Quote:
Originally Posted by Daox
Frank and PaleMelanesian are completely right. You won't see any improvement in FE from replacing your muffler. Your car will just be louder, thats all.
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Not true.
Lower back pressure means less work for the engine to expel gases from the cylinders on the exhaust stroke.
A common misconception is that that an increase in manifold vacuum due to increased throttling to maintan the same power offsets any gains due to more work being required by the engine to pull the fresh air charge into the cylinder. Since the crank case is connected to the intake manifold via the pcv valve any increase in the manifold vacuum increases the vacuum in the crank case eliminating any extra work needed to pull the fresh charge into the cylinder.
The crank case vacuum does however increase the work required by the piston to expel the spent charge again however this is countered by the increase in power due to the higher pressure differential across the piston on the power stroke.
Another side benefit is that the mass of air thrown around by the under side of the pistons in the crank case is reduced (I can't remember the technical term used to describe this effect). At higher rpms this reduction in air mass reduces drag on the engine.
The end result is a net gain in power (actually a reduction of a losses ) for the same amount of fuel burned.