I wanted to clarify a few things. First, a higher octane gas actually burns SLOWER, not faster, than a lower octane gasoline. Think of gasoline as a MIXTURE of various hydrocarbons with varing burn rates. Parts of a lower octane or "cheaper" gas starts to burn earlier if too much heat is applied. The only reason you would need a higher octane fuel is when you have a higher compression engine. The more squish you have on the fuel under pressure, the more heat is generated potentially causing the fuel to pre-detonate early, before the spark plug even sparked. This is the so-called "pinging" that we hear or hear about. This pre-detonation can hurt power (since it happens too early in the power stroke) and in some instances, can damage an engine.

There are a few other factors that contribute to an engines ability to withstand pre-detonation. Obviously the choice of materials plays a part. In a piston engine, aluminum heads allow for at least an extra point in compression ratio without any ill affects because of aluminum's ability to dissipate heat quickly over cast iron. Other detonation tolerance factors include the amount of spark advance an engine runs, and also what I really wanted to touch upon briefly---Cylinder head design. In piston engines, three basic configurations exist: wedge heads which are, obviously enough, combustion chambers shaped like a wedge with a disadvantagely spaced spark plug; hemi heads with hemispherical combustion chambers with an advantageously, centrally, placed spark plug for more even combustion; and the very compact, efficient pentroof design used on multivalved engines. Now throw all that you just learned about piston engines out the window (almost) and go back and study the rotary engine. It has many inherient advantages over a piston design but not in every area....Where is it's flaw? I'll answer that for ya....It has a very poor combustion chamber design. Study it. It has a very elongated shape and in order to get a full, complete burn, it needs to do so with two spark plugs. Those two spark plugs are there to compensate for this weakness.

Knowing what I just told you about combustion chamber design, do you think a rotary engine needs a SLOWER burning, i.e., higher octane fuel? Combustion needs to start as early as possible. Spark plug number two is just for clean up. Save your money and only run 87 in your engine. The one caveat is if we are running under boost. Otherwise, no need for more. Granted, the extra detergents that better grade fuels have might help here and there to clean your car's fuel injection system but you are really not maintaining anything any better or any worse.

What about a compromise and run 89? Or perhaps run 87 and and every 5th tank full or so run 92 to get the benefits of the extra detergents?

I thought any detonation was really bad for a rotary engine considering how thick an apex seal is. Maybe the lower combusion chamber pressures of the NA rotary helps a bit. To maybe toss in a point, my parents' V6 Accord is rated to run on 87. They get gas at Costco which is the worst gas you can probably buy. It detonates on costco 87 octane, and when I ran some 91 in it and reset the ECU, it still pulled timing quite a bit...

Anyway, if you want to be safe, just run gas from 76 if you life in California. Their octane ratings are spot-on, or better. Chevron is a good brand too, especially if you like keeping your engine clean.

I run 89 and premium once and a while. I have had some rough idling, hesitation ONLY when the engine is not fully warm so am I right to assume it's not because of the gas?

OKay

I went to the dealer about my recent idling issues and was told it was normal. Here, in FL the weather/temp changed and that is the most probable cause _ I am told.

ALSO, NOTE TO ALL OWNERS

The dealer's Mazda senior servicewriter came over when He heard about my idle questions and told me to stop using PREMIUM - that it was causing problems with idle.

As I have said from the beginning, I believe the rotary runs better on regular. I informed him that I learned about the gas grade early on.

Skagen - I have used regular since the beginning of August exclusively. I was experiencing problems using premium with missing and rough idle and soot heavy on the pipes. Since I switched to regular the soot has greatly diminished the idle improved and the missing nearly disappeared - until recently when the temp here in FL dropped down into the 40's or so - the idling got rougher BUT NOTE what the dealer told me to do above.

