Hi, I'd like to find out about the stock ECU on my 87 Rx-7 Coupe Sport. Specifically, I'd like to know what I can do in terms of running different rotors, irons, porting etc., on the stock ECU if at all.
Where should I be looking? I guess I can check if there's a guide to performance tuning.
I guess it's likely not a huge deal to go to a mega-squirt or something, but I'd like to understand the stock system and have an idea of its limits.
Thanks for any help.

 

Generally the stock ECU dumps waaaaay too much fuel in for best power, in order to protect the converters.
It also measures airflow directly, so making the engine flow more air will result in an equal amount of too much fuel to be added.

I've heard of racers in classes where stock computer is mandated, of dropping fuel pressure down to 23-25psi, and it was still not pulling enough fuel out for best power, but that was the limits of how low the regulator could go.

I had a friend with one of those old dial type piggyback controllers (like a stereo equalizer but for your fueling) and he kept running quicker times right up until he had the fuel dialed as far out as the little box would allow.

 

its old and slow too.... the trouble with the stock S4 ecu's is that it runs a reasonable AFR, with mild timing (the S5 runs much richer), so once the AFM is 100% open, it stops compensating, and it goes lean.
if you were to put a wideband on a stock S4, it runs in the 14's up to about 4000rpm, where it dips into the mid 12's and stays there up to ~6500 or so. (the S5 is the same up to 4000, but then it linearly goes to 10:1 by ~7000rpm)

so basically the stock ecu works ok if you have an intake and catback, but much more and you're missing out