The supercharger is an M90 Eaton unit off of a 3.8 Build V6. It'* designed for 90 cubic inches of air for every revolution.
It has a rear centerline air entrance, not a top entrance. In the pics you can see where the air enters the rear, has to climb up above the centerline of the rotors to enter the rotor chamber. Then it is discharged out the bottom front of the rotor chamber.
This is the rear entrance.



This is a close up of the entrance where the air has to climb up over the rotor centerline and into the chamber. You can see the bypass air return hole in the pic at the center of the ramp. They use a vacuum operated bypass valve to release the pressure in the manifold back into the supercharger entrance to bleed off pressure during cruise and low rpm engine speeds for more gas mileage.



This is looking into the rotor chamber. You can see the air entrance in the back wall of the chamber.



And the rotors and front bearing plate. Twisted 3 lobe rotors.



MArk L

This is a view of the bottom of the case. You can see the front bottom discharge V and the silencer slots. The bigger round hole is the bypass entrance to the inlet. I'm not sure what the smaller hole is for nearer the inlet. There are several little holes for engine water to circulate into and out of the throllte body and some EGR pssages .



When porting the case you want to grind back the sides of the V discharge opening just slightly. Supposedly , do not lengthen the V as it goes too far back into the rotor area and the supercharger loses efficiency. In the pic, the bottom of the discharge opening (large open end) has two casting bosses sticking into the air flow. These are extra deep bosses for the bottom nose/bearing plate/case bolts. All of the porting I've seen illustrated shows cutting that all the way to the front case line, eliminating the extra boss material and then radius that inside corner. Then shorten the bolt to fit. I will do that but I will either drill and tap for larger 3/8th bolts (for more grip) or use the 3/8ths bolt plus heli-coil the threaded area.



I'll also open the inlet from the stock 70MM opening to hopefully 80 or 90MM.
I mentioned a blow off valve. I would use one prior to the throttle body to release pressure when the throttle is closed, just like a blow thru turbo system.

Mark L

I know many of you are very familiar with this unit. The above posts were copied from other boards I frequent that know little to nothing about these blowers. I added them here so the members of this board can see what I'm doing. If anyone here has good info on porting or otherwise getting a lot more out of these blowers I would be more than happy to read about it. I want to build this 4 cylinder pretty radically which means some pretty well flowing heads. In the Pontiac world, high flowing heads usually comes at the expense of velocity thru the ports. Which could be a death sentence to any low end torque on the Pontiac based 4 cylinder. I wanted a blower to enhance the heads, even at low RPMs, like just off idle. I know the M90, like any blower, has a limit on how much they can efficiently flow. I want to see how much I can get out of it for the 4 cylinder. Because I'm using just the blower, mounted on a plate (in place of a manifold), and using no throttle body on the inlet, I can get all of the flow possible from the inlet area. I can open the 70MM stock inlet as far as I can go, and I can use the bypass inlet to get even more air in. Plus a couple of other openings into the inlet area that will not need any of the original sensors etc. So they are all potential air inlets. All that will help the efficiency of the blower by lessening restrictions at the inlet. Does any one know if porting is done to the opening right above the rotors? Or if there is a limit on how much the rotors will use even if more air is allowed to come it??? Meaning is there a point of diminishing returns? I've looked into porting of the bottom discharge and see where they open that up. I've seen warnings about opening the narrow end of the V, that it gets too far into the rotors and loses efficiency? Is that true? I'm going to cut out the big bolt bosses at the big end of the V too. If the bosses at the end of the V is totally removed, is there any evidence the material left in the case is not enough for the bolt torque? If it presents a weakness, I'd drill and tap for 3/8ths bolts and use heli-coils in the shortened area. That would add a lot of strength with the shorter bolt boss that is left.
I did have an additional thought. Since the air is discharged at the big end of the V, I see where it must be impinging on the casting that crosses the open end of the big V, right where the bolt bosses are that I need to remove. If that cross piece is cut way down, or even removed between the bolt bosses, using the bearing plate as the front of the opening, I wonder if that would add some additional flow coming off of the rotors??? Just an observation. Since I'm mounting the blower on a plate, I can make the opening anywhere.
Anyway. I hope I was clear enough on everything. Any help/advice you guys may have will be welcomed. Other than I've read or observed on U-tube, modifying these blowers is new to me. Thanks for any help guys. It is much appreciated.

Mark L

I got one down in the basement I'm working on also. I got the inlet done and the bottom part mostly done. I got some pictures posted here... but I don't have then all posted yet. Just the inlet...

https://www.gmforum.com/general-gm-c...-303944/page2/

Only work that can be done to an M90 is port/polish the inlet and the outlet. And if you know someone that can weld aluminum, weld shut the 2 smaller outlet ports. This doesn't increase power or anything, but does increase the sound of the whine.

Thank you for the link Mike. I have this vid for a gen 5 M90 but I don't know if the big differences in the inlet shape is his porting or if it is just a polished stock shape??? I've never seen what the differences are between the Gen 3 and the Gen 5. But it is a good video.


I'm happy to get any info and experience from you guys I can.

Mark L

No, keep it stock shape. Nothing is gained by increasing the size of it because your still limited to the diameter of the TB inlet. Only thing we need to do is remove the rough sand casting and make it smooth to increase velocity and reduce friction temp. That'* all.

I'm interested in how your going to mount this thing to a 4 cylinder. So, if you wouldn't mind, start another thread about the */C project in the General GM chat please.

Mike, I can increase the air capability at the inlet by using the bypass hole to add additional air. So it may become an issue of actual rotor/rotor chamber flow capabilities. Or the actual flow at the blower exit opening. It'* nice that I am not limited by the stock installation. I don't need any of the bypass or egr etc item openings to functions so they are all additional air leaks (air sources, sounds better LOL) into the case. As long as they are filtered. Plus, even if I have more flow capability at the inlet, it makes the blower more efficient just not having any possible vacuum incurred trying ingest the air.
Back on the Catalina. I received the 4 inch polished stainless tubing to make my turbo out exhaust system.





Should look good and flow great. The turbos have a 3 inch exit but turbos like to expand out because of the turbulence at the exit. So mine will be 3 inch out to the 4 inch stainless. Then 4 inch all the way out.

Mark L

Just had to take a shot of my garage. How many guys do you know have a built 4 cylinder, an unbuilt 4 cylinder, a built 400 bare block, and a built '60 389 twin turbo motor??? In a '63 Catalina Beam Axle gasser style build? Huh?? Huh??? Not many I bet. LOL.

The build 4 cylinder is from a '62 LeMans restoration project that Keith picked up. I'm storing it for him out of the elements.





I just had to take a pic of the garage just oozing Pontiac-ethnicety.

on the blown 4cyl what are you using for the fuel control, carbs?

It'* funny. That 4 cylinder looks just like a V8 minus one whole bank. Ironic that GM built a 4.3 V6 based exactly off of the Chevy 350 V8 minus 2 cylinders.