I HAVE noticed that there is a marked difference in the idle of the engine when cold (very smoth) versus a fully warmed up engine (rougher idle) A fresh tank of cool gas makes the engine idle smoothly too. It seems that the engine is more sensitive to temperatures than other engines - it seems.

i3man - I am not sure that the higher octane detergents are needed. IF regular gas burns better and cleaner in the rotary than the slower burning high octane, that would negate any reason to have the detergents.

Hey Rod,

Thanks for the info man. I appreciate it. I just don't get why the manual says to use premium shit when friggin regular is better for it. Weird. Don't make any sense. Gonna drop by my dealership as well and ask. Thanks again man. Keep ya guys posted on what my dealership says.

Running a higher octane gas every 5 fill-ups MIGHT be a good strategy to gain the benefits of a fuel with some extra detergents. Detergents are tooted as being good for keeping fuel injectors clean and really, it's in dispute how much. If indeed the extra soot or carbon build-up is resulting from higher octane gas---Which I suspect it is, well, then I think I would even second guess the above prevenative maintenance I just suggested. Again, we have to keep in mind we are not talking piston engines, but a rotatary engine with different characteristics.

Also worth mentioning is why did Mazda relocate the exhaust port on this engine from previous years? We know it was emissions related but lets look at it a little closer. Before the advent of fuel injection, this port used to carbon up very badly in it's present position that we have here today in the RX-8. A carburetor just did not have the accuracy back then to precisely meter the amount of fuel this engine needed. Too rich a condition resulted in carbon build up. So Mazada moved the exhaust port where the apex seal sweeps. Good, good, good from the stand point of eliminating the carbon build up from it's original design, but Bad, bad, bad from the standpoint of wearing the apex seal prematurely. Also, the extra overlap resulted in poor emissions from sweeping unburnt fuel out the door and into the atmosphere. Knowing that the rotary engine will never again pass tighter emissions with this compromise, Mazda again rediscovered relocating the exhaust port BACK to it's orginal design debut. Why could they now get away with doing this and not before??? Because we have modern, superior fuel injection systems that precisely meter the exact amount of fuel this engine needs. No unburned fuel means no carbon build up.

However, do you think an engine's computer is smart enough to know what octane level is being put in it? In a word, NO. All an engine has is a knock sensor. If the octane level is too low, audible pinging tells the comptuer to retards spark. That pinging you hear is pre-detonation. The fuel is burning too quickly and too uncontrollably before the spark plug can start timed ignition. What caused this? HEAT.

What happens if the octane level is too high? Do you think the comptuer will be able to compensate? How? There in lies the problem.

Also worth mentioning is that there are alternative fuels that have very high octane levels. Alcohol is one. A normal piston engine running alcohol can run 14:1 compression ratio without detonating. Why? Well, alcohol is just ether with a water molecule attached to it. In essense, you are burning a fuel loaded with water. Why does this effect octane? Because the water molecule allows for alcohol to burn SLOWER than normal gas. Alcohol motors make great power but they suck at gas mileage since half it's fuel composition is water. However, why do they make good power? Alcohol, although it has a slower burn rate, consider that it has a very controlled, concentrated explosion that happens 'ALL AT ONCE'. Gasoline, on the other hand, is a mixture. In order to have burning 'AT ONCE' you have to ensure that all parts of the mixture have the same burn rates. Parts of lower octane gases burn early and other parts later. The charge isn't 'at once'. Not good for any high compression application that will run hotter than normal.

In summary, what I'm saying is given the very elongated combustion chamber of the rotary engine, this engine doesn't need compact combustion. The higher octane characteristicly burns too slow and the ECU has no means to compensate otherwise---Hence, carbon build-up, a rough idle and overall lower performance.

Don't believe everything you read. Most people on forums, including myself, are not experts on these engines.

The reason the manual suggests the use of higher octane gas in the Renesis than previous 13B rotaries is the compression ratio. The Renesis has a 10:1 compression ratio compared to 9.7:1 for the 13B non-turbo rotary.

