bigger compressor wheel in a gm-8 turbo

[Jim P]

Has anybody installed a bigger compressor wheel in a gm-8 turbo?

I bought a cheap gm-8 turbo on ebay. The bearing had gone bad and the compressor wheel rubbed on the housing pretty bad. The exhaust wheel never contacted the housing. I was planning to just put this exhaust housing on my gm-4 turbo but then I got to thinking about boring out this compressor housing a little and installing a bigger compressor wheel.

Anybody have any thoughts on this?

[Dvldog 8793]

Sounds like a JP-9 turbo!
Neat idea keep posting....
L8r
Conley

[rjschoolcraft]

The contour of the compressor housing is critical to maintain efficiency and proper performance. It must be matched to the impeller contour.

[gmctd]

Only the rotating assembly - turbine wheel\shaft, bearings and compressor wheel - will interchange GM-4 to GM-8.

Compressor will easily make 20psi Boost, as is.
Turbine side is the problem area - becomes increasingly restrictive as Boost and exhaust volume and energy increases.

Some turbo shops trim the turbine blades and outlet diameter as an effort to allow more flow.

Check the -8 compressor inlet area, where the fan touched - if it's not scored, it's re-workable with the -4 rotating assembly.

To achieve original balance (rotates at 100,000rpm), mark one compressor blade and corresponding turbine blade, re-assemble to same index.

[Jim P]

GMCTD,

So the center section of gm-8 is different than the gm-4? I thought the only diffence was the exhaust housing but what do I know.

I e-mailed a turbo parts supplier and they said it is possible to install a bigger compressor wheel and that it would increase performance but I am just taking his word for it.

Could it be possible that with a bigger compressor wheel, the turbo would not have to spin as fast to make the same amount of boost? If it don't have to spin as fast, maybe I could back off the turbo master a little and lower the exhaust pressure a little.

[gmctd]

If you'll take the -8 out to the truck and hold it near the -4, the differences will be readily apparent.
With a little closer scrutiny, the subtle differences can also be seen.

That is an exhaust gas turbine driven compressor -compressing air takes power.
Bigger wheel on the air pump will require more power input on the turbine end.

The turbine end is the limiting factor for the amount of power that can be made with hot chips, injectors, gears, charge-air coolers, etc.

[Jim P]

I compared the two turbos side by side and the only difference that I can see is the location of the vacuum canister. The gm-8 points up and my gm-4 points forward.

I am not saying that there is not a difference, just that I don't see any.

The compressor wheel on the gm-8 wore into the housing about 1/32" all the way around.

[gmctd]

Ahh - the heat shields are hiding the differences.

The compressor scroll is two-piece, so that inlet area is replaceable, but check the local boneyards - you should be able to find another complete rebuildable turbo for around 75 bucks.

GM-5 and -8 are identical, but for the wastegate shaft and bushing in the exhaust elbow.