Port and polish the plenum
I've wanted to do this for a while whether or not it'll show any power improvements.
The inside of the plenum is fairly rough and isn't machined or cleaned-up after being cast. At least it doesn't look like it. The pics show some of the work I've been doing on the upper plenum. Pretty simple and quick to use a barrel sander and some small grinding stones to take off the "sandy" surface and make it a lot smoother. I think it's valuable to the the inside "bell" part, too.
The first two pics show inside views with the first being un-polished outlets and the second showing the other side having been polished. The third pic shows two outlets with one polished and one not. The polished one doesn't look that scratched in person and does feel a lot smoother.
The original surface is pretty rough with grains of metal sticking up. It's not a case of polished vs. dimpled as some surface texture can be good.
I also did some work on the lower plenum, to. (No pics) Mostly just smoothing the inside of the casting with a Scotchbrite pad and getting it clean. I did notice that the throttle body didn't line up well on the plenum. Some work with a half-round file got that to match. I cleaned up the front well as that's where the air is being turned upwards to the throttle.
Next up is to polish and port the runners a bit.
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1980 928S Euro x2
1987 944 N/A
Last edited by GlenL; 04-28-2018 at 09:32 AM.
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