Is it really important to get 91 prenuim?
 

Them gas price...

It's important to AT LEAST use 89 octane. Anything less and you are risking premature engine failure. Gas prices, or engine prices choose your pick.

Why stop at 89? Why not get 91? That's what Mazda recommends. They make the car.

There were posts a few years back by people who said they got better gas mileage on 87 than 91. RIWWP had some interesting posts where he pointed out that, since octane need relates to compression, a worn engine with low compression might run OK on 87 but a healthy engine really needs 91.

Rotaries may be different, but in general when you use lower octane than recommended timing gets retarded based on what the knock sensor detects. Personally, I'd rather run an engine on its design curve rather than against the knock sensor.

Ken

Under the assumption that you drive 12,000 miles per year at an average 19 MPG:

You would consume 632 gallons of gas each year. Assuming that 87 is constant at $4/gallon and 91 is the usual $0.20 more than 87 (i.e. $4.20/gallon) the difference between filling up with 87 all year long vs 91 is $126/year. Which breaks down to $10.50 extra a month. (And obviously none of this includes the price of the oil consumed by the engine :lol:.)

While I admit I'm not that well versed in mechanics, I would rather pay $11/month extra than retard the timing, and potentially risk apex seals (particularly with unknown carbon deposits quantities and locations).

Series 1 RX8's have only 1 knock sensor on the rear rotor, which I've seen people call it both "worthless" and "up to the task" depending on the tread. I can't seem to find a definitive answer to its suitability for the task :banghead:.

I figure it's only $4-5 a tank more if that so why risk it? Especially when Mazda recommends the 91

Ive ran quite a bit of 87 through mine 15-20 tanks. Only because i had no other choice during hurricane season. My first engine lasted 138k.

Theres no real way to know for sure, but i dont think it will hurt. Boost basically increases compression levels and most people running boost only use 91 or 93.

I ran for 9.5 years on 87 octane, ~85k miles on that gas.
My compression test at 88k miles when I had it checked was well within spec (lowest face was 8.8, highest was 9.1) on a mazda rotary test tool (250rpm).
I never once had issues with pinging and very rarely had misfires and when I did they were related to bad coils or plugs.

Not all engines will handle gas like this.
I've heard of others who can't run 87 because they get pre-ignition.

I ran 87 octane for 60k km..engine now have 160k km on it..stock

I don't track and i DON'T "redline a day"..

No hot start/ cold start issue..

Premix every fill-up..

Octane is a measure of resistance to combustion. They say to use 91 because rotaries typically run hot, and that should give some extra insurance to pre ignition.

This is kind of the wrong car to think about gas prices. You might save $4 by not buying premium, but you still pay $80 to go not very far.

I just purchased an 05 rx8 and I believe the previous owner was running 87 unleaded. To swap to 91 octane would you recommend draining the fuel system first or run the tank to almost empty and fill it with 91?

All octanes are compatible. You can drain and re-fill if you want, but it typically isn't that easy for most people. If you have 87 in the tank, keep the RPMs low and avoid full throttle until you can get at least 91 in there, if not better. Half a tank of 87 mixed with half a tank of 91 is roughly 89 octane, which still isn't really safe enough.

A healthy renesis can not safely run on 89 octane, and 87 octane can be dangerous to the point of sudden engine destruction from detonation. Yes, it has happened plenty through owner or dealer ignorance. 89 octane is only 'safe' if your engine is already low compression. 87 is only 'safe' if your engine is almost completely dead.


Basically, because octane is a resistance to detonation, and a higher compression engine heats the air more, making detonation more likely, higher octane is needed in higher compression engines. However, when the Renesis is failing, it is fundamentally lower compression (they do a compression test to determine pass/fail after all), so a weakening engine can run at lower and lower octane. A perfectly healthy Renesis with compression scores in the 8s will shatter it's seals if you go full throttle high RPM on 87 octane at sea level.

Keep in mind that the highest compression face you have in the engine is the real determining factor here. You can have 1 dead rotor and the other rotor with 2 dead faces and 1 perfectly good face, and that perfectly good face will blow it's seals from too low of octane, sending seal shrapnel through the rest of the rotor and housing.


So get over to the highest octane that you can until you have fully converted, then shift down to 91 octane if you want (it's safe on a good engine), and you may want to have a compression test done to see how poorly off your engine is to have been running 87.

Quote:

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Originally Posted by ken-x8 (Post 4604264)

Why stop at 89? Why not get 91? That's what Mazda recommends. They make the car.

