Carbed Turbo Compressor Housing Mods


Active Member
I think everyone agrees the compressor housing on the '78 to '83 carbed turbo is a terrible design. The 3 bolt flange, commonly referred to as the "foot", that mounts the turbo to the intake manifold, seems to get a lot of the blame for lack of performance. The truth is the entire housing is too small and is flow limted by it's size. But adapting other housings would be very involved with numerous obstacles to overcome to make one work. It can be done and I've been trying to figure out the best way to do it. But for now I'm going to work with a stock housing. You can see in this picture the size difference between the '78-'83 housing, an '84-85 housing and an '86-87 compressor housing.
The outlet hole inside the compressor housing going into the 3 bolt flange "foot" area is very small in diameter. Airflow is directed into the middle of the foot through a narrow neck and doesn't take advantage of the 2 inch wide exit hole into the intake manifold. After crashing into the wall of the foot, airflow then has to make a sudden 90 degree turn going down into the intake manifold. The sharp top corner angle of the foot creates more turbulance.
I decided to see if there was any room for improvement by porting the exit of the compressor housing into the mounting flange or "foot" area. In order to really make any flow improvement, the sides of the housing at the neck needed to be welded up so that I could widen the neck without breaking through to the outside. And I DEFINATELY would have.
I used 7" long cutters to port the sides wider and also started the downward 90 degree turn downward further up inside the compressor housing. I used one with a rounded nose and one Christmas tree shape to get the areas cut down. It's tedious because you can't see inside while your grinding with the cutter inserted at such a sharp angle. You have to cut a little and then stop to look and feel inside. I've never ported anything that allowed for so much material to be removed! I had aluminum chips everywhere. I followed up with cone shaped sanding rolls to smooth it as best I could. Although you can't see it in the picture, the airflow now starts the 90 degree turn downward about 1 1/2" up inside the housing and the neck is much wider with the airflow having a straighter shot and directed to the entire 2 inch exit hole in the flange. It's not pretty but should be an improvement. Here is a comparison of the ported housing with an untouched housing.
I also filled in the dead space in the upper corner of the foot with Epoxy. This should help direct the airflow downward more smoothly instead of crashing into that sharp corner. I should have done this last because the heat from welding turned it dark colored.
There's room for opening up the hole in the mounting flange also. Laying a gasket on the flange shows it can be made larger and still seal against the intake manifold. Of course the intake manifold itself needs to be opened up the same to match.
I have often wondered if any of the snake oil carb spacers would help in atomization with our setup. It would be fun to try all the different oils;) Keep up the good work.
Maybe try putting that intake squared up on a drill press and using a hole saw bit to cut out the opening.
It's easier than that. Just line the gasket up on the opening of the intake using the 3 bolts to line it up and hold the gasket in place. Then scribe a line inside the gasket. Use a die grinder with a cutter (made to cut aluminum or cast iron depending on what intake you have) and grind back close to the line. Don't use the die grinder to cut all the way to the line. Leave some material. Once you are close to the scribed line, switch to using a half round hand file to get the curvature of the hole even and then a die grinder with sanding rolls to smooth it out.
I don't think there would be an advantage to a carb spacer since the fuel is inhaled through the turbo. The problem with draw through turbo systems is fuel puddling in the intake manifold.
The pontiac 301 housing was used as a better solution a bit ago. It has to be modded to fit. Has a a/r .60 vs a/r .42 compressor housing. Notice the foot difference.
I have the 301 cover on mine. Started tinkering with my stock cover porting it just for fun...but I have yet to see anyone post anything much good with it. Stock carb cover ported vs stock 84-85 cover wonder if it's anywhere close to same flow with the tapered neck and "foot"