So the extra RPM buys you powerband. Turbos take X amount of exhaust gas to spool and create boost. Being able to rev out higher is just buying a bigger powerband. Its not that the revs spool the turbo so much as it buys a longer time in boost.

A larger displacement engine with smaller turbos will spool faster and run out of RPM or turbo efficiency sooner, almost to the point of acting like a larger displacement motor like modern production cars. This is great for the street, lots of power low in the rev range where most of your driving happens but in a racing application, that matters a lot less because the engine is always high in the rev range. Like I said, it really comes down to use case. AutoX requires a different powerband than drag cars, etc.

 

Thank you for the explanation. This makes a lot more sense to me now as I had no idea that turbos were that encompassing. However, when people say they "turn up the boost" are they essentially saying that they are raising the upper limit of the turbos spool ability? I am assuming that there is some point like all wheels have where rpm will create a catastrophic event.

Thanks again.

 

So a turbo can spin fast enough to have a catastrophic failure sure, this is controlled by limiting the exhaust gas through the turbine side with wastegates. When people are turning up the boost, they are simply pushing more of the bleed off exhaust gas through the turbine side to increase the pressure created by the compressor side.

Boost is controlled by controlling the exhaust gas running through the turbine wheel.

 

The way I see it when you turn up the boost that is dependent on the effiency of the turbo at that pressure ratio. The tuners job is to tune the Evo so it makes enough boost to keep the turbo in its optimal effiency island. When you go past that pressure ratio and run the turbo close to the choke line then its being overspun and failure is immient. It will make more power but it wont live through that abuse for long. That's how drag racers using turbos will blow them up. They wanted to put down a run that no one could match and turned up the boost or closed the wastegate and gave it everything that the motor could handle.

 

I have seen a lot of cars at the drag strip lose the hoods on the cars from over spu. And over reved motors. To restate what was said before, you really need to know your application and what you are going to do with the car.