ADVAN_EVO

Hey there one and all. I was trying to look into the effects that a smaller turbo and bigger turbo would have on a motors' internals as the same boost pressure?

For example: 16G @ 15psi vs 25G @ 15psi;
A) Would the bigger turbo at the same boost pressure put more stress on the motor as I assume more air mass is going in or would the smaller turbo at the same boost pressure put more stress on the motor as the intake temps are higher, causing the air volume to increase as well?
B) Is there a general way to calculate the pressures that a motor in stock form can handle (e.g. 4G94 DOHC) based on cfm or psi?

I've been trying to find the answer, but all I can see as far as the magical search button shows is "how much psi can stock internals handle" and those sorts of topics. Not one yet about comparing boost pressure effects across multiple turbos and their effects on engine internals at similar boost pressures.

If you know of a link for one, posting it would be appreciated, since I'm clearly having difficulty finding it.

RoadSpike

iTrader: (5)

In the end cfm and boost pressures are pointless. What breaks a motor ends up being two things in most cases where the tune of the car isn't at fault.

First is heat, heat is bad causes knock which breaks pistons and such and generally melts things that are weak in the system. Going lean will do this so fuel is important here also a good tune to prevent knock happening in the first place :P. In high boost cases you can reduce knock with lower octane fuel by simply injecting a lot of it into the chamber acting as the worlds most expensive coolant allowing more timing and less knock. This can be overdone of course but any good tuner will know what the limits are.

Second thing is cylinder pressure but you'll know this as engine torque. Torque is the killer of all things mechanical. An engine can only survive so much torque before rods snap or bend in some cases and further down the line the transmission will fail at elevated torque levels. You may be thinking a big turbo means more torque but usually that's not the case.

Engine volumetric efficiency drops rapidly, with stock cams, after only a few thousand rpm and this means you can throw more boost later into the game assuming your intercooling can handle the flow with zero bad effects creating a lot more power at the high range is easier for this reason and secondly because the charge of hot air has a lot less time to contact the piston before it exits making for a colder denser charge and hotter explosion for more power.

So realistically you could kill a motor with a small turbo MUCH faster at the same boost level than a larger turbo which will be colder due to the size increase and also has the effect of being less sudden in boost which is easier to both tune for and cooler/easier on the motor.

All that said I think 250-300wtq is probably a realistic perhaps lofty goal for a 4g94. Evo motors routinely make the 400wtq range without fail some even high as 500wtq without fail. With proper tuning and timing control you can sort of stall the torque a turbo would make by limiting either boost or by limiting the timing at an area to reach a maximum safe torque level and still make good power. You'll see this technique all the time in pump gas high boost evo threads the torque line will be very flat since that is the appreciatable heat limit the gas can handle.

ADVAN_EVO

Thanks a lot for the info. Definitely changed my perception of tuning.