Higher compression means more chance of knock/ping/detonation, hence the recommendation.

I'm going to be sticking with 91 octane because I don't want any chance of detonation in the motor. There have been other folks who ran 87 octane who reported (what sounded like) detonation when under heavier loads. That's good enough for me to shy away from it.

Simon.

Yes, I have heard that sound with 87 octane fuel. The only beneficial effect of the "Tornado" device (yes, I bought one just to get first-hand experience) is that the "marbles rattling in the engine" sound stopped even when burning 87 octane, and it has not returned under any conditions I have encountered since.

I wonder if the tornado was a sufficient intake restriction to alter the air-fuel ratio a little bit, making it less susceptible to detonation? Interesting...

I have a question. Does it matter if you put in 93 and then halfway through, you put 87? For some reason, I remember hearing that you should empty out the gas tank and then fill it up with another octane if you're gonna switch to a different octane? Fact or fiction?

Fiction. We (Navy) mix different grades of jet fuel in our planes all the time with turbines that have much more finicky tastes than a Mazda rotary, and these varying fuels have widely different flash points from each other. If you have half a tank of 91 and half a tank of 87, you've effectively diluted the one half and enriched the other for a tank of 89. Molecules will mix around regardless of octane rating for that particular gasoline.

BTW, FWIW I'm running 89 with no issues or problems whatsoever. It's not as expensive as the 91/93 my old car required and doesn't carry any possible side effects that only a very few have experienced so far on 87 (and I do mean very few). I'm just splitting the difference, and so far things are great. I figure once every 5-7 tanks I'll find a good detergent 91/93 to run just to keep things clean, but in the meantime as others have said there's no point in giving away good money for octane that isn't necessary. Note - this isn't the cheap bastard in me talking but rather a simple matter of chemistry.

I believe that is fiction, but I'm not a petroleum engineer.

Hey, I see you are in Upland. Hi, neighbour!
The Shell station on Mountain Ave. and Eighth St. (next to Home Depot) had pretty good prices today, but they were packed.

Hello everyone,

I believe this info will be alot of help to you

i have a service technician that works here, which worked for Mazda in Japan.

He has top of the line experience with Rotary Engines. He can seriously take apart an engine and put it back together in 2 hours right in front of you... I wish he can get on here and talk to you guys direclty, but his english is not that great..

Anyways he has suggested that for American Rx8 owners to do this..

3/4 of your gas usage, use 87 octane

1/4 of your gas usage use, 93 (premium)

What this means is , lets say you fill up 4 tims a month. Of the 4 times, fill it up 3 times with regular.. on the 4th time, fill it up with premium.

He tells me that rotary is obviously not a piston engine, therefore it doesnt need the prem. gas. Hwoever you use the prem gas to clean out the engine . of course if you fill up only 3 times a month or so forth, try to work out the figrues but thats the general ratio

Be consistent and use the same gas company, hope this helps . I have to tell you that all of us guys here, all do that and we have great mileage and no flooding issues at all

Good luck

Z

one mroe thing, DO NOT USE SYNTHETIC OIL !

[forget it]

relax dude, give people more credit to thinking for themselves..

people who "blindly follow" are just as guilty as those who spread disinformation.

If somebody wants to say something with the intention of helping others, let them say it and leave it to the individual to decide whether or not the advise is legit.

unless you're proclaiming yourself an expert on this stuff, their word is just as good as yours, so chill out.

Here is an article you might find interesting from sindicate writers also known as Click and Clack:


Dear Tom and Ray:


What is the formula on the gas pump all about? (R+M)/2 -- what's the R? What's the M? -- James

Tom: It's all about octane, James. Octane is a hydrocarbon that, when burned in an engine, has a very high resistance to engine knock, or pinging.

Ray: But since it's very expensive, you never actually burn octane in your engine. The gasolines we use contain no octane. The octane rating simply measures how closely a gasoline compares to pure octane in suppressing knock.

