extrude hone do it in that order. Because I ported a manifold that was already extrude honed, it took a lot more work for the finished product to have a smooth surface finish.
I would like to fart around with a shorter runner plenum and probably will when time permits, stay tuned.
since August 7th, 2002
Last Updated: 08/08/02 07:50 PM