I'm running a S5 13B with an '84 distributor and a MegaSquirt ecu controlling fuel in an SCCA EP racecar. Engine is a big streetport. I have the Mazdaspeed main pulley which has timing marks at 0, 5, 10 and 20* ATDC.

I have almost 0 experience with rotary ignition and even less experience with distributors - I've been told by some people that I should have my timing light with the dial on 0, and aim for ~0* leading (TDC) and around 20* ATDC for the trailing - is this accurate? Others have said that I should double the desired timing on the dial and set that way. I know how to rotate the dizzy and adjust the trailing timing.

What are some optimal 13B timing numbers? How should I be using my timing light?

Totally confused, please help!

 

I think that Paul Yaw said 26 leading and 10 training for 13B (24 and 10 for a 12A).
If you run a search I am sure you will get the details. I will see if I can find it.

I have been watching your post in the Megasquirt section and I am intrigued. I am planning on something similar to your setup.

 

Thanks. Are those 26/10 numbers ATDC @ 4000rpm? Should I be using a timing light at 0* and going off the marks on the pulley?

 

Yikes... the search function is off tonight.

Well, here is Paul Yaw's site (Timing page) - http://www.yawpower.com/pultime.html
He says 26 leading and 16 trailing.

 

The total advance numbers of 26 on the leading and 16 trailing are BTDC numbers, not ATDC numbers. If you still have the springs in the mechanical advance then you need to rev the engine to at least 4000rpm to set the total advance.
The 26/16 BTDC numbers are a good place to start. You can start there and then adjust it slightly up or down to see where yours runs best.