In a V8, each piston fires individually. I don't think you can get RPM from just the sound frequency, as the frequency is dependent on many factors. What RPM are you trying to determine? Idle RPM? Redline RPM? RPM in an engine is almost meaningless unless paired with the rest of the drivetrain.
the pulses per second coming from the exhaust is in direct relation to the number of cylinders present.
all that is needed to determined crankshaft RPM is the number of cylinders and the pulse frequency.
however, i have made one important assumption here, and that is we are dealing with a 4 stroke cycle.
this is why i said 8 cylinders, i have never seen an 8 cylinder engine that WASN'T a 4 stroke cycle.
a single cylinder 4 stroke engine delivers one power stroke for every 2 revolutions of the crank.
4 stroke cycle engines are commonly referred to as 4 cycle engines and commonly found in cars.
the other type of engine, the 2 stroke cycle delivers one power stroke for every revolution of the crank.
this implies more power can be had with the same displacement.
this however comes at the price of increased pollution.
these engines are commonly found in chainsaws, weedeaters and less commonly in lawnmowers.
they usually require oil to be mixed with the gas.