What do the GM-X (GM-3, GM-5 etc) Turbo designations mean?

[Sand]

My 95 TD has GM-5 engraved on the turbo. I hear about GM-3 and GM-8's but cany find info on what it designates. Are they sizes, flow rates, revisions, housing sizes, production years, just for fun?
Thanks
Sand

 

[NVW]

Welcome.

GM-3 was 1993, GM4 I think was 1994-96, Gm8 was 97 on. Gm5 was for vans AFAIK.

There were improvements made with the revisions.

 

[Will L.]

Welcome to the site.
Afaik each number went in order to new model changes.
Gm8 was last one made.

Gm5 were used on vans and trucks, but rare- idk why.
Gm6 vans, truck and Hummers.
Same turbo even though t3 flange and centermount versions.

What kind of rig do you have? Just learning, or do you have repair/ upgrade plans in the works?

 

[NVW]

Forgot about the GM-6

 

[Sand]

Hi Will. Im here from the Hummer Forum at your recommendation. Its a 95 K2500 4x4 with the 6.5 TD. Just checking everything out and researching what I have to see where I'm headed (H1 is on the horizon) The truck is at 430 000K and wont have much resale so I may just keep it and slowly cannibalize it for the Hummer as needed. Thanks for the info.

 

[Will L.]

Oh yeah, and the hummer with a 6.2 and th350 transmission?

Wanting to add a turbo and go to 4 speed transmission?

 

[Sand]

4speed yes. Turbo if the 6.2 is strong - why not. Free power. Or buy whats left over of your 4 speed when you go allison....

 

[Will L.]

Years away, if it happens.

 

[6.5L]

Has anybody every actually figured out what the difference between the turbos was? Was the inducers/exducers changed? Wheels changed? Housings changed? Why GM-1 vs GM-3? Both identical? Why switch wastegates? Any actual factual history that anybody knows?

 

[JayTheCPA]

I do recall that the turbine's flange is different from generation to generation. IIRC the lower the number, the more restrictive (in comparison) the flange where GM-3 had the most restrictive flange and GM-8 had the least.

And, No, am not stating that the GM-8 is the solution as it too is known for high restriction in comparison to aftermarket turbo's.

 

[Will L.]

@Sand
I'm a jerk for forgetting this-sorry. If you are thinking about upgrading your turbo: Dennis is he guy that came up with the much loved ATT turbo many use. I spoke to him a bit before about adapting the ATT to use in Hummers. He was happy to work through what it would take.

My 95 was n/a, and sits about 1" lower to the frame than turbo Hummers. Unwilling to raise my body at the time, I walked away. I have regretted it every since.
Once I finish my body upgrades and redo my interior, my next improvment will be the ATT, even if Dennis is busy at the time. My timeline is looking like 2 years from now, unless my turbo lets go sooner.

I've driven several pickups that have the ATT and I and Greenmeanie (here @greenmeanh1 ) are convinced it would be the perfect turbo for the hummer.
If your interested, you guys should figure this out.

In heavy loaded trucks it runs great with no inner cooler, and improved power for all, mpg for (most?) everyone.

Leroydiesel.com is a vendor for them, and tunes for them if you have a ds4 IP.

 

[Paveltolz]

Has anybody every actually figured out what the difference between the turbos was? Was the inducers/exducers changed? Wheels changed? Housings changed? Why GM-1 vs GM-3? Both identical? Why switch wastegates? Any actual factual history that anybody knows?

FWIW, GM-5s aren't all that rare and the differences in the models has been discussed as mainly a casting change in the geometry of the exhaust elbow making the GM-8 a flow little better. Air Intake, Exhaust inlet and outlets, compressor wheel measurements etc. are the same between those two at least. Waste gate actuators are mounted vertically on the 5 and 8 too. I posted the measurements for the GM-5 and 8 here in post 409 if anyone is interested in my attempts at 'helping' the knowledge pool.

As for the ATT, mine performed quite well...on a Orionthade's truck which is a 3500 5 speed manual. He, AKDiesel and other members drove my 1500 with it and we all felt a little underwhelmed. The thoughts Orion and I had was that the turbo likes being run with resistance, heavier loads/trucks. I noticed it pulled better loaded with a couple of pallets of sod. Orion said he couldn't believe it was the same turbo on his truck the difference was so amazing.

 

[WarWagon]

As for the ATT, mine performed quite well...on a Orionthade's truck which is a 3500 5 speed manual. He, AKDiesel and other members drove my 1500 with it and we all felt a little underwhelmed. The thoughts Orion and I had was that the turbo likes being run with resistance, heavier loads/trucks. I noticed it pulled better loaded with a couple of pallets of sod. Orion said he couldn't believe it was the same turbo on his truck the difference was so amazing.

Some Dyno's are known to have trouble loading a turbo. I can hit 18 PSI with the ATT on a grade here wound up yet on dyno days we couldn't get it over 14 PSI. Both the 1993 or the 1995. The Dyno accelerates too fast and holding the engine at a specific RPM was not well controlled on that dyno. As I recall some runs had more power in OD than 3rd... Without the grade or trailer my peak boost is around 15 PSI. (The WG controlled HX40II would hit 24 PSI, generating the same HP and TQ as the ATT, on or off the very same dyno.)

It comes down to the GMx is so small it will boost off airflow alone. Turbo's are supposed to boost off of heat and this is what it takes to wind them out. Light load may not hold the engine at the same RPM to allow the turbo to completely wind up from exhaust heat. Altitude and other factors introduce boost creep where you set the Turbomaster on a GMx at 12 PSI and hit 14 PSI going past the GMx choke point at altitude on a long grade. It's really things heating up and the extra 2 PSI comes from heating the air rather than moving more air "mass".

 

[FellowTraveler]

Some Dyno's are known to have trouble loading a turbo. I can hit 18 PSI with the ATT on a grade here wound up yet on dyno days we couldn't get it over 14 PSI. Both the 1993 or the 1995. The Dyno accelerates too fast and holding the engine at a specific RPM was not well controlled on that dyno. As I recall some runs had more power in OD than 3rd... Without the grade or trailer my peak boost is around 15 PSI. (The WG controlled HX40II would hit 24 PSI, generating the same HP and TQ as the ATT, on or off the very same dyno.)

It comes down to the GMx is so small it will boost off airflow alone. Turbo's are supposed to boost off of heat and this is what it takes to wind them out. Light load may not hold the engine at the same RPM to allow the turbo to completely wind up from exhaust heat. Altitude and other factors introduce boost creep where you set the Turbomaster on a GMx at 12 PSI and hit 14 PSI going past the GMx choke point at altitude on a long grade. It's really things heating up and the extra 2 PSI comes from heating the air rather than moving more air "mass".

The turbine likes being wrapped, covered secondly but loves both together, it becomes critical to let the turbine cool down "EGT under 300 deg. f before shutdown for longevity."

 

[Will L.]

After reading Paveltolz remark on the GM5, I reread how I wrote that, let me restate: The GM 5 is not rare on pickups. It was used on vans and Hummers rarely-most vans and Hummers use the gm6. The wastegate difference with the centermount body had issues and was short lived. Most 95 Hummers are N/A, 95 was first year of turbo option for Hummers, Afaik only a few 1995 Hummers and vans got the GM5, but most '95 Hummers got gm6.

 

[GM Guy]

What I have allways heard is that GM1-8 has the same turbine and exhaust wheels, and it was all just various changes in the housings. Gm5-8 had a bigger less restrictive exhaust housing, so many feel that the Gm5-GM8 is alot better than GM1-Gm4.