It has too. It is more than likely part of the programming. PCM has to know how many cylinders there are, to give the correct information to the cluster. (don't think it cares about what fuel those cylinders are burning though.)

 

but if its the same signal for a v6 v8 and 6bt from pcm to tach then the pcm doesn't care about the amount of cylinders

 

The needle needs to move the same amount for X RPM regardless of engine. However, the number of pulses the PCM sees from the engine needs to be multiplied by the number of cylinders so the pcm can feed the cluster the correct info. Otherwise, if you upgraded from the V-6, to a V-8, your tach would read higher than it should, because of the extra pulses generated by the crank sensor.

 

a crankshaft position sensor measures the rpm of the crank... so how dose the # of cylinders have an affect on rpms?

 

The cps reads the little holes in the flexplate that speed past. For the six cylinders, six holes go past for one crankshaft revolution, for the V-8s, 8 holes go past for one crankshaft revolution. The PCM determines engine rpm by using a formula, number of holes/number of cylinders over time equals engine RPM.