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GM-8 GM-4 Hybrid pushing 28psi

vegtech

vegtech

WVO Enthusiast
Messages
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11
Location
Tilton, NH
Soooo when I rebuilt the Yukon with the New Teds motor I figuered i would upgrad my GM-4 to an 8 that i had. I bought a rebuild kit off Ebay. I had a problem with one of the bearings fighting me but i got it. After the first ride I was getting a strange noise from the turbo. took off the intake boot to find a butt load of shaft play. Apparently i F'ed something up. As i did not have another kit on hand, and I had thermal coated the exhaust side of the 8, I took the center of the 8 off and put the 4 on. I tried to put the snail shell from the 8 on the 4 center but it would not fit so I put on the 4. I put a few turns on the TM and went for a ride. I was amazed at the spool up and max boost. I was hiting 22psi no problem. I backed it off to 18 for a few thousans miles, but then i got curious and cranked the TM down. It builds 28psi and pulls hard!
 
No one ever said spooling was a problem with those turbos It's the back pressure they produce at that level of boost that causes problems.
 
That is very true, the motor WILL NOT BE HAPPY. Just cause it will make that kind of boost does not mean its a good idea, your drive pressures most be soaring big time. The gm turbos are out of their efficiency range at less than half that amount of boost. These stock gm turbos are junk and you are just asking to blow the bottom end of your motor. What were your egts? Im assuming your running an egt gauge because of the tuned ecm you got.
 
Looks to me he used in the end the complete GM 4 bolted to the exhaust elbow of the GM8.
I would turn that TM back a bit till max 20 lbs.
I been running mine at that boost level since rebuild 40 k ago,runs and pulls like a raped ape,EGT's never been in the red yet.
3.5" straight pipe.
 
The redline of the GMx turbo's is generally around 14PSI. More than that is generating extreme intake heat in excess of 300 degrees. Anything near the intake will melt and you loose efficiency from high intake air temp and high drive pressure. You are overspeeding the turbo RPM and run the risk of mechanical damage from high stress. The turbo going boom will take the engine with it and maybe the hood.

A larger turbo will give you better results after it spools up...
 
Not to hijack this thread. (he should get the point by now)

Is there any info on these GMx turbos as in flow rate, cfm, lbs per min?
Been looking can't find any good info.
 
Yes it the 4 center and front bolted to the 8 elbow. Normal egt on The highway at 75 is about 400 if I stomp on it for a long time it will 800. And before someone ask, the prob is a half inch under the turbo in the manifold.
 
in the truck in my sig, I made a turbo master when the air pump let go, the way its set up, isn't the best for fuel efficency but it works, at cruising speed it runs about 4-5psi, it maxes out at about 10-11psi when you stomp on it, but when I pull a trailer up a long grade it will gradually climb until it defules at about 15psi, works ok for now
 
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