Many Chevrolet Corvette owners express this philosophy on exhaust piping size: the bigger, the better. As you will see, this is not necessarily so.
When moving exhaust gases away from your Corvette and you, the
objective is to keep them hot. Why? One reason is so that they flow out
of the exhaust pipe more quickly. It is matter of relative weight. The
cooler air is, the denser it is, and thus the heavier it is. In a
larger pipe, the gases have a longer chance to cool off. Theoretically,
therefore, with larger exhaust system piping we are asking our Corvette
engine to push out a larger amount of relatively cooler, heavier air.
Overly-large piping also affects header tuning. With the larger piping,
exhaust pulses achieve a higher level of exhaust-gas entropy
(essentially de-energized exhaust gases), which will throw off our
header tuning calculations. This is because the vibrational pulses will
line up differently than in a smaller pipe. Coating the exhaust system
with insulation, like a ceramic thermal barrier or header wrap, will
reduce this affect slightly, but this is an expensive way to go.
Realistically there is no set formula for determining piping size. It
all depends on the exhaust system: how many kinks and bends, muffler
configuration, exhaust gas temperature, etc. As a general rule of
thumb, engines producing 250 to 350 horsepower choose 3 to 3.5 inch
piping. Four hundred to 500 hp, choose 4 inch.