A question for you all. My 1994 6.5 has a GM-4 turbo on it that is in good shape. I now also have the opportunity to buy a new GM-8.

Is the GM-8 so much better than the GM-4 to warrant changing it out? What would I have to modify to change it out.

If your 4 is working, I wouldn't change it. The advantages if any wouldn't justify the time spent swapping them.

The -8 is much more efficient - if you can get it cheap, it's direct bolt-on, will reduce EBP over the -4.

Unfortunately, we don''t get much choice, maybe a Holset offa dodge Cummins - takes some work, but several say it's worth the trouble for hiway work.

Sell your -4 to a '93 mech FI (samo, samo, no sweat, GI) guy for half what you get the -8 for, everybody's happy!

First off, the GM-8 is barely if at all more 'efficient' than the GM-4. Same compressor, wheel, shaft, cover, bearing housing, only a slightly different exhaust housing and different actuator location.

Second, a GM-4 won't work on a mechanical truck without modding an actuator to make it work, the GM-1 on 92-93 has a longer actuator arm because the WG is closed at about a 45 deg angle, so if you put a GM-1 actuator on a GM-4 the wastegate will stay WIDE open.

I like my 8, now if i could just get some more burnable 02.... lol, you should here these things sing @ 20 PSI! Now what i'd like to know is will a bigger turbo generate less heat and give you more 02 for combustion?

yes. I think grape had a pretty good explanation of what it takes to get the right one.

A question for you all. My 1994 6.5 has a GM-4 turbo on it that is in good shape. I now also have the opportunity to buy a new GM-8.

Is the GM-8 so much better than the GM-4 to warrant changing it out? What would I have to modify to change it out.
In and of itself, a GM-8 holds little realistic advantage over a GM-4.
However, since your GM-4 is now eleven years old and no doubt a little tired, a 'new' GM-8 will definitely make an appreciable difference.

Its a direct bolt on replacement, no mods needed whatsoever.

Very cool. Thanks all! My GM-4 is really only 4 years old according to the previous/original owner. But since the opportunity has arisen, I will take advantage of it. Now to just get the damn turbo bolts off :eek:

Shouldn't be that bad, might be a good idea to pick up new gaskets for the exhaust manifold and drain pipe when you do it.
It also helps if you have a second person to hold the downpipe up while you reattach it.

Very cool. Thanks all! My GM-4 is really only 4 years old according to the previous/original owner. But since the opportunity has arisen, I will take advantage of it. Now to just get the damn turbo bolts off :eek:

Saturate with Aero-Kroil or PB Blaster, if you can't find that oil of wintergreen from the drugstore works also it creeps; then from bottom side thru thru the wheel well (rubber skirt & tire off).

Access nuts from bottom side with a "wiggly" socket, use a "wiggly" universal socket, (a std socket on a universal joint is too floppy and you loose some of your torquing ability) and as many extensions as you need to reach, then take zip strips to the torsion bar and around the extensions for the 3rd hand to hold all of that up to the nuts, while re-positioning yourself.

Then take breaker bar apply force then rap on breaker bar with hammer to shock nuts loose while applying torque with the breaker bar, at least that is worked on mine.

Second off - The GM-8 is a definite improvement over the GM-4 on both ends.
Decreased EBP post-turbine, pre-turbine, and increased compressor efficiency are resulted.

The wastegate assy in the GM-4 turbine 'snail' was relocated to the exhaust elbow in the -8.
The reduced 'snail' length allowed opening up the exhaust adaptor elbow by removing the 180deg fold-back
Also opened up the wastegate exit path, allowed freer wastegated exhaust flow.

Result : lower EBP in the elbow, post-turbine, increased flow thru wastegate, and reduced EBP in the turbine.
Not insignificant, when combined with a freer flowing exhaust system and upgraded PCM.

A radial, or centrifugal, compressor housing usually consists of two parts - the scroll housing, and a plate to seal the open side, which is also attached to the center section containing bearings, wheels, and drive shaft..
The plate acts as part of a diffuser, which creates pressure by reducing the gas velocity after it slings out of the high-speed impeller wheel..

The GM-4 is of the parallel-wall diffuser type, meaning the scroll is cast in two pieces, with an integral vertical separator, which will be parallel to the main sealing plate, but spaced apart from it at a dimension depending on designed flow rate and pressure ratio
The -4 compressor has a multi-stepped inducer throat area, and is limited to a more specific flowrate range by the dimensions of and between the parallel walls.

The -8 is of the scroll-diffuser type, where the shape of the scroll acts as the diffuser, and which, with the inducer throat steps removed and smoothed, is more efficient over a broader range of engine flowrates.

The inducer throat area is where linear flow into the compressor begins to whirl, in anticipation of becoming radial flow into and out of the spinning wheel.
Any turbulence here, as caused by the stepped -4 surfaces, can mean easier 'stall', where the compressor wheel, spinning faster than inlet air flow rate, begins to cavitate.

Turbine rpm surges as load decreases, and - well....exploding turbochargers are not generally considered spectator sport.

A compressor wheel and a turbine wheel may be used in various sized housings for various flow rates and pressure ratios, where even the shape of the housing will raise or lower the expected output.

Or, various size housings may be used with same wheels to alter turbocharger output, as required, desired, and inspired..

So - it may be seen that the GM-8 is of improved efficiency over the -4, even with the same size and shape wheels.

Remember - science, without reason, makes no sense.:cool:

Maybe on a modified engine there could be an advantage between the -4 and -8, but from the factory @7-8psi the 1994-2000 have the same power reguardless of the turbo. Having said that if I got a -8 for free (anyone want to give one up) I'd try it just for shiz and giggles. They definately sound different when connected to a 4" exhaust my turbo is not nearly as loud (unfortunately) as CR's.

Yes, but the upgrade - cooling, PCM, Inj Pump, turbo - was planned to meet the proposed 215hp power upgrade.

However, GM's intent, seemingly, was always to restrict power, with 7-8psi Boost, no charge-air cooler, and etc.

imo

I was thinking about upgrading to a GM-8 turbo since my GM-4 is about 11yr old not doing it anytime soon still working on getting the gauges. And was wounding if you could just bolt the GM-8 on in place of the GM-4. So I guess I just want to make sure that I read this thread correctly in saying the only thing you need is a GM-8 turbo to do the so called upgrade, and nothing else. Is that correct?:confused:

From what I can tell, you will need the GM-8 wasegate actuator or a turbo master.

Yes GM-8 with GM-8 actuator; the PCM is dumb to what turbo is on the end of the WG solenoid ,or buy or fab a TM, use caution how far you tweak TM without WMI or IC.