Actually on a four stroke one pulse reads out as two revolutions at the tach, not
four. Whether a two stroke or four stroke, it takes a stroke up and a stroke
down the cylinder to rotate the crank one revolution. A two stroke fires every
time the piston comes up while a four stroke fires every other time the piston
comes up. But what you are saying is true, the error would be 2:1.

wrote:

On Fri, 19 Mar 2004 15:11:59 -0600, "RB" wrote:

Any reason I can't use a 4 cyl automotive tach for my 4 cyl o/b motor? The
auto engine is 4 cycle, and the boat engine is 2 cycle.

I'm just wondering what it take to make it useable on the o/b.


2 more cycles. The tach works by counting ignition pulses. In a 4 stroke motor
those pulses occur once in every four strokes per cyclinder, so 1 pulse reads
out as 4 revolutions on the tach. In a two stroke engine, they occur every
other stroke.

BB