Tom: So a fuel rated 93 octane will resist knock and pinging like a mixture that's 93 percent pure octane.

Ray: There are two different methods used to determine the octane rating of a fuel. There's "R," which stands for "research." The research octane number (RON) is determined in a lab with a test engine running at 600 rpm, which represents a low-compression, low-knock situation.

Tom: Then there's "M," for "motor." The motor octane number (MON) uses a test engine, also in a lab, at a higher rpm. That's supposed to represent higher-speed, higher-temperature operation, where knock is more likely.

Ray: Why is the second one called "motor" when it's also, technically, "research"? Done in a lab, too? We have no idea. Why not L+H for "low" and "high"? Or S+F for "slow" and "fast"? Or if it's going to be meaningless, why not S+R for "Siegfried" and "Roy"?

Tom: Anyway, the way they get the number on the pump is by averaging "R" and "M." Or, put mathematically, "(R+M)/2." So, if the RON of a fuel is 93 and the MON is 87, the octane rating you see on the pump is 90.

Ray: And remember, all the octane rating tells you is how much knock protection you get. A higher-than-necessary octane rating doesn't keep your engine cleaner, make the car go any faster, make your engine last longer or keep your hairline from receding. It just costs more. So use only the octane required by your manufacturer to prevent knock, and no more.


© 2003 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.


--------------------------------------------------------------------------------

Also found this about octane at altitude.

Tom: And what allows you, Bob, to use lower-than-recommended octane is the altitude in Colorado. Up where you live, the air pressure is lower. That means the pressure in the cylinders is lower, too. So by going up 5,000-plus feet in elevation, you've effectively lowered the compression of your engine, and that means you can use lower-octane fuel without making the engine ping.

Ray: At the altitude where you live, 85 octane is the equivalent of 87 octane at sea level. So, by all means, go ahead and use it, Bob. Along with getting winded more easily, it's one of the fringe benefits of living up there in the mountains.

Wow, I wanna know what ferrett said.

so theres no need for 91+ octane on the 8s?

im considering the 89 grade instead of the 87, to be on the safe side.

I usually use 89. I think it's a good compromise (plus I have stage1 - wouldn't go any lower). Once in a while I'll throw some 93 in there, tho.

nothing outrageous.. he was just sick and tired of hearing people say things he felt was misinformative, and wanted posts like that squelched.

I am so impressed. Plenty of you guys are explaining the technical operation of the rotary & why low octane works on the atmo engines.

Personally, I have been preaching this for years after reading over Curtiss Wright test results from the 70's where they explained the negligable power differences from the lowest octane through to the highest.

The thing to remember is that the rotary engine combusts its fuel in a different manner to a piston engine. Many of you guys are realising this. Again, well done.........

I have run lower octane fuel in all my atmo RX's & high octane in the turbo ones. High octane fuel in a atmo rotary (10:1 compression or not) only helps the petrol station companies bottom line & not your cars.....

REgards

My 8 wouldn't idle when it got below freezing. Talked to my dealer who got up with the area Mazda rep and she (the rep) said to use 87 octane and it worked. No idling problems since. 17mpg city and 20mpg hwy. Also see thread:
https://www.rx8club.com/showthread.p...threadid=18646

I have filled my 3rd and 4th tank with 87 octane now. On my 3rd, I used 87 from Chevron station. On my 4th (current), I used 87 from 76 Station.

BTW, how do you spot when your engine "knocks"? Ever since I had my 8, I always here this "tock" sound in the engine bay while stop at a parking. Finally, I opened the hood and waited for it. To my surprise, it was just the cooling fans activating!

Is there any other way to watch out for a knock? What about from the Tach, does the rpm surge significantly? Please advise.

