Therefore, if I have stock NA engine, racing coil-overs, a full roll cage, racing seats and harnesses, short shifter, nice stereo system, Delrin bushings, competition sway bars, performance brake pads, aftermarket steering wheel and shifter, body kit, cool paint job, aftermarket spoiler, aftermarket wheels, competition radiator and oil cooler, and ricer stickers all over my car, then I must need REALLY high octane.

Not such a great way to determine the octane requirement, eh?

No.

Yes, it is fine to use 87 because porting does not raise compression. Since none of the RX-7 rotors have all that much compression, the ignition timing is the main factor that could increase the fuel octane requirement in an NA engine.

 

Man, I am so happy that I am not the only one to post this kind of info. Great job Evil!!!! Thanks for helping!

 

You get a bit better mileage by using 87, because it burns easier. As was said, the lowest octane you can run on your setup w/o spark knocking is what you want, no matter what the setup is. The reason high-compression or high-boost engines run higher octane is because it doesn't burn as easy or quickly, and resists pre-detonation (pinging, spark knock, same thing) and so allows more boost or timing.

 

No problem. I hate to see so much BS floating around about fuel, especially when it sucks money out of the wallets of poor rotorheads. Sometimes it turns into what seems like an Evil Aviator QA session, but I don't mind spending a bit of my time so these guys are not as ignorant as the other car enthusiasts. It would be nice if one of the forum chemists helped me out now and then, but I guess there aren't too many of them who specialize in POL.

Could you maybe hook up a forum function that shocks anybody who posts anything about octane other than AKI and cost?

 

False

Yes, assuming that cost is a factor.

False

True (ignoring the incorrect terminology of "pre-detonation")

Not bad, 50%.

 

i heard they used to run 87oct in the mazda factory race cars in the 70's-80's

 

Lol, well what are the correct answers then? And what is the difference between spark knock, pinging, detonation and pre-detonation? I help a lot of people out w/ this kinda stuff on other forums and in person, so I need to make sure I'm giving out proper info

 

If your car isn't tuned to run 93 octane, then there is no need to run it. I run 85 octane in my seven. they don't sell higher octane like that in higher altitude areas.

 

Actually I noticed an increase in MPG when I switched from 93 to 87 with premix. From 16MPG average to 23MPG average.

 

Spark knock = Pinging. Once the combustion of the air-fuel mixture is initiated by the spark plug, the flame front is supposed to spread smoothly through the combustion chamber. High temperatures or pressures can auto-ignite a portion of the mixture prior to the flame front reaching it, causing the pressure waves to collide and rapidly increase the pressure inside the combustion chamber, creating a pinging or knocking noise. This is also called detonation.

Pre-detonation = A common incorrect term made by combining the terms "pre-ignition" and "detonation".

Pre-ignition = When a hot spot in the engine ignites the fuel-air mixture prior to the initiation of the spark, or otherwise without the spark, such as run-on (dieseling).

Gas mileage has nothing to do with the octane rating.

A fuel with an 87 octane rating will DETONATE easier than one with a higher octane rating, but it does not necessarily BURN easier.

Also, there are several types of octane ratings. Two fuels could have the same Anti-Knock Index number, but one could have a higher Motor Octane Number. Example:
Brand A "93 Octane" (97 RON + 89 MON ) /2 = AKI 93
Brand B "93 Octane" (94 RON + 92 MON ) /2 = AKI 93

 

The higher the octane of the fuel, the more compression is can withstand without causing detonation... This is why you can safely turn the boost up in a turbo engine if you are using a premium fuel. Most people should know that it is always advisable to run high octane fuel in any turbo, or high compression N/A motor. I won't fill my tank with anything below 98.

 

No, it is not necessarily advisable to run high octane fuel in ANY turbo motor. The 87-91 RX-7 Turbo is rated for a minimum octane of 87 (R+M)/2 in the USA. I ran my slightly modified 88 TII for years on low octane fuel without any problems. The fuel in your country tends to be less stable, so see your owner's manual for the minimum RON required.

Yes, higher boost, higher engine load, advanced ignition timing, higher temperatures, lower humidity, higher atmospheric pressure, and engine hot spots will increase the octane requirement. Higher octane fuel will not hurt anything but your wallet.

 

meaning mods too the motor sorry.