Tach question... Kinda

[Seventy4Blazer]

the crank shaft spins at twice the speed of the cam

the distributor spins at the same speed of the cam.

the tach reads what speed?

thanks.
Grant

 

[tRustyK5]

The dist spins at half the cam speed, so the tach reads one engine revolution as one engine revolution...


Rene

 

[Seventy4Blazer]

yeah, it has been a while... but i THOUGHT the cam gear was the same size or LARGER than the dist gear....?
i dunno. ill look again when i go back to my shed.
rgant

 

[BigBen]

Doesn't the crank spin at twice the speed of the cam?

To complete one combustion cycle:

You need two piston strokes per cylinder (2 crank turns) for one valve stroke per cyldiner (1 cam turn) and one distributor turn (1 spark per cycle).

I thought 2 crank = 1 cam = 1 dizzy as far as RPM.

But maybe I'm dizzy on this one....

I'd guess that the tach compensates and gives you crank rpm.

 

[tRustyK5]

Hmmm, the more I think about it I'd say that's right.

4 cycles, takes two full revs to fire all 8 cylinders. To run through the entire firing order once the crank must go around twice. So, Crank X2 = Cam X1 = Disty X1. Tach must double each rev it counts...

Rene

 

[BigBen]

My best guess on the tach:

The terminal off the distributor gives one pulse per coil firing. (Guess)

I had to set a DIP switch on my tach so it knew how many cylinders were firing.

I'd bet that since it knows it is an 8 cylinder it counts every 4th pulse as an "RPM"....(or every 4 as a revolution) giving two revolutions per 8 sparks of the coil.

We dould verify this if anyone with an oscilloscope wants to check the output off the distributor. I'd bet that if you took the freqency of that terminal in hz (pulse/second) divided by 4 and the multiplied by 60 they'd get the RPM from their tach...... But I'm just guessing here.

 

[Seventy4Blazer]

yes, you are right..crank does spin twice the speed of the cam..... i messed up... working to much and letting my mind slip.
ill fix the origonal post... sorry again.
grant