And last thing... another difference from 91 and 87 is the smell of the exhaust gases. With 91, I always smell unburned fuel. It was like half of the injected fuel just went out without burning at all. With 87, no more funny smells.

my heads about to explode, so confused...

so, should i stick with the 91+ octane that states in the manual OR use the 87 like other fellow rxers???

Wondering how much temperature plays a part in this? In Texas would it make sense to run 87 during the coldest part of the year and 89 during the summer?

Why not try it for yourself? I'm happily running with 87 octane. If you don't like it, switch back.

BTW, the manual states: "You may use a regular unleaded fuel with Octane Rating from 87 to 90 (91 to 95 RON) but this will slightly reduce performance."

rx8cited

My dealer in (Earnhardt in AZ) has said to run 87. I have since the 2nd tank I have 2000 miles now. And it has run better at idle with no performance difference.

WOW this thread is still going! To add to the info, the dealer technician told me to use 87 octane gas when I mentioned occasional idle roughness. I told him I already used it.

I don't know about your owner's manual, but mine says explicitly that 90 octane is the lowest to run in the 8. It's a high compression engine and it needs the extra octane. Running high octane in a low compression engine will burn hotter and extended use can reduce engine life. In our 8's, though, the performance of the engine will suffer if you use gas w/ an octane of less than 90, and isn't perfornamce why we bought these cars?!?!?

Killer...Oh good grief, didn't you read the first 8 pages??, Geeez! I think Mazda didn't really know, still don't what exact Octane rating should be used...it was still a new engine when they wrote it.

Also, they changed the flashed the ECU before import so it would burn a richer mixture to pass future emissions standards and that by suggesting higher octane fuel was a way to ensure it would.

I have yet to put higher than 90 octane in it and it has never run or idled rough, the gas mileage depends on how I drive it, it's never got over 16 MPG yet and there are no pings.

Killerking1964,

Please quote your manual and version - I'm interested in exactly what it says, not your interpretation.

Why is Mazda telling people to use 87 octane if it's bad for the engine? Ummm, they want us to buy a new RX-8 in a few years ...... yeah, that's it :).

Many of us are quite happy with the perfomance we get with 87 octane, it's cheaper, in some cases the engines run better than 90+ octane, and Mazda says it's okay to use it. Okay, we know you'll blow us away with your high octane fueled RX-8, but that's alright :).

regards,
rx8cited

I checked the manual for you guys, and even contacted our service tech guys, which also own and maintain their own rx8's

They all use 87 octane gas for the reason of Rotary Engine does not reuire a high octane premium fuel

However they use premium fuel once every month or so, to clean up the engine.

From what the say and being one of them is a Rotary expert, I would trust their word. I am also currently using 87 octane and I see no loss of power , and in fact.. I see an increase in mileage..

Good luck guys

z

im trusting your word:cool:

Ok... this analogy isn't totally spot-on, since we're not talking about a Renesis.

But...

I guy I know who runs well in GT3 on a National level, also runs on a shoestring budget. I'm not sure what he's running now, because there were concerns about passing "fuel tests" with oxygenated fuel.

But... for years he ran his full-on Drummond race motors on 87 octane from the local gas station. Had no problems.

Again... not a Renesis (actually as 12-A), but just showing how a highly-tuned rotary can run on low-octane gas.

FWIW...

I been using 93 since i bought my RX, never had a problem, Why i will put cheap stuff in my brand New car?? Is like buying a Brand New house and putting Cheap Paint that will Fade in 3 Months....Now for the experts here, and theres alot of those, 5w-20 oil is the same cheap or expensive? will you put cheap oil in your car just to save a few bucks now and spend alot more later?

Using higher octane fuel than your engine needs to prevent knock makes about as much sense as offering your car salesman a tip. It's just throwing money away and does not benefit your car one bit. But if it makes you feel good...

Started using 87 octane after I had about 1000 miles on the car. 2 full tanks and no problems. MPG increased from 15.3 to about 17. Maybe a little smoother idle, but that could be my imagination.

