Hello everyone. I am in need of help in resolving a friendly wager between a co-worker and myself. He owns a stock 84 GSL-SE. He is convinced that he is required to run premium gas in this vehicle and I disagree. Fortunately when he purchased the vehicle, it came with the owners manual. After refering to the manual, he believes that it supports his theory while I am convinced that it not only supports my theory but clearly states that Mazda recomends 87 octane. The following quote is taken verbatim from the owners manual.

FUEL REQUIREMENTS
Your new Mazda is designed to operate only on Unleaded Gasoline of at least 91 Research Octane "(RON + MON)/2 = 87" when the engine is adjusted to factory recommended specifications. Unleaded gasoline will minimize exhaust emissions and spark fouling. (end quote)

Co-worker's point of view is that "91 Research Octane" = premium is required.
My point of view is that the "research octane" or "RON" is only part of the formula that is used to calculate the anti-knock index that is posted at fuel stations. Not only did the manual include this formula, it also went so far as to say that it was equal to 87 anti-knock or more commonly 87 octane. He is convinced that the formula is clearly there as reference only as to how anti-knock or octane values are arrived at. My belief is that if it is there only as reference, then why did Mazda assign a value of 87 to the anti-knock. Obviously (Ron+Mon)/2 is not always equal to 87 as there are mid-grade and premium fuel to take into account also. I figured that if it was there for reference only, Mazda would have just included (RON+MON)/2 and not "(RON + MON)/2 = 87".
So this is what I am thinking:
(Ron+Mon)/2=87, according to manual Ron is 91.
(91 + Mon)/2=87, with the addition of the Ron value, the Mon or Motor Octane Number has to be 83.
(91 + 83)/2=87, Mazda is recommending 87 octane or anti-knock as the fuel requirement right? Makes sense to me but for all I know I could be way off base. Any and all help will be appreciated on this matter. Thanks everyone.

 

Almost everyone I've asked about this subject told me that the lowest octane works best with a rotary. In my case though I get vibrations with low octane gas, really noticable on the highway. My car was pretty much set-up for 92 octane or more. My timing and engine adjustments were all done with high octane. So I notice a difference with different grades of gas. I have a gsl-se.

 

You are right. The 91 Research Octane is the RON in the equation. 87 is what's on the pump.

From Shell:

"RON, MON and Road Octane:

RON stands for a fuel's Research Octane Number. This value is measured under fairly easy test conditions. The MON, or Motor Octane Number, is a tougher test measured at higher engine speed and temperature. And the Road Octane Number - the octane number that relates most closely to actual driving conditions - is the average of these two values, i.e. Road Octane = (RON + MON)/2. The Road Octane Number is more frequently referred to simply as the octane number. "

The number on the pump is the Road Octane. 87, for his car. Any higher octane is simply wasting money. A stock normally aspirated rotary will not knock at even lower octane levels.

-John.

 

IF you need some "official" proof,
Howstuffworks.com:

http://environment.about.com/library...D&terms=octane

 

Thanks again for the input guys. Unfortunately this guy has a problem admitting that he is wrong. First thing he did was accuse me of replying to my own post. That didn't take long to shoot down, but then he said that I should not have included the quote from the owners manual as it backed my point of view. When I pointed out that if it backed my point of view then why are we still having this discussion he made an idiotic remark about the time frame of the replies to the post. They were too close together. Informed him that I had no control as to when people replied. Then he went so far as to say that how do I know that you guys know what you guys are talking about. And the funny thing is he added "They could be just like me." He didn't catch his own insult. At this point the only thing I could give him was a whatever dude. He wants me to get something from Mazda. Have a feeling that if I did he would only accuse me of writing it myself so I'm not even going to bother.
I don't care what he feeds his car, I'm not the one paying for it. And I wasn't questioing whether his car needs premium or not, I was simply stating that Mazda did not design the GSL-SE to require premium. It just bothers me that he realizes that he is wrong but continues to deny it with some stupid excuse. Well I'll see if I can get him to join this fine rotary community you guys have going here. I'm sure he will make a fine member. I myself will have a family of 4 in July so a two seater just will not cut it for me. I do drive a 929 though. Hopefully I will be able to get rid of some bills in the near future and free up enough to invest in a 1st or 2nd gen. Rx. If I get a 1st gen, it's gotta be a gsl-se. The ol'lady can have the 929. : )
Thanks again.