[Travis Q]

More on backpressure (its time for a coffee break this morning, I'm dragging).....

To boil it down why backpressure is important in a drag racing application, it all has to do with max engine speed. In nearly any drag racing scenario, engine speed allows for a greater window with converter or clutch tuning, and equates to more power strokes per second. If you have two engines that make the SAME peak/average power, but one does so at 1000 higher rpm, and the race car is geared to take advantage of that additional rpm, that will be the faster car, even though the power number is the SAME.

A turbocharged engine with a high backpressure number can make the exact same power number that one with very low backpressure can. It can make the same average power over its operating range that a low backpressure engine can over its operating range. The difference is what that operating range is! High backpressure forces you to run a camshaft with very little overlap. Small overlap numbers means that the engine isn't going to rev. Also, high backpressure applications (like street cars) are not overly sensitive to valve timing events, since the cams are already so small. You can put nearly anything in it, and so long as the overlap isn't too crazy, it will run just fine.

Low overlap engines act a lot more like a NA engine, because there starts to be a small amount of scavenging on the overlap cycle. So these engines can sometimes be a lot more picky about camshafts.

The bottom line is that for the typical drag race engine, it's not worth losing sleep over backpressure, because most guys don't really want to spin their engines over 8000 rpm anyway, or don't have the components to take advantage of it if they did. The gains from reducing backpressure aren't going to come from a huge gain in horsepower; they are going to come from the additional engine speed afforded by a low backpressure combination. And for a typical T/S application, a high backpressure combination may not be all bad....a high backpressure combination is usually one that has a very responsive turbo combination, which means that it will be really easy to get the turbos to follow the boost controller.