GM turbos,,

[94GMC2500]

Hello, is a GM 8 turbo any better than a GM 3?

[Robyn]

YES.

The 8 is a better unit, but there are aftermarket ones that are much better.

[94GMC2500]

I have heard it spools quicker and moves more air with less heat. Is that true?

[Robyn]

Pretty much.

The issue with the GM X turbos is the nasty convoluted flow through the outlet, wich still holds in heat and unwanted back pressure.

A clean radius outlet is the ticket to solving a lot of the issues with the 6.5

Being a precup engine, the 6.5 has most of the combustion event in the heads as opposed to the Direct injected engines like the DMAX, P stroke and the Cummins.

This causes lot of residual heat to be left in the heads, which does not help a thing when it comes to making big power.

Reducing back pressure and retained heat helps a bunch.

When we consider that GM designed the 6.2 (Later the 6.5 ...same basic engine) as a fuel miser to get their CAFE numbers up back in the 80's the idea of big power was not on the list of THINGS TO DO.

When the Dodge Cummins came along and then the Ford P stroke the game changed...GM NEEDED TO GET SOME POWER so they could remain competitive in the market...hence the turbo starting in 1992.

Trying to get a clean install GM took the easy route and ended up less than optimal on the HP part, but the 6.5 was not intended, nor was it a good candidate for big power.

300 hp under the best conditions is about all you can expect AND KEEP THEM ALIVE for long.

The job of rejecting heat in a truck chassis (cooling system) is not going to allow high power. The radiator and airflow is just too limited.

The 6.5 turbo in a boat with unlimited cool water to take care of the heat changes the game entirely.


ADDING A REALLY GREAT TURBO is still an issue as the engine design will not handle high boost pressure.

At about 10-12 psi the need for aftercooling comes up, and then when you head up in boost from there the head gaskets are just not going to deal with the pressures.

The 6.5 does not have enough head bolts to hold the heads on tight and at this point we have basically reached the reasonable limit of the engine design.

With more $$$$$$$$$$$$$$$$$$$$ things can go farther, but still the point of diminishing returns.


Bottom line.

Get good airflow and maintain EGT AT OR BELOW 1000 DEGREES F (at the ports) at full throttle on a hard pull and you are going to get the most reasonable service possible out of the 6.5

A FEW GUYS HERE have used some aftermarket turbos and had great results.

The GM X will be fine for average running, but its not the best choice.

[94GMC2500]

My truck has a 3 on it, I have a 8 laying here, I was wondering if its worth the time and effort to put the 8 on it.


I was reading that the gm8 has a better flowing elbow than the gm3 did.