posted October 01, 2000 11:32 PM                

I have a question about forced induction vs. NA with forced induction. You have to lower the compression ratio to prevent pre-ignition, doesn't that lead to lower thermodynamic efficiency (when not under full boost, i.e., most driving) vs. a NA engine, since the combustion temperatures are lower with the lower compression? In other words, aren't NA engines more efficient for passenger cars than forced induction engines? Or does better thermodynamic efficiency not always translate into better MPG?

ANSWER:

Wow, ask a simple one, why don't you?

I don't think I'm qualified to give you a definative answer on this, but I'll take a stab at it.

First off, forced induction engines lower the mechanical compression ratio to prevent detonation, not pre-ignition. No, I'm not being picky, they're two different things, pre-ignition being a premature ignition of the mixture due to a hotspot (normally glowing carbon deposits on an excessively dirty combustion chamber). Pre-ignition is fairly benign as such things do, detonation can slag your engine.

But yes, that lower mechanical compression ratio does reduce the amount of power produced per cubic centimeter of displacement when not running under boost, and you're right, most daily operation is not under boost conditions. So that would mean that the NA engine should be more efficient.

However . . .

If I remember correctly, the extra compression didn't make all that much difference on it's own - ten to fifteen horsepower on a Pontiac 6.61 moving up a point or so. The real benefit that all those 60's muscle motors got out of the extra compression was the ability to run bigger/longer camshafts.

Side Note: As camshaft duration and lift increases, you need more compression to make use of it, at least on big V8's.

Secondly, the turbo motor is much more efficient under boost - and when you are under boost, you're accellerating hard and burning more fuel, so the turbo gains efficiency when it's most needed - so it may make more difference in overall efficiency.

I guess the easiest way to tell is to the EPA MPG ratings for a Civic Si, and then compare them against a turbo-Civic Si, and see how they compare.

Hope this helps.