MPG seems to be improving as the engine wears in too. :)

On my third rotary car since 1985 - always used low octane 87 to 89 gas without performance loss or detonation IMHO. Using high octane in RX-8 at present but considering changing to low octane when car is broken - 5K at least.

Flooded the car once - and one very hard start - shortly after the flood. Driving and starting well now. Hopefully not an ongoing issue.

The only reason I use 91 in the rx8 vs. 87 in the other N/A rotary car is the higher compression that the Renesis engine has vs. the standard 13B from my '90 rx7. If it were the same compression ratio I'd be definately using 87. Given that it's not, then I take the recommendation in the owners manual and use 91.

no issues with the car to date.

Thats a horrible analogy. 87 gas does not equate to cheap paint. Please learn what octane levels mean before you corrupt the minds of some of the new owners here.

I didnt suggest putting 87 octane fuel to save money, god knows If I wanted to save money.. Id buy a Hybrid Fuel Cell car.. Saving $$ is just a side benefit of using 87 octane fuel vs 91+

If you use 93, thats fine.. continue it but I can almost guarantee you that your not doing anything to improve the performance, reliability, or mileage with such high octane fuel. Rotary is not a piston engine, it does not work the same and does not need such high level octane.

but as someone stated here, its a mental issue.. you think cuz you put the highest octane, you think ure feeding it Red Bull juice.. Thats the results of great Marketing

Z

Guys,

I also wanted to suggest to you that though I dont think MAzda did this on purpose, but if you notice in general alot of manufacters tend to always suggest higher octane fuels for your car..

I believe Honda, is probably one of the first makers that specifically designed the new primeline models (Accords..) to use regular (87) fuel..

If you compare to Camry, Maxima.. they all want 91+

I think in the eyes of the car manufacters, its an industry mental standard that a buyer would feel their car is "better" because it uses higher gas..

I know this sounds crazy, but im telling you.. it works. Do you know how many times, I tell people.. oh the 6 speed accord coupe, all it needs is 87. and they say" Wow it must be slow then.

After a demo test drive, they realize thats obviously not the case..

my cynicism leads me to the following unvalidated, biased, but probably true criticism:

car companies conspire with oil companies to milk consumers

true or not true?

I don't believe there is conspiracy amongst these groups; obviously they are both about profit. If that were the case for example, they would encourage more oil changes and not suggest that everything is rosey stretching to 7500miles.
The auto manufacturers are often struggling to stay afloat, far less make a profit in this current world economy. They both have their strategies for money making and are influenced by many factors including world and market specific environmental concerns. They must communicate and co-operate; but conspire?

Umm...my family has had 3 generations of camry's, none of them suggest premium, only regular. As we leave the lot, the dealer even tells us, don't put premium, regular is actually better for camry engines. Honda accord, the new one, I went Honda about a year ago to look at the new two door accord, dealer says regular gas works just fine and so does the sticker.

By the way putting regular gas it doesn't affect performance instantly. The question is whether or not it'll affect your engine in the long run. That's like saying if I eat a Carl's Jr. double bacon cheeseburger today, I'm gonna feel like crap. No, it only happens after years of eating that shit, and you have a heart attack later. Same thing, just cuz you put regular gas a couple of times doesn't mean you feel a performance change right away, it'll take some time for residue build up to take place. The question is, does that happen to a rotary engine with just regular gas?

Exactly ........ show me the scientific evidence ........ otherwise its just hearsay.

I am currently using 89 myself. I was using 91 previously but feel it is overkill as well, especially when the car is blatantly running too rich. Last thing I need is for slower burning petrol. I should probably drop down to 87.

I was reading an article on using lower then recommended octane and the engine retards the ignition to handle the lower octance hence the reduced performance. However he went on to say if the retarding(?) system should fail you could have major engine damage happen to your car from detonation. I am not sure if this applies to our cars but wanted to offer this info.