Quote:
Originally Posted by Daox
Its pretty accurate. If you compress too much air in the engine you have problems with preignition and pinging, thus the need for most super/turbocharged engines to use higher octane fuel. If you don't use this higher octane fuel you have to sacrifice optimal ignition timing and thus efficiency. Another remedy for this problem is to reduce the compression ratio which also lowers efficiency.
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Yes, lowering the compression ratio of a NA engine lowers efficiency. The reason why is because your cylinder pressure is reduced. With forced induction, you have two compressors increasing the cylinder pressure (the supercharger and the piston) so lowering the compression ratio of the piston doesn't neccessarily mean your cylinder pressures have been reduced.
You're good at explaining the reasons why a typical supercharged engine has lower fuel efficiency, and I concede that. However, as I've already stated, superchargers are typically used for performance reasons, not fuel efficiency. I still stand by the statement that a properly designed engine can be made more efficient with a supercharger. Pistons are inefficient at compressing air and a lot of heat is added. An efficient supercharger running at low boost through an intercooler on an engine designed to utilize it should be more efficient than a NA engine.