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Xnke

Positively Displaced

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Xnke    21

A turbocharger or centrifugal supercharger is NOT going to blow the throttle open, as they are dynamic displacement compressors and vary pressure and flow based on demand. A roots or screw type postive displacement blower WILL have the ability to do so...either by having a pneumatic "balance" point in the system, or by just bending your throttle plate around the shaft. I have seen and suffered both, and the latter is a well know problem.

 

On a 6500rpm throttle closure, the supercharger displaces 62ci per rev and is turning 14,995rpm, moving roughly 460cfm of air. It doesn't stop on a dime, your engine takes a real number of cycles to decel, and with the throttle closed that 460cfm WILL find a place to go.

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Xnke    21

Going to crib from the miata boys again...the car drives very nicely from 2krpm and up, but has the idle sag and stalls at lights if the idle is set lower. This is apparently caused by the very large throttled volume. Not sure hoe the Supercoupe got away with it, but one positive solution appears to be going back to the dual synchronised throttles, or back to the (imo dangerous) post supercharger throttle. I'll be setting up my old synchro setup again in the next few days and once that's done, tuning will take the priority.

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Xnke    21

Things are good. New cam makes more power overall than the wide-lobe-seperation cam, but it breaths too good for the supercharger...Making 140kpA at 3800RPM but it falls off slowly to about 135KPA at 7300RPM. I'm moving as much air as the little M62 can move...and then a tiny bit more.

Also noticed that doing back-to-back-to-back-to-back, i.e., just drive back to the starting line and stage up again, 1/4 mile drags, the IAT's and coolant temps get lower each pass...started at 122* IAT and 181* coolant temps...8 full-throttle passes later IAT was 109* and coolant temp was 166*. No time slips, we were running untimed tonight. Was crossing the markers at 5200RPM in 4th gear, so that's just about right.

Found out what happens if you overfill the gearcase on the super though...the slightly higher hot pressure will force out the excess all over the engine bay.

 

Don't worry about blocking the silencer holes...at WOT it'll still drown out the exhaust.

Edited by Xnke

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Xnke    21

Started with a standard GM Series II 3800 M90 supercharger, gutted the case and started chopping. Got it cut down, then went to the stock shelf and started fitting stuff up.

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All cut down on the back end, anyway. Getting things cleaned up and this made it VERY easy to do the intake side porting.

Now you can see how we're going to work this. Raising the inlet up and opening the bore from 65mm to 83mm.

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Getting things sealed back up on the inlet side, all the intake porting is done, just need to run over things with a cartridge roll or two and clean up the burrs and divots from the carbides.

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Much more direct path into the rotors, will be noisier but that's easily dealt with for 99c at the shooting store.

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A few good shots of the porting, both inlet and outlet ports. It doesn't take much to push an M90 up to 600CFM inlet flow, but it does take some work to get one higher than about 650CFM...Stock this housing pushes 520-540CFM.

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Not much to it, a little cleaning in the case and all those "score marks" wipe right out. they're not even deep enough to catch a fingernail, but the carbon and oil deposits make them look like someone went at the case with 36 grit paper!

Might try to find another rotor pack, as this one has some miles on it. They're not terribly hard to find and this one will work for a while yet. It's not leaking nose drive oil, so the front bearings are still tight enough to keep the seals from going to shit. I've rebuilt the rotor pack in my M62...it's a shit job unless you've got the special jigs to do it. I probably would be money ahead to pay the 500$ to have a rotor pack re-coated and rebuilt. However, good condition M90 rotor packs are all over and are far less money than a newly rebuilt one.

I still need to cut down the nose drive to the proper length, then it's two ball bearings, one snout seal, and two case bearings to replace. A new baseplate will have to be cut and I will be refining the method used to make the charge pipe, hopefully making it flow better than the current charge pipe and also lower profile...that way I can get it up off the lower radiator hose and alternator.

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Xnke    21

Progress on the M90 front.

Had to do a little sculpture to get the case in place.

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The old PCV tower had to come off, and be blended in.

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Now the snout was too long, so I mounted it up and turned it down to the proper length, and cut a new bearing recess.

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Nearly finished...need to replace that seal. It's got a big cut in it right across the lip; that isn't going to fly.

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Yes, that is the shaft from the M62...I am having a custom hub cut for an interchangeable pulley system, since the M62 keyed shaft is pretty much unsupported by anyone.

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Xnke    21

Wow, haven't updated this thread in a long while.

 

Haven't driven the car much, it's developed some suicidal tendancies, problems like electric fan relays rusting up and not turning the fans on and the like.

 

That and completely bald rear tires that need to be replaced...at 9lbs of boost it was plenty of fun.

 

Now it's time to work on the NVH problems...mostly the incredibly intolerable whine of the supercharger, and fitting a complete interior. Right now, I'd never want to drive this more than an hour at a time...I'd be deaf by the time I got anywhere.

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