Spud's "The Definition of Ghetto-Rigged" M90 Supercharger Build Thread (lotsa pics)

[Spudchucker]

Originally Posted by williaty

You're right that the IC is fairly free-flowing, but so is everything else, really.

If you set up a series of pressure gauges, you'd see that almost the entirety (I'm acknowledging that some heat is shed in the piping here) of the pressure drop is over the intercooler itself. This is because the IC sheds heat from the air. Since the volume in the intercooler is fixed (since it's made of metal), the pressure of the air MUST drop as a consequence of the drop in temperature.

Now, the engine takes in a fixed volume of air (Volumetric Efficiency). However, if you increase the pressure, or drop the temperature, the engine will take in an increased mass of air for the same volume. The question that's relevant to your injectors is wither the drop in temperature increases the mass of the air faster or slower than the drop in pressure decreases the mass of the air.

I know the air mass stays the same regardless of temperature. What I am talking about is the fact that the piping on the way to the engine will be long enough that the air will compress easily over the space, thereby bringing the pressure down. It's like turning on a leaf blower in a full size room. Sure, everntually it will be able to bring the place to a decent pressure, but it will never get very high with the room constantly bleeding out the air every 5 seconds (which is longer than a good pull)

[Sarra]

Bad logic detected... I'd suggest modifying the pulley sizes, to reduce the airflow out of the supercharger. And, this is the exact reason I'm going with an M62 on my WRX build instead of an M90.

[Spudchucker]

No I want 8 psi, my car has forged rods and very solid heads- pistons are really the only big weak point. I just gotta wait a week or two til I have a bigger fuel pump, so I figure for now I can just constrict it a bit and deal with it lol

[Skidd]

^ Just remember Spud, your using a Fixed Displacement SC. Unlike a turbo, or a Centrifugal SC, you can't really just "limit" the pressure. A Fixed Displacemnet pump will just keep building more and more pressure until .... something... Either a hose pops, or the belt slips, or even jumps.

If you really want a "short term" hack to limit boost, you need something that can bleed the excess air. Do you have a bypass valve? You could always just rig it to not close all the way shut. That would give the air somewhere to go.

[Spudchucker]

Originally Posted by Skidd

^ Just remember Spud, your using a Fixed Displacement SC. Unlike a turbo, or a Centrifugal SC, you can't really just "limit" the pressure. A Fixed Displacemnet pump will just keep building more and more pressure until .... something... Either a hose pops, or the belt slips, or even jumps.

If you really want a "short term" hack to limit boost, you need something that can bleed the excess air. Do you have a bypass valve? You could always just rig it to not close all the way shut. That would give the air somewhere to go.

Yeah that's a good point. I was originally gonna make a popoff valve (wouldn't be too hard) but I decided it was too much effort for a short-term thing. So I ended up taking Williaty's off hand advice about limiting how far the TB can open, and it's running a LOT smoother (til it gets warm.. no intercooler til the weekend, bleh..) with the plate being limited to opening no farther than 65%. Once I fine-tune it a bit more, I will have a fully functional "the other white EWG"


And onto the weird part- My car has officially gone schizo lol. Since I'm working without a wideband for a couple more weeks (hurry up and pay me, work!!!) I have to tune it with a lot of the old fashioned methods, and stock sensors. I't better everytime I reflash it, and I'm really starting to get the hang of ghetto-tuning it, but man this thing put out some weird numbers today. It's not knocking bad or anything (only knocks if I blip the throttle, so I just need a bit more tip-in, and a bit lower of an AFR to go with it) But yeah, check out some of these numbers I got..

(keep in mind, all the vitals are perfect.. no knocking, good temperatures, etc)

-minimum base timing clocked at -30 (impossible)
-maximum base timing was a whopping 127.8 (wayyyy impossible, I'm looking through my huge log for where it saw that one)
-idle AFR was 15.5 or so (leaner than the 14 I requested, will compensate next tune by making it rich)
-on the bright side, by limiting the TB opening angle, I was able to keep the G/Rev down to 1.30


I really need to get the intercooler in, it starts bogging down pretty bad after running for 10 minutes while not hurting any vitals, I can only assume it's because the intake temp is getting a bit high. Looks like I got a busy weekend ahead of me!

[mattej]

Are you using an EGT gauge?

[Spudchucker]

Originally Posted by mattej

Are you using an EGT gauge?

That would be one of those things I don't have til my fat backpay check comes through (pleeeeease be wednesday )

I know it's running a bit lean (doing a lot of fixing, it's getting better in all areas everyday), so the temp is a bit higher definitely, but it's not bad enough that I'm alarmed or anything.. After 8 minutes of running the car in stop and go driving, I can put my hand on the supercharger or the intake manifold and it's maybe as warm as a hot tub, and I don't think I could touch the manifold after that drive before the install- Seems like that big ol' hole in the hood is good for ventilation

I'm gonna be looking over my logs really close tonight and see if I can't get this tip-in thing right (IE: able to idle without having to "learn" first, and not so rich my exhaust makes you high) It'll be a fun day tomorrow

[reddevil]

Dude, put some bigger injectors in ASAP, that motor is going to go south soon.

[Illnastyimpreza]

wow.... not thats a DIY. Whata you think the full potential of that thing is ? I would like to see you run a Crank pulley the size of Reddevil

[Spudchucker]

Originally Posted by reddevil

Dude, put some bigger injectors in ASAP, that motor is going to go south soon.

I am completely ready and willing to grind the ends off my 280s like you did, but no one ever responds when I ask if my fuel pump can handle the larger size. You've done it, did the pump work? Did you have to pinch the regulating line? Just say the word and any tips you know and I will have my injectors ground, tuned and running within the hour. I am going crazy not being able to get a bigger fuel pump, but I'm dead broke right now and it's not an option. I'll be loaded once my backpay arrives (hopefully Wednesay) so I'll be doing a lot of catch up then.

Also, keep in mind I've tuned the TB to only open halfway as a temporary measure (it's pre-compressor, so that's safe), and I never even hit half throttle. It's just really annoying not being able to do anything with my potentially fast car right now





IllNasty- What's my potential right now, or what's my potential when the other side of my hood has an M90 sticking out as well? When my injectors are up to spec, I'll be looking for mid to high 200s for torque across the band at the crank, not sure what it would be at the wheels.

Once I beef up the internals and slap another M90 on top, it'll be an easy 25 psi through two throttle bodies with rediculously low RPM boost. That's later though, I gotta make sure she's running to spec now before I think that far down the road. Oh, and the beauty of the M90 is that I don't need huge pulleys. In fact, I had to get an underdriven crank pulley to start out with just to make sure it was safe The level it's putting out with the stock pulleys is perfect, I just have to get more gas shooting in the engine to make it run to spec