CincyScott

In a sleepless evening (too much caffeine) I was pondering maximum boost pressure and whether it was a function of the cylinder head or the turbo itself.

So here's how I got here: in a stock turbo car, the turbos provide a certain level of boost pressure into the cylinder head. Just to keep things simple, a tuner can control the rate at which the turbo spins in order to produce additional boost pressure, therefore more air and fuel and horsepower.

So, what is it that ultimately decides the amount of boost pressure our engine can handle? Is it a function of the size of the turbo, the speed of which it turns, or is there really a terminal point in which you've crammed as much air in as it can get?

I think it's obvious that turbo size and speed control boost, but how do you know how much pressure an engine can handle? Is it mathematical or as simple as testing until you have parts blow up?

Kevin

When we look at this engine platform and Motronics.. You must thihnk in terms of what is the limiting factor, before you lift the heads or burn a hole in the pistons (extreme cases)..

Knock retard limits timing and maximum boost.

Exhaust gas temperature limiter restricts the small KO4 turbine

Intake air temps restrict boost pressure levels..

Fuel injector limits.

If the above items were removed, one could strap on GT35's on each custom header and boost 35 PSI, providing that you replaced your rods, and oring'd your cylinder heads and anchored the heads to the engine case. And that would require running race gas, C16 and or E85.

sidwin

compression ratio is usually key in determining how much boost on pump gas. i read somewhere that the 997 has a 9.8:1 ratio. The lower the ratio the more boost a motor can run before detonation. The higher the ratio, the more power an engine will make but the more suspectible it will be to detonation which is the point where gasoline will compress to the point it will ignite without an ignitor source(spark plug) Detonation is the engine killer in addition to high EGT and lean A/F. Now if race gas was added or methanol injection, the boost level where detontation occurs will also be lifted. Then downside of that would be the rods and other engine components. That's probably why you don't see anyone try to turn the boost up to 20 psi on a stock engine. I'm guessing 14-15 psi would be the max i would want to push pump gas. The shock wave sent due to detonation is simply an amazing destructive force. I've seen pistons pitted, valves pitted, rods bent...

Ewok

I just typed up an essay, and when I clicked submit, it said my session timed out and I lost the essay.

Well, I won't type it again. sidwin is right though. It's all about detonation. Increasing temperature (via boost or compression ratio) will bring you closer to detonation. Higher octane or water/meth injection will take you farther from it.

The temp increase compounds when you compress compressed air.

In reality, the typical turbocharged engine can handle more than the max boost of a small turbo but less than the max boost of a large one. If someone ever says "the engine had more boost than it could've taken" it's either because of poor tuning or asking too much of the engine mechanically. PERFECT tuning can get you to really high HP numbers, but it's like walking a tightrope. And if you can get the engine to actually fail due to mechanical stress and not detonation, the carnage would be spectacular!!!