Octane requirements
 

I'm a little confused. I thought that the owners manual stated that the 8 must be run on premium 91 octane gasoline, yet I'm running across posts on this forum where owners state that they're using 87 octane. Is Mazda wrong about the octane requirements? What's the consequences of running lower octane fuel? Pinging? Detonation? Nothing???

1.3L :confused:

yes.. some people that run lower octane will have a squeek sound.. almost like a belt squeek.. It will get louder with RMP's ..

Incorrect. Mazda says 87-90 Octane is OK, but they recommend 91.

i second that one bowman...

you bought a high performance car, you really shouldnt be cheap with the gas.

I run mine with stock tuning on 87.....no problems, and no pinging. When I lean out the A/F the octane needs to be 92-94 or else it will ping. No advantage to run high octane if it is not pinging.....except to flush the cash :D

The manual says 87 is fine. Search "octane" for a full discussion of the pros and cons of 87 octane. In short pro better mileage, con possible risk to engine.

I can't notice any difference with 87 octane. I might if I were hauling up a steep grade or out on a track, but for regular flat-land driving, it seems to make no difference in performance, I don't have any funny sounds or pings, and my gas milage is actually better. For that matter, I don't think that any production car "must" use premium fuel. That's oil company hype. Maybe the pros out there will disagree, but unless you're on a race track, it seems to matter not one bit.

The benefits of high octane fuel are seen under high stress conditions. If you're just puttering around town, then using high octane isn't necessary as the computer can adjust ignition timing in case of detonation. However, if you like spinning it up to redline and feeling it push you back in your seat, or if you're towing something with your '8 or traveling through hilly areas, you really should use the minimum recommended octane (91, IIRC). The computer can adjust for lower octane fuel, but even the computer has its limits and it's generally better to be safe than sorry.

Most new production cars will run on lower grade fuel, you're right there, but most high performance ones will not run to their full potential. Octane ratings are not merely oil company hype but are actually very important considerations when designing an engine. A class on internal combustion engine design here at my school puts a good deal of emphasis on octane ratings and choosing the right grade fuel for the job, as do all of the engineers and professors that I've ever talked to who actually work on engine design and really understand the physics and chemistry behind it all. Mazda's engineers certainly took fuel grade into consideration when designing the Renesis, although they, like all engineers designing a product to be sold to a largely technically ignorant populace, are overly cautious when delineating safety precautions.

You may not hear any knocking when you redline your car at WOT, but this doesn't mean that the computer isn't pulling timing and robbing you of some power. It all comes down to whether or not you value the performance of your car over a few dollars a week in cost savings. The way that I look at it is that if you buy a performance car, then why handicap it by skimping on fuel? Is the extra $3 a tank really worth it? Although I don't believe that it is, if you do then go ahead and buy 87 octane. Your engine will probably survive fairly well even on low grade fuel. However, I would be unwilling to risk thousands of dollars on the belief of a few people that 87 octane will cause no damage at all during high load conditions, so I would be wary of getting on the throttle too often if running sub-par fuel.

You guys need to realized that the higher octane rating means that it has a higher resistance to ignition. Meaning that it takes a higher temperature and pressure in the combustion chamber before it'll ignite.

Higher octane fuels are especially important in high compression or forced induction engines. Reason being is that lower octanes will pre-ignite before top dead center, (detonation) causing a disturbance in the 4 stroke cycle. The piston will start to rattle back and forth within the cylinder against the sleeves causing the sound we all know as engine knock. This can cause extreme piston damage, and in some cases can throw rods, and bend crankshafts and valves.

As far as the rotary is concerned, my educated guess would be to use higher octane fuels. Detonation is a problem that I'd rather prevent than have to pay for later. Who knows what kind of damage detonation can cause on the apex seals, the rotor itself, or the eccentric shaft.

There's probably not really that much daylight between us on this. Although I'm not convinced about engine damage on 87 octane, on the other hand, I'm very much "inclined" (no pun intended) to go with 91 in the Rocky Mountains, where I often like to travel by car where toodling in the normal manner around town doesn't work. Then again, regardless of where I happen to be driving, I often wonder if the octane I'm supposed to be getting is what I'm really paying for. Depends on the Department of Weights and Measures (or the equivalent agency) in the place I'm in.

