I'm trying to use a tach/dwell/voltmeter to adjust the idle speed on my rotary. The tach meter has scales for 6 and 8 cylinder engines (for 4 cyl multiply the 8 cyl reading by 2). Of course there is no 2 rotor scale!

Here is how I figure to convert the reading for the rotary. Would like it if someone who knows could double check me.

I think the meter measures coil pulses (sparks) per minute (frequency). I'm just talking here about a single coil operation--the leading coil for a rotary. For a piston engine, every 2 revolutions gives one spark per cylinder. Thus an 8 cyl engine will have 8 sparks per 2 revs.

In a rotary engine, the rotor rotates at 1/3 the output shaft rpm which equates to the rotor rotating 1 rev in 3 output shaft revs. Also, each rotor rev has 3 sparks.

This gives 3 sparks per 3 output shaft revs or 2 sparks per 2 output shaft revs (per rotor).

Since the engine has two rotors, we should measure 4 sparks per 2 output shaft revs.

In my opinion, it looks like I use the 4 cyl scale (multiply 8 cyl reading by 2).

Hooked it up and using the 4 cyl scale appears to match closely with the in dash tach--so this looks right.

Scott

 

You're absolutely right. A twin rotor engine has the same firing frequency as a 4 cyl.