What size gasser would the GM-4 work best for? I don't know the calculations. I have been toying around with the idea of swapping my GM-4 for the GM-8 on my blazer but there is nothing at all wrong with the 4. Thinking of putting it on a 2.5L Subaru flat 4 banger but I don't want too much lag. Kind of off topic but a lot of you guys know some chit about turbos especially the GM-x series!:cool_shad

Well let's see the gm-4 supplies enough air for a 300 ci engine at 3500 rpm that would mean a 300 ci gasser at 7000 rpm would require two of them. I have a 318 ci sb overbored .040" in a power ram 4x4. I've thought about putting the two I have on when I'm done upgrading to bigger turbos.

I don't know but can a GM-4 be spun safely without self destruct at exhaust flow in excess what a 6.5 flows at 3500rpm.

I'm thinking there's one turbo per bank not seriesed. that would keept volumemetric flow relative to a 6.5L.

Well let's see the gm-4 supplies enough air for a 300 ci engine at 3500 rpm that would mean a 300 ci gasser at 7000 rpm would require two of them. I have a 318 ci sb overbored .040" in a power ram 4x4. I've thought about putting the two I have on when I'm done upgrading to bigger turbos.um a 6.5L has 398ci so roughly thats 400 ci not 300 ci.... so by that math it should be good for up to about a 4.3 considering those dont go above 6krpm.

BUT to really know if it would work would require the use of a turbo map and comparing your engine flow rates (lbs of air/min) at a given psig/psia at a given rpm at a given intake temperature in Kelvin and then seeing where those points fell on that turbo map.... Turbo surge is a very very bad thing..... and turbo inefficiency defeats the pourpose...

:offtopic: You're assuming the GM-4 turbo fit's the 6.5L. It doesn't, that's why people upgrade to larger volume turbos. I would assume it'd work on a 350 chevy in a twin configuration as well but 400 ci would be alright as long as you're not running 7000 rpm constantly.

So what would be the flow of a dodge 318 overbored .040 at 9:1 cr at 6000 rpm? I can add up the number's just not sure how many veriables I'm not factoring in.

I bet the GM 4 would flow great for the 6.5LTD if we did a 10* or 15* clip of the exhaust turbine....;) Supra guys have been doing that to their stock ct-26 turbos for a long time to help increase the top end flow... (their 7mgte's in the mark III's have the same issue that we do with their turbos - they are horridly innefficent above 14psig and cant flow enough exhuast above 5500 rpm when the 7mge will wind up to 6700)

What CI is that 318 after the .04" over? It will make a small difference and what psig do you want to run with it?

I think I'd have to index the bore and stroke, then work off that. Ideally it wouldn't make more than 15 psi at 6500 rpm or so.

I'm putting a pair of GM-8 turbos on my Jag V12. Displacement is 5.6L, so each turbo has to provide air for a 2.8L engine up to 7000 rpms, running 15 psi boost, with high-lift cams (minimal overlap), ported heads, etc. I found a compressor map for the GM-8 (also known as the IHI RHF6) and did the math... it looks like it should work perfectly for my application, but I imagine it will be a little big laggy.

I suspect that putting a GM-4 on a Subaru 2.5L would be fine for mild (<15 psi) boost levels. Don't forget to add an intercooler, or else all that boost will be wasted as the temps go up. For reference, there was a guy I saw who put a Holset HX35 (stock turbo on some Cummins turbo diesels) on a 2.2L Dodge. He had some lag with it, but not nearly as much as you would expect. The Holset is a significantly larger turbo than the GM-4 or GM-8.

Remember when sizing turbos that boost (pressure ratio) is only part of the story. What is really important are the CFMs. You can run a GM-4 at 20 or 25 psi of boost, but the intake air charge will be superheated and you're not going to get usable CFMs. As temperature increases, so does pressure for the same volume of air. When sizing turbos, I tend to lean on the side of larger as opposed to smaller. I would rather have more lag and then have a turbo that will have no problem keeping up with my demands at any point. It's easy enough to compensate for the initial power you want when flooring it by just having a large enough (i.e. torquey) engine. Plus, larger turbos give you more room to expand! :)

I've heard of Mustangs running dual GM-8s and making 700 hp or so with the setup... on my Jag I have a modest goal of 500 hp. That said, I won't complain if I go put it on the dyno after tuning and then find out I'm actually making 650+ :D

remember the CFM isnt the whole story - the CFM will vary according to its ambient temperature post turbo. Ideally you look at pounds of air per minute to get the best idea....;)

QM - bbl doing tsome calculating