b'Eight'

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.

i3man

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?

areitu

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.

cruzdreamer

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?

RodsterinFL

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.

skagen

Hey Rod,

Thanks for the info man. I appreciate it. I just don't get why the manual says to use premium **** 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.

b'Eight'

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.

sferrett

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.

bobclevenger

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.

sferrett

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...

JaChTsai

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?

S3/P3/E2

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.

bobclevenger

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.

RX8Z

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

RX8Z

one mroe thing, DO NOT USE SYNTHETIC OIL !

sferrett

[forget it]

Squidward

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.

mdsbuc

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.


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

mdsbuc

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.

SCiMMiA

Wow, I wanna know what ferrett said.

Tirxer

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.

SCiMMiA

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.

Squidward

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

DMRH

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

JD32

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