I want to install a Sunpro tach in my '83 RX-7. My thinking is that since a rotory fires twice for every revolution of the crank then I should hook it up like it was a four cylinder piston engine. Sound logical?

 

twice every revolution? I'm pretty sure it fires 3 times per rotor each revolution of the e-shaft. So thats 6 total... am I wrong?

 

You're partly right. Yes, each set of plugs will fire three times for each revolution of the rotor (or a total of six times per revolution of the rotor as you state) but the crank turns three revolutions for each revolution of the rotor.

So, each rotor will have the plugs fire once per revolution of the crank while the rotor turns 1/3 of a revolution. On a two rotor engine that means two spark plug firings per revolution - one for each rotor.

Hence, my thoughts of connecting it like a four cylinder piston engine that will have two spark plugs fire for each revolution of the crank.

If I'm in left field here please give me your input.

 

4 cylinder is correct

 

Yes, 4-cylinder setting is correct. Distributor-run RX-7s can use the negative terminal of either coil, because there is no wastespark. 2nd and 3rd gens, as well as direct fire leading plug conversion 1st gens (wastespark setup) need to drive the tach from the neg terminal of the trailing coil (or "tach" terminal of an ignition box, like an MSD).

 

A neat idea when installing an aftermarket tach...connect it to the leading coil. Coupled with the stock tach (connected to the trailing coil), this gives you a good diagnostic tool...especially when tracking down an intermittently failing igniter.

 

Oh, btw, an Autometer Ultralight 3 3/8-inch 10,000RPM tach (~$93 at Jegs) can be made to fit an '81-83 cluster, in place of the stock tach. I did this in my 83 RX-7 with peripheral port 13B, when I needed more than the stock 8K gauge. Pretty slick looking and very functional...plus you don't have the "I've got a big tach to compensate for my small *****" syndrome, like most ricers.

I made a fiberglass spacer -- ID (inside diameter) matches OD (outside diameter) of tach, OD of spacer matches ID of stock tach hole -- and used the regular mounting bracket that came with the tach to secure it to the cluster, then I connected the wires to the appropriate places on the cluster so that it was plug-n-play. Surprisingly simple.

Note: in the picture, the tach reads ~1,200 RPM...this is normal; when the ignition power is "on", the tach reads zero, just as it should.

 

Nice work!

 

looks nice