Choosing head flow for turbo...theory discussion

Long story short, I just got a free GM 3.1 V6. I plan on giving it some boost, but need some head help. I think I'll just go super cheap and keep the iron heads on it now, but it brought to mind a question I had.

There are basically three heads available for this engine; a Gen1 iron with inline valves (what I have), Gen2 aluminum with splayed valves, and Gen3 aluminum with splayed valves. Flows basically look like this respectively: Gen1 183/130, Gen2 200/160, and Gen3 219/148. As you can see, they step up intake each time, but exhaust goes way up with Gen 2, then back down to in between with Gen3.

The iron headed engines also use a flat top and larger chamber, while the aluminums used a small chamber and dished pistons. Iron heads make about 8.5:1, and later choices made 9-10:1.

So, as you can see the irons have some decided advantages; lower compression, a touch stronger for boost but not a big deal, and they let me use my current intake, and they're free. But I digress...

- Let's take this example I know that if I run a PR of 2.0, (and assuming a magical 100% IC efficiency) theoretically I'm flowing twice the mass of air in the ports per that 183 CFMs. So, at what point should I worry about adding flow to the intake side of the heads? Should I worry about it at all with a turbo on this engine?

- What should I do with exhaust ports when setting up for turbo? I know more flow AFTER the turbine, but is exhaust port flow more or less of a concern for turbos? How much or a role does velocity play in getting the turbine moving? Should I port the exhaust like nuts to get more flow, or keep them modest to keep velocity up before the turbine? On an NA car, I would port the intake and exhaust similarly. Porting for a turbo, do I give more emphasis to the intake porting, or the exhaust?

Thanks. My goal is 250 hp which should be easy. Some folks over at 60*V6 forums are making 400+ :naughty:

For 2.0 street/strip application put far more emphasis on the intake side than exhaust. Longer cam, higher flow heads, ect.

On your 3 particular heads I would not go with the gen 2 heads because of the exhaust flow/intake flow ratio is not as suited for turbo. choice 1 and 3 would be good. I am kinda of a tightwad, so if it was me I would use the iron heads and make it up by using 2-3 # more boost. Another option may be to use the iron heads with the gen 3 pistons to see what low compression it might be? 2.0 is lots of power increase,,,,,

I sorta used the 2.0 as an example. I'l probably run more like 10 psi. Thanks for the advice. I also would love to keep the free iron heads instead of buying aluminum heads, intake, and associated pushrods/rockers. I'm currently looking for a free car to put it in. MG, Triumph, Chevette, LUV :)

So, is there a formula for this? Since my current heads at 183 CFM intake are currently supporting 130 hp, 10 psi after the intercooler should put me around 210-ish, right? So if I port the heads and make 160 NA, that same 10 psi should make around 260. Those are obviously generalizations that assume many things, but just for the sake of comparison.

So, my next question would be; I can increase duration or lift on the intake at the expense of increasing overlap. I can combat the overlap with a wider LSA, but I don't want to go too far; maybe 114 to retain some vague hope of torque while not under boost. Is there some sorta magic formula to help me match flow and cam duration for turbo? It seems that making the flow with porting is wiser than with duration, true?

Your hp figures may be right maybe not. Your are correct about the many many variables.

The second part I have to think about for a while,,,,regardless there is always a trade off for the cams and ports. I am not sure about the flow vs. duration. There must be some calculators somewhere.

Duration can be increased as large as you want so long as you don't pick up too much overlap.

I'd open those ports up to flow the best VE close to yoru max RPM. Hence more whp at less boost.

Duration can be increased as large as you want so long as you don't pick up too much overlap.

I'd open those ports up to flow the best VE close to yoru max RPM. Hence more whp at less boost.

With an efficient turbosystem you can use large overlaps aswell. This basicly depends on how high boost levels you are using in comparison to the exhaust pressure when the exhaust valve is about to close.

With a very efficient turbosystem you may need to limit the overlap since it may lead to air/fuel being blown out into the exhaust manifold.

As always, large valve overlaps will result in a poor low end.

So, as you can see the irons have some decided advantages; lower compression, a touch stronger for boost but not a big deal, and they let me use my current intake, and they're free. But I digress...

Iron is of course heavier than aluminum but aluminum also conducts heat better. The latter mean that you can use more boost or a higher compression ratio before detonation occur.

Also note that head flow depends on the port sizes and the specific flow. Specific flow depends on the port coefficient. This will tell you how well a port flows under certain static conditions, it will however not tell you if the ports are suitable for the dynamic conditions in your application. It's easy to make the ports a bit larger than they need to be used because that seems to be favored by the flow.

Ok, good stuff. I'm glad I asked. Now I guess I should start getting parts together :)