QUOTE(KBanks @ Jul 25 2007, 09:27 AM)
Why do you want higher RPM's? Wouldn't it be better if you could get the same efficiency at a lower RPM?
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Well, if you only have a 4 banger to start with, you want a decent rev range. A mid sized 4cyl with high output is going to be inherently weak down low.
Strap a turbo do it (like the 4B11T) and you have to add turbo lag to the equation. The turbo won't fully spool until around ~3000rpm. That gives you about 4000rpm of usable power band assuming a 7000rpm redline. If you want to increase your power band by say...1000rpm, you have two choices: 1-increase the redline by 1000rpm, or 2-decrease the spool time by 1000rpm. (Or you could do a little of both).
Point is, it's easier to make the engine rev higher than it is to make the same turbo spool 33% faster.