LIM polishing

[OH2LS]

I'm going to have some quality time with my parts before I top swap so I'm wondering if anyone has ever polished all the rough casting surfaces inside of the supercharged LIM including the base and the center divider. This will be already machined for an intercooler so I have plenty of room to work in there.

[harofreak00]

Yes it has been done before. You won't see as much gain as an L36 engine would, but there is always room for improvement.

[SuperSLE]

I've done it on my new one Joe. Too bad I never got around to it on the one your getting. Something for you to do I guess. . You've seen the head porting thread on the PDX site. I would get that kit and go to town on it.

[OH2LS]

Yup, that's what I was thinking about doin. I may not get the top swap on the car this year. I want to take my time with all the items and make sure everything is ported the way I want it and ready to go. Maybe I'll make a debut at BSB/WCBF 09

[agrazela]

I certainly could be wrong, but I doubt smoothing the "plenum" section of the L67 LIM would matter nearly as much as smoothing the inlet and the runners.

But if you've got the time and the gumption, then what the heck.

[SuperSLE]

I think he is going for more polish everything in there so that the air flow is as smooth as possible. I agree though, I think that if you aren't getting it machined out, the bar in the middle isn't going to benifit you being totally removed, maybe just smoothed over and polished as well. It probably has some sort of function in there with air direction I am sure.

[OH2LS]

I'm planning on leaving that center divider in there, I'm just going to polish all the surfaces on the inside.

[willwren]

Don't over-port! .95x1.95" maximum, then smooth the runners. The air velocity is too low anywhere else in the plenum for polishing to matter.

[SuperSLE]

Good to know. I just polished the runners on mine.

[willwren]

L67's (either series) don't really need porting. Just smoothing. The runners are very well-matched from the factory (as are the L27), quite UNLIKE the horrific L36 ports.

[PDXGTP]

I'm not 100% sure but the bump your talking about, I think its a water path thru the LIM.

Again, I'm not 100% positive but I believe dyno tests have been completed and identified that smoothing or polishing the bump and the center of the LIM realized zero gains in HP.
I believe the tests also identified that the LIM was hotter inside and the bumps actually provided some cooling properties because the air could grab onto it.

If you think about it, even under boost it shouldn't be able to help airflow at all.

I'd simply focus on the Intake runners, that's where these engines are starving... always trying to get the intake working better.

I also can't prove if it actually helped on my own car or not.
It's been so long since I did my car that I can't remember what the work looked like. Next time I'm inside my motor, I'll apply some of the technics that I've learned over the last few years.
The fact of the matter is, I had my best ET with a stock blower and what ever my current intake might look like.

[willwren]

91-95, that bump is a water passage. 96-03 it's just a bump.

Thomas is right on the potential cooling of the bumps in the plenum. Rougher surface equals more surface area. Believe it or not, the air trapped in the lifter valley is likely cooler than the plenum (boost) charge under WOT. That LIM needs to cool down just like the supercharger needs to. Polishing the runners for increased port velocity is one thing, but I wouldn't polish the inside of the plenum.