I am probably just not thinking here, but does a tach measure crankshaft RPM or camshaft RPM?
The tach signal is a pulse for every spark right? But the camshaft is rotating at half the rate of the crankshaft. So does the tach multiply the tach signal by two?

 

Actually the tach recieves 4 signals per crankshaft revolution, so I guess it divides by 4.

 

Or in my case, doesn't do ANY of that.

 

The tach is driven off of the distributor shaft...which is running at camshaft speed. But, the bevel transfer gear in the distributor changes the ratio and the tach is calibrated to it, so that the engine crank rpm is displayed on the dial.

 

I have an '81 so all electronic. There are 8 pulses per one revolution of the distributor, which is two revolutions of the crank. Where is this bevel transfer gear at? Is this in the points style distributors?

 

The gear is only on pre-75 cars that have mechanical tachs with tach drive distributors.

RPM is measured at the crankshaft.

 

Yes, the mechanical stuffs are fairly obvious, just like a speedometer....

but the electronics is not so obvious....the pulses generated are of course much faster as engine RPM increases....and so the space between pulses is much less...obviously, while each pulse has substantially the same characteristics because of the 'tuning' of the coil, the capacitors, and points, or in the case of a HEI with electronics tachs the same thing as the points, just more reliable and consistent with the magnetic pulse and solid state switch sending unit.....

the job of the tach after that inline filter is to take those pretty equal pulses which firing at 600 crank rpm is only 5 per second per cylinder....so obviously 40 pulses per second.....and through 10x that speed at 6k crank rpm we obviously have 400 pulses per second....

note that all of this is down at the lower regions of the audio spectrum which makes for ignition noise in AM radios/amps so easy to pick up

so the tach will filter out on a electronic thing called a pulse integrator....making a average voltage outta that raggedy looking pulse train....all those frequency variations through engine speeds....

then the tach displays the meter movement equivalent of the averaged out/integrated pulses......giving you a RPM indication....

keep in mind, that along with other stock instrumentation of the era, much less 30+ years, the accuracy of tachs as well as other electronic instuments is somewhat questionable.....

for instance, I have fired my HEI and looked at the tach, and know for certain that at 1500 rpm and lower the tach is 100% right on the snot locker accurate.....but at 3000 rpm, it's ~300 rpm high same all the way through 6000 rpm so at 3k it's 10% at 6k it's only 5% but the same error factor in hard numbers.....



I have a HEI with FI, and a '75-77 type factory tach....

 

Let me see if I have this correct:
With the distributor rotor spinning at a constant 100 rev/min, then the crank is at 200 rev/min, right?

There are 8 pulses per rotor rev so 8 x 100 = 800 pulses/min or 13.3333 pulses/sec = 13.333Hz = 0.075 sec betweem each pulse rising edge.

So does the pulse integrator integrate the time between pulses or the time between pulses?

Still unclear how the rotor rate of 100 RPM becomes the crank rate of 200 RPM.