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I'm stopping at 89 for the low point and would not advice to go any lower, I didn't say not to use anything above 89.
"You may use a regular unleaded fuel with Octane Rating from 87 to 90 (91 to 95 RON) but
this will slightly reduce performance."

Of course we know using anything lower than 91 isn't good but 89 is where I would draw the cutoff point, I wouldn't use anything below that.



Wanna know what sucks? Difference between 87 and 91 here is about... $0.50 on average. Granted 91 octane is only about $3.70 something but still, ridiculous gap in pricing.

Thanks ill look into it. Can any place run a compression test on the vehicle?

No. It needs to be a place with a rotary engine compression tester. Most Mazda dealers have them, most independant rotary-specific shops have them (but there are only a handful around the US, so don't expect to be close to one).

See the new owner's thread for why you need a rotary engine compression tester, why the test should have been done before purchase, and what the scores mean: https://www.rx8club.com/new-member-f...4/#post4533733 (link jumps to the right post on the compression information, allowing you to skip the rest of the critical new-owner information if you don't want to know about that stuff)

I don't understand people who buy an RX-8 and then worry about the price difference between regular and premium gas. As was pointed out in a post in this thread, it's not like it's enough to make or break your bank account every month. I even use higher octane gas in my 2014 Mazda3 daily driver, which gets 33,000 miles put on it a year, just because most recent model cars will advance engine timing automatically to make extra power on higher octane gas. This was stated explicitly in the 2012 Ford Focus Source Book that I read while waiting for my pre-ordered 2012 Focus Titanium hatchback. My feeling is that if you can't afford an extra $10 or $15 a month for 91-94 octane gas in your RX-8, you probably should have chosen a different, more fuel efficient, car.

I always thought that's what you got when you filled up with 89 at a station that has 87, 89 and 91. The station has two tanks, one with the lowest octane and one with the highest. The pump mixes them. Goes back to the days of Sunoco's "custom blended" gas from 190 to 260.

Ken

I've seen that reference around all over the web, but I have never seen anything to prove that it is actually what happens. I suspect that the truth is somewhere in the middle, like "some pumps do that, other stations have independant tanks"

I put 94 in it always, forever... I really don't car about the extra $4/week, I spend more than that on coffee in a day...

I use 93, which is what they have for premium in my neck of the woods. I don't drink coffee, so I'm probably ahead of the game. :)

Ken

I really don't think the tolerances are that tight that running 87 will just cause the seals to go bad and in fact, I know it won't.

My post above was from memory and my memory was wrong.
I remembered I posted when I had my compression test done on this forum so here's that post:
https://www.rx8club.com/series-i-tec...esting-179414/

Of those 80k miles, all but the first 15-20k were running 87. I redlined that car practically every day and had more problems with the transmission than I ever did with the engine (plugs/coils).

So I don't see how it's possible that for 60k miles, all of which the compression was likely higher than those readings, the engine didn't completely crater, nor was my engine completely dead and hell, it barely ever misfired except when the coils were going bad.

Octane is resistance to combustion and the shape of the combustion chamber in a rotary causes incomplete combustion as it is. Using lower octane gas allows for a more complete burn of fuel, which yields better gas mileage and better power assuming the engine computer is happy with what it's seeing on the sensors and doesn't pull timing.

Now I'm not saying everyone should go out and buy 85 (if you can find it) or that every engine will behave like mine, but the fact is, I ran exclusively 87 for 8.5 years on a VERY healthy engine and had no issues whatsoever and never had my engine replaced or even repaired for any reason.

I don't doubt you, but I am also too wary of taking a single case and applying it across the board. We know that the RX-8 has a knock sensor, and we know that in general it is very effective in pulling timing to save the engine. Does this mean that every knock sensor is equally effective? Is every engine just as easy to save? I don't know the answers to those questions, but in light of the basic danger of too low octane and how most owners have no idea how well their engines will tolerate lower octane, I can't ever provide a positive recommendation to use lower than 89, and quite a bit of reason against it.

As far as my advice about throttle and RPM, yeah, he may be fine. But is it really worth the risk? No, I don't believe so.

The manual says 87 is fine but I wouldn't go under 89.

A full tank of 91 octane also feels like it last longer than anything else.

Them compression test, if I want do one just make sure mine is in good shape, should I go mazda dealer? Are they thrust worthy at doing it? Or should I get a tester and do it my self? No clue...

A factory compression tester costs about $2k new.
If you don't have a shop in your area that works on rotaries, then the dealer may be your only option.
Are they trustworthy? That depends on the dealership...