In many of the more advanced, and high compression engines today the ECu can detect lower grade fuels and retard the timing of the engine. So in a way some of the posters are correct that if you do not need the full 238 hp, and you are just tooling around town you are using little of the engine. If you need the higher hp, the ECU will retard the timing if it senses the lower grade of fuel and you will lose hp, but it will protect your engine.

A while back, Car & Driver ran a new Honda Accord on 87, then 91 and found that the Accord ran about +10 hp more on higher octane fuel. Whether or not that can be acheived on most high performance engines (high compression) is debatable.

Here's a question, what about the people running 93 octane? Per the manual, the only allowed octanes are 87-91.

87-91 octane?

Man, I just learnt something new today.

I wasn't aware the octane rating of fuel in the US was that low.

Here downunder, Mazda Australia recommends our cars to run on at least 95 RON.
Most 8 owners here run it on 98 octane which is readily available.

Octane rating is specified differently in the US vs Europe. From the manual "91 [ (R+M)/2 method] or above (96 RON or above)" and "87 octane (91 RON)".

So when someone in the US [ where we use (R+M)/2 method] says 93 octane, it's probably 98 RON. Our 87 is 91 RON and our 91 is 96 RON.

rx8cited

Thanks for clarifying.
Phew, for a minute there I thought the US was a third world country. :)

OK some observations here...

I'm in the UK so we have a choice of 95, 97 (BP Ultimate and most other "super" type fuels) or 98 (Shell Optimax).

Mine's really sensitive to what kind of fuel goes in it. It is not happy at all on supermarket crap and I don't drive it over 5-6k at all when I am forced to buy a tank of that.

Delivery is smoother throughout the range running 97 and I'd say there was a little more power at the top. With Optimax in, response past 7k revs is DEFINITELY better.

So basically, I stick to 97 fuel and I only put Optimax in if I know I'll have a chance to see the benefit.

In Germany Shell sell "V-power" which is 100RON! It absolutely FLEW with that in. :D

It also helps if you can stay consistent - I guess because the ECU doesn't have to keep relearning.

ok here in the states (east coast) we have 87 89 and 93. 87 is usually 20c a gallon cheaper than 93...say you drive 12000 miles in a year, and to make this a high range estimate you only get 12mpg. 1000 gallons a year...by switching to a lesser fuel you've saved a grand total of 200 dollars in one year. if this car is that prohibitively expensive that 200 dollars makes a big difference you really shouldnt be driving it. i would hate to be someone down the road buying a used rx8 that didnt have anything but the best used in it. you owe it to your car to treat the way it was recommended to be treated.

I've noticed that by using the "regular" grade (87-89) gasoline around here, I actually get better mileage per tankful (180-200 per 13.4 gal). Maybe it's because when I have a tank of Shell V-POWER I'm wanting to prove their "cleans as you drive" theory, by driving the crap outta my car... you know... shifting at 9K, WOT in every gear... :D With premium gas (any brand - except generic) I usually get about 150-160 per tank... but the last part of your quote is why I've switched to premium only... plus the fact that I drive this car where the engine is happiest (above 5000rpms), it's an RX8 not a cargo van.

How many litre of fuel do you have to save to replace a busted apex seal?

The chances are remote but it is there and I think a few cent a litre is a sense of security in my heart/mind :rolleyes:

High RON/octane will save fuel and have a better environment too :)

Here's what it says it the manual: "You may use a regular unleaded fuel with Octane Rating from 87 to 90 (91 to 95 RON) but this will slightly reduce performance."

I'll trust that Mazda knows what they are talking about. I don't mind the reduced perfomance, so I'll be sure to report back when my engine fails due to using "less than the best" Shell 87 octane in it.

rx8cited

Well said.

I do think it is more important to prevent detonation with a Rotary motor than with a piston motor.

My 8 will see nothing but Premium gas in it.

better safe than sorry

I have only used 91

my point is you only save about 30 dollars a month by switching to regular....if something did happen trust me it'll cost a few years worth of savings to fix.

93 octane is fine. The way the ECU work in the US is that it functions optimally at 91 octane because in many states that is the best you can get, unless you spend $5/gal and get race fuel at limited gas stations. So while the ECU can sense lower octane and retard the timing (lower hp in the process), it won't advance the timing when you 93 octane. The 91 octane map is the best it is going to get for our cars from the factory. Some aftermarket companies have made 93 octane maps for other car brands. For example the Porsche aftermarket tuner Evo Motorsports has 93, & 91 octane maps for their 600 hp + 911 Turbo cars.