T-von

Yes depending on your driving habits. If you run the engine as it was intended & and use the full rpm range all the time, then 91 octaine is the best fuel choice. If you granny drive around all the time, it's better to run the lower octaine. Rotary's knock mainly happens in the upper rpm ranges under load (6k and up). If your not driving your car to it's potential, then it's pointlss to run the 91. That's all I'm saying. Think about this, why run a race fuel if your just cruising on the highway? You would actually have less power because race fuel takes more spark to burn. That's a fact.

Lets not forget where talking about a NA rotary here. It's not boosted or anything. My turbo charged Fd will safely run 10psi on 87 octaine has been driven around on 87 octane for thepast year.

T-von

Not! Mazda isn't dumb enough to program the ecu for only one specific fuel grade. 87 is perfectly safe in the Rx8 as it was for my turbo charged 94 Rx7. Do you understand anything about tuning & A/F ratios?

T-von

I disagree see my example below.

When I was running 91 (which is the Mazda recommended fuel for my Fd) when it's really hot out and have the A/C going, my fd sometime would struggle to idle. When the compressor would kicked on is when it happened. Sometimes the engine would even die the hotter it got and I would have difficulty starting it. When I switched over to 87, I never had this problem. The engine didn't bog down as much and it was way more responsive. I also had more power throughout the whole rpm range. This is why I believe 87 is more explosive. If this isn't the case, please explain to me why my car is more responsive? I'm always open to being educated.

AdRoCK3217

Your personal examples do not override chemistry and physics...

nycgps

iTrader: (1)

It depends ...

CarAndDriver

It lives....

zoom44

never been to axial flow?

zoom44

http://www.faqs.org/faqs/autos/gasoline-faq/part3/


if your car tried to die when the load changed on a particular grade of gas then you you had a tuning issue of some kind. i would guess- never having seen your car. id like to find out tho. i ll ask around to others who may have seen this issue before.

T-von

? More info please! I'm not the only one who has used 87 octane in my fd. So it's not just "my personal examples" of why I know it's safe to run in an Rx8. I also speak of the generalized experience of others.

T-von

Elaborate please!

T-von

My Fd was otherwise stock except for down pipe and modified stock air box. The tuning is all factory. There were no issues with my car other than the fact that when it started to near the 100k range is when I started to see these problems. I know the older rotaries will last well into the 200k range, but the 3 peice apex seals in the 86 up models are really near their end when you get in the 100k range. Anyways the fact is 87 octane gave my car (and other Fd owners who have used it) more power in stock form. Regardless of any articles, I'm all about real world testing and experiments.

AdRoCK3217

On similar tunes, as long as you can avoid detonation, lower octane will almost ALWAYS result in higher horsepower numbers. This does not say that it is SAFE to run, however..high boost and high compression applications should always run higher octane fuel, not so much to make more power, but to avoid detonation.

I would say it is fine to run 87 in the humid summer, but downright stupid to run it in a boosted or high compression application in the dense-air winter.


Again, your experience, or even the experience of a few people you know does not overcome physics and chemistry. Individual results will occur.


Discussions like this are why it was once said: The RX7club is the worst thing to ever happen to the RX7. Don't make it happen for the RX8's..

AdRoCK3217

The 3-piece apex seals are NOT near the end of their life around 100k..the small end piece will generally roll out of it's groove upwards of 175k miles..IF it ever rolls out at all..

Around 100k, if your FD was like.."most"..your turbos were on the way out. Your engine was probably taking a dip in compression, due to said turbos baking away at the coolant seals. Because of this loss in compression, you would have less ignition "punch", and you could run slightly lower octane to help combat this.

I'm all about reality.

bottoms up

one of my friends bought a 2007 4runner and had many problems with it and the dealer gave her another suv!! that was also only a few weeks after she bought it though. worth a shot...

bottoms up

i have a 1994 4runner over 200k miles and toyota did alot of recall and warranty work...i didnt even have to pull the bitch card they were very helpful. i love my rx8 but ill swear by toyota for reliabilty (sp?) as well as service. i havent had to deal to much with mazda yet and hopefully wont have to but not alot to go by to compare the two. just gotta put my .02 in for toyota

Icemark

The octane rating is the gas's resistance to detonation. Higher the octane number, the more the air/fuel mixture can be compressed without exploding pre-maturely.

Now with most rotary engines, a lower octane (which is easier pre-ignited by things like a hot spark plug tip) will often produce more power, however you must avoid at all costs detonation (or pre-ignition).

While detonation in a piston motor is not generally destructive in short doses, in a rotary engine it can kill a motor in only a single ill timed event.

Running a boosted or high compression (anything about 9.9:1) rotary motor on low octane such as 86 or 87 RON can easily cause detonation and apex seal failure.

, that would be incorrect. It is not the 3 piece seal that has the problem. Many 86-88 models (turbo or NA) have no issues with the seals. Where the the problem comes from, is with the lighter rotors on the 89+ models. The lighter rotors use a less dense metal, that will deform because of seal movement (often caused by minor detonation). The seal itself can be still in spec just fine... not even half worn out at 100K miles, but if the slot in the rotor has expanded, then the seal can roll out. Using too low of an octane will result in that problem.

