Lowest octane you can go is 87, provided you haven't done something that would require higher octane use, like advanced timing over 17*, turbo etc etc.

 

^ what he said ^

 

on my 91 sr20 I use 93 octane and I get 29-32 mpg. If I use 87 I get in the mid 20's, lowest being 23 mpg. But that's with advanced timing, intake, header and exhaust. If you haven't done any of this, you can go with 87 safely for sure. The debate rages on as to whether or not advanced timing requires higher octane(ie, 91-93). You'll be safe with 87 though.

 

I suppose the need for higher octane fuel on higher-than-17* of timing is necessitated by the fact that you are going beyond factory recommended specs. At that point, you are risking engine-knock, which we all know is not an exciting thing to happen. Therefore, to avoid a risk like that, going higher on the octane gas helps to neutralise any issues. The car itself retards timing if there is knock, but you'll notice the power-loss and will have to go back and set the timing all over again.

So if you are not great with mechanical aspects of cars, I would advise you to leave the car to factory specs. This way you won't be sweating it everytime you take the car out. Just my 2 cents.

 

The only reason you run higher octane fuel is because you raising your compression ratio advancing the timing. Therefore you will need a fuel with higher octane so you don't run too lean and get knock or detonate the engine. So, if you have the timing advanced I would suggest higher octane, sometimes it can also be good to run high octane, ocaissionally. Be it the fact that higher octane fuel cleans the fuel system. So it can also help on cars without advanced timing, but you don't have to run it constantly. PEACE

 

The only reason you run higher octane fuel is because you raising your compression ratio advancing the timing. Therefore you will need a fuel with higher octane so you don't run too lean and get knock or detonate the engine. So, if you have the timing advanced I would suggest higher octane, sometimes it can also be good to run high octane, ocaissionally. Be it the fact that higher octane fuel cleans the fuel system. So it can also help on cars without advanced timing, but you don't have to run it constantly. PEACE

C/R is not affected, what happens is that you are making a spark a little earlier, which means more combustion in the chambers, which means more power. The max factory recommended timing is 17* on the SR20DE, beyond which you will create knock unless higher octane fuel is used. If you do get to the point of knocking, the ECU will retard the timing to prevent it from causing damage.