Originally Posted by Ole Spiff

A 12% loss of performance on a PISTON engine, not a rotary. And there seems to be some misconceptions about what the octane rating really means. Octane determines when the fuel ignites; the energy is the same. Higher octane gas retards the ignition of the fuel so that it doesn't ignite too early in a high-compression engine. The higher octanes start burning at a slower rate than a lower octane, thus the probability that not all the fuel will burn completely leaving some deposits inside the engine. This is why the high octanes have all those detergent additives put in by the gas companies; to help remove those carbon deposits.

Lower octane fuels burn with the same energy that a high octane fuel does. The reason they're not usually used in "high performance" cars is because they can ignite very quickly which can be bad in a very hot combustion chamber with a piston compressing the fuel to high pressure levels; it can "pre-ignite" before the spark plug fires. This is why gas octane is increased which means the burn point is raised (retarded basically) to make sure the fuel doesn't ignite until the plug fires. This will give the "best performance" under high performance conditions....the track obviously. For street use however, and regular daily driving, high performance isn't the main issue. When the engine is NOT being revved up to "high performance" levels then the maximum combustion conditions that burn high octane fuel efficiently don't happen. So what happens to the fuel? It doesn't burn efficiently so it leaves carbon deposits inside the engine. A lower octane fuel ignites earlier, quicker, giving it a chance to burn more efficiently and completely (thus the better mileage many notice) which gives better performance for the normal driving performance levels that are typical in everyday driving. Start running the rpm's up to redline and it won't perform as well as the higher octane fuel; HOWEVER....how many times do you do that driving around town? Buy the gas for the driving situation you anticipate.

I used to run 87 exclusively but I've now had to go up to 89 because of add-ons to my car. For everyday driving, 87 or 89 works fine. For high performance situations, by all means run 91 or higher.

Originally Posted by Gerael

struggling to make ends meet but you got an rx8....

not that i know that much about new york... but don't most people commute and not even have cars?

Originally Posted by Krankor

So all of this makes complete sense to me, but the one thing that doesn't add up: so why does Mazda *insist* that you should use the higher octane stuff?

Krankor
05 Shinka 6MT

Originally Posted by devious12

Purchased the RX8 before I lost my good job and then had to start over again. And I live on LongIsland so I commute everywhere. Believe me I wouldn't have said anything if the .30 a gallon wasn't an issue. I'd like to look into Mazsport for the managment system, maybe I can get better gas with that a RB intake setup?

Originally Posted by Ole Spiff

They don't "insist" on it; they just "recommend" it for best "performance"....notice it doesn't say best efficiency. Under high performance driving, such as on the track, absolutely run premium....on the street however, it's a different story. The only real issues with running premium would be cost, (which is actually minor considering the income level you have to have to own a car like this, however you DO save money running lower octanes); and the biggest issue for me which is carbon build-up. Engine deposits do exactly what commercials that talk about it say it does; rob your engine of efficiency and performance...not to mention emissions issues.

Running 87 or 89 (or whatever is available in your area) will allow the fuel to burn more completely and efficiently which means less carbon deposits and build-up. This helps everything inside the engine including the expensive spark plugs this engine uses. IF you run the engine up to redline all the time and are pushing the performance....then 91 octane gives the best performance under those conditions. However we're not talking about a hugely noticable difference.....just best if you measured it...as in track times while racing. If you take 2 cars which are identical, run one with 87 and the other with 91 and do quarter mile runs, driver skill would make more of a difference in times than anything else.

All that being said, if you want to run premium....do so and enjoy your car. If you want to run the next octane rating down, do so and enjoy your car. If your car runs fine on 87, do so and enjoy your car. It doesn't really hurt anything to run a lesser grade (unless your car is one of those that just doesn't like 87) and you'll enjoy a cleaner engine over the long run.

Originally Posted by Krankor

Well, they *do* insist that you should use premium (which is different than insisting that you use premium). After first reading about this issue on this site, I asked the service guy at my local Mazda shop about it, and he said that Mazda makes a *big deal* to them that it should be premium. That's what gave me pause.

Anyway, thanks for the excellent explanation. And thanks to others who have also shed useful light.

Krankor
05 Shinka 6MT

Originally Posted by mauricio

Regular gas sucks

Originally Posted by Fanman

Let me get this straight...you say because you don't have as high paying job as you did before $ .30 a gallon is a big deal for you, but then you go on to say you want to buy $2000 in aftermarket parts for your car. That would buy a lot of premium gas.