AdRoCK3217

Ahh, it always feels good to be backed up by someone with credibility!

T-von

100% agree

In all honesty how could you say it's stupid without your own testing? I've fully boosted my fd (11 primary spiking to 12 psi) in 40degree weather without issues with 87 in the tank. I know for a fact it's safe because, I understand how super rich Mazda's factory tuning is under full load. Because I undertsand this, I know my margin of error. Your basing your opinions on what you think is best without testing. Is it good old fashioned paranoya? I know for a fact there is a ton of that here on this forum form people who really don't know anything about rotarys and there real limitations. Don't take that the wrong way, I'm just talking about people here in general who are really cluless. My 91 vert has 9.7 compression only .03 less than the Rx8 that's not that much differance. If we were talking 12 to 1 compression ratio, then I would agree. 10 isn't that high. It's all in the tuning period.



Go figure. I could easily say the same about this forum. Regardless of what people say, there is a ton more experience on that forum than here. I'm just trying to pass on my personal long term experience/knowledge on to the Rx8 community in hopes that these flooding issuse can be resolved and avoided. The last thing I wanna do is spout off BS info so even more engines can be replaced. Hell am I the only person here that can't stand the fact that Mazda chooses to have the mechanics replace the engines with low vacuum with remans (which are nothing more than someone elses engine that's been opened, cleaned/decarbonized and put back together) just so the next person can granny drive that engine to the point that it also will need to be replaced once again? This pisses me off simply because it gives the rotary a bad reputation of non-reliabilty when the problem is so easily fixed.

Sorry for venting!

T-von

I understand this fully!

Understand this fully also!

Your right but answer me this? Which engine has the potential for much higher knock in a lean condition? 10psi boost rotary or NA renesis.

I have never heard of any situation of a "STOCK" NA rotary loosing an apex seal due to detonation due to fuel selection of a lower octane. However, if it's the old 3piece apex seals with really high mileage, then I wouldn't doubt it.


I'm aware of the change in the casting between those models however, I guess the engine in my 91 vert is a freak of nature. I pulled that engine apart at 146k and my rotor to apex seal grooves where perfectly in spec. I've put nothing but 87 in that car. So why is mine not expanding? I've also bought a total of 6 S5 NA 9.7 compression rotors (3 of which im putting in my 20b) and not one of them has had this expansion you are talking about. The only expansion I've ever seen with any rotor is with the 1st gens (12A and possibly 13b) with the heavier 3mm apex seals.


Lastly I should have elaborated better when I made the statement that the 3 piece apex seals are near their end near the 100k range. Truthfully they are. I didn't mean in terms of wearing out. Regardless of what the specs say, the top peace of the apex seal gets very thin and gets very....very brittle making them extremely weak in this mileage range. They can snap pretty easily. This is what I was meaning. I consider any rotary with 3 piece apex seals with over 100k a ticking time bomb boosted or NA.

T-von

Credibility is earned. It's all about who you know and who you are comfortable getting info from. Just understand that it doesn't always make them right.

zoom44

exactly right. his compression had dropped over time so he was able to run lower octane.

T-von

The reality here is your assumptions are completely wrong about my Fd. Do you actually realize the very small percentage of Fd's that actually have original engines that have over 100k? Were talking in the single digit percentage. It's probably less than 2%. My turbo's are perfectly fine and don't leak any oil. My coolant seals are in perfect working order. Absolutely zero leaks. I understand how to properly maintaine my car. Period!


I do believe my compression was lowering a bit. I already said earlier that 87 helped me out due to my higher mileage. Tell me something I don't already know!

Also the reality is 87 is safe in the Rx8 whether you want to believe that or not. The higher than normal A/F ratios of the engine tuning plus the knock sensors will more than compensate for the lower octane. No one has yet to prove me wrong.

T-von

You really have no clue don't you. There are Fd guys who have been running 87 in the cars early years without issues. Mazda is't dumb enough to program the ecu for strickly only one specific fuel grade. They had to program a margin of error just incase someone did infact put in the lower octane. Otherwise Mazda would have been replacing Fd engines on a regular bases from secondary owners who didn't know better. Not everyone reads their owners manual. This is exactly why the stock A/F ratios are so rich under boost. If lower octane is used, the extra fuel being dumped in will compensates for the lack of octane to help control knock.

zoom44

yes yes yes of course mazda runs em rich- i have a clue and have had for a long time . the clue i needed in the beginning was that you were running stock maps. i read that a post or two ago. at first i assumed you had had some tuning done to the car- hence my "then you you had a tuning issue of some kind" post

T-von

Well now it all makes since. All this time I had been talking about stock Fd's and Rx8's running 87. I didn't realize you had misunderstood one of my post. My fd has a downpipe and modified stock air box w/ K&N filter. I had to install a boost controller to get the boost back to stock levels due to boost creep. With the FD, the so called 3 mod rule is nule and void provided you keep the boost at stock levels with the stock ecu however, I completely understand that it's stupid to run 87 when modified on stock programming.