Has anyone noticed a difference between low and high octane gas in their N/A? I didn't think it made a difference (unless it detonates), but I talked to a friend who said his 83 RX-7 12a wouldn't even start on 85 octane, but would run great on 91. My car (89 GTUs), on the other hand, starts and runs fine on 85.
Is there a difference, and, if so, why?

 

High octane fuels burn slower. Run 87 or 85 or whatever. If his car wont even start on 85, there is something seriously wrong with it.

 

Do a search. This question has been discussed hundreds of times on here, so you can find out all the info you want about it.

Basically...
Stock NA = lowest octane possible is best
Turbo = highest octane possible is best

 

Due to the short time the charge is in the engine, you want the most volitle, fastest burning fuel you can get without pinging.

 

i've noticed that my GTU runs better on 87 than on 91. smoother and nicer running idle/low revs.

 

Mazda made the turbo ii to run on 87 also. There is no reason to use anything higher if you are close to stock.

 

Yes, running high octane on a stock TII is a waste of money unless the engine is in very poor shape and knocks with the recommended octane. The octane rating is an anti-knock rating, not a power rating or engine starting rating.

Also note that at high altitude the octane requirement is decreased. The Series 4 (86-88) RX-7 is rated for 87 octane, but your 85 octane will probably work just fine at high altitude. Some US gasoline manufacturers sell their gas as a higher octane rating (87 may be sold as 89 for example), but others keep the same rating, so this can get confusing.

Also note that the Series 5 (89-92) RX-7's are rated for 89 octane, and other countries use a different octane rating than the US, so be careful of what you read on the internet.