will my stock Tach work on my 350 conversion?

[bryanjeep]

will my stock Tach work on my 350 conversion? If so what wire is it that goes to the new chevy distributor Hei?

[Inspector-Gadget]

Hello bryanjeep,

I am thinking that you are facing more than wiring difficulties in using your OEM tach with a SBC 350.

I believe you will be facing tachometer incompatibility issues as well. Unless the OEM tach is programmable via DIP-switches or some other means, I will say that it would be doubtful that your original tach will accurately indicate RPM even if correctly wired.

Why? Because four, six and eight cylinder four-cycle engines produce differing numbers of ignition pulses per revolution (PPR) of the crankshaft. The tach has to be internally set-up to match engine PPR to accurately indicate engine RPM.

This is why aftermarket tachometers are usually PPR programmable via DIP-switches or some other means. The programmable PPR feature allows more or less universal application of the tach.

For example, here is how a certain model of a Stewart-Warner aftermarket tach would be programmed to accommodate 4, 6, 8, 10 and 12 cylinder engines:



If you are going from the four cylinder engine indicated in your sig to an eight cylinder SBC 350, then obviously the PPR of the 350 will be incorrect for the OEM tach.

Unless the tach or some other electronic sub-system of your YJ is adjustable to accommodate the PPR of an eight cylinder engine, my guess is that the OEM tach will not accurately count and display engine RPM.

I will guess further that if your OEM tach works at all, lacking a means to accommodate the PPR of the eight cylinder, the tach will indicate twice the actual engine RPM.

Regards,

Gadget