I was on half asleep and operating on automatic when I filled my tank this morning and before I realized what I was doing I had poured over half a tank of 87 octance gas into my recently aquired rx8. So I had heard that certein cars are ok with lower octane gas but how do you tell? Will the check engine light come on? will the car be prone to detonation if it gets too hot? or will I just get shittier gas mileage and less power?

 

hopefully this is the correct section to post this thread.

 

nothing will happen, if you look in your manual it says 91 is recommended, but if for any reason you are low on gas and the only gas available is 87 octane, then you are okay to use it,

 

^ Yep. Modern ECU's automatically retard the ignition if lower octane is used, to protect the engine from ping (aka knock, pre-detonation, dieseling). Of course you lose some performance as a result.

 

+1 to the comments above. Try not to make a habit out of using low grade gas, but in a pinch you'll be fine.

 

thank you!!

 

You can always add octane booster as well if you are in a pinch at a gas station that only has 87 and/or 89 (most gas stations sell octane booster). I've had to do this before when outside of major cities.

 

Since this conversation came up, is there any benefit to pumping 93 octane in your 8 and then add octane booster to that? Just curious ...

 

NO. As we've cited, using a lower than specified octane will cause the ECU to retard timing and sacrifice some performance, but there's no advantage to you in using higher than required octane (just an advantage to those trying to take your money).

Now, if you have a custom tune that requires 93 you should use it.

 

cool, gas thread and no flames.

 

My husband did this a month ago.
I was driving he was sleeping. I pulled into the station he stumbled out (how chivalrous!) and proceeded to put half a tank of 87 in before he realized what he was doing (woke up)!
All seems fine.
I kept my mouth shut, that helped :^)

 

I always use Chevron 89 octane with techron and it never had no problems so far. No pings and knocks whatsoever. Anyway, if gas gets more cheaper than usual, then i will use the highest octane that is recommended by the manufacturer.

 

You've got an '05, where the owner's manual says 87 is OK at lower performance. Owner's manuals in recent years say something like 87 in an emergency.

No idea if the change in wording came from the engineers or the lawyers.

Ken

 

The previous owner of my '04 8 used 87 exclusively, and swore he had no issues.

So far, I haven't had any issues with 87!

 

I don't have the owner's manual, but it says premium gas on the fuel cover.

 

But your not saving that much money per tank or even per year. Generally its only 10 cents more per grade, so if your filling up a tank your looking at a $3 difference between lowest grade and highest grade when buying 15 gallons of fuel. So if you fill up 1 time a week thats $12 a month and $144 a year.

 

That explains a lot.

You should download one from mazdausa.com. The electronic version is really nice - much handier than the printed version.

Ken

 

For those of you above that have said that the engine gets retarded when you use lower octane. What makes you think that the engine can determine the grade of fuel that you are using? If it could retard the engine, why would Mazda even bother telling you to use a higher octane?

 

The timing gets retarded. I suppose that happens because it reads some of the threads on this forum.

All cars work that way these days. They have a knock sensor. If octane is too low, the engine will start to ping, the knock sensor will sense it, and the ECU will adjust the timing. In the old days if you picked up a tank of bad gas and the engine started to knock, you'd lift the hood and retard it manually. Nowadays they do everything for us.

Retarded timing is not optimum, so it costs power. Again, in the old days guys trying to squeeze the most power out of an engine would advance the timing as much as possible, just limited by knock. If you use the proper octane on a modern engine, you at least get the timing that the engineers designed into it.

Ken

 

Right...and that is my point. In order for the engine to get retarded you must have a knock/ping/whatever in order to get retarded. If this does not occur your car runs just as if it were running on race fuel.

 

Agree

Furthermore it's been proven (so I've read from Jeff) that the knock sensor in the RX8 is worthless and does not work at all.

He has cited that during testing an engine would knock itself to death and the knock sensor would not pick up a thing.

I just read thread the other day ... lemme try and find it.

However that all being said .... I would have to agree that there is no mechanism in the RX8 that physically detects the Octane Level in the Fuel .... ie) like the O2 sensor detects Oxygen in the exhaust stream

Things that do effect timing
Load
RPM
IAT
Baro

edit:
Ok found the thread and the post .... Post#133 in this thread ... https://www.rx8club.com/series-i-tech-garage-22/octane-what-actually-means-does-190858/page6/
Yeah so I didn't quote exactly ... sorry

Another good little article
http://www.motorcycle.com/how-to/wre...tion-3420.html

 

wcs - *whew* glad to know I'm not alone on this one. I'm not crazy...WOOHOO!

If you do run 87 or 89 on the regular, like I do, I would recommend some preventative seafoam activities to help cutdown on the chance of a misfire or knock.

 

If the knock sensor doesn't work, and people running 87 don't hear any knocking, does that mean the 8 really can run on 87? And the premium requirement is just boilerplate from something else, inserted by whoever pasted together the owner's manual?

Ken

 

I believe that Mazda put it in the manual to help control the amount of misfires that would lead to possible engine failure. Once you get a good amount of carbon buildup you run a higher risk of a misfire. With the amount of people that complain about having to check the oil I don't blame them for avoiding the whole fight over having to seafoam at least once a year.

 

I have driven easily a million miles in my (street and race) seven rotaries since 1973 including all three generations of RX7 and now a 2008 40th Anniversary edition RX8, so I have been around the block on this octane issue. Yes in an emergency you can put 87 octane in an RX8, BUT you also risk more than just poor engine performance. You also risk transmission issues as smooth shifting and transmission performance is designed for the engine operating at its optimum, as power is sent to the transmission . And any prolonged use of that 87 octane can lead to engine damage. Don't make a big mistake and think you are safe here. The engine has a higher compression ratio than any NA 13B before it, and it needs that minimum 91 octane to keep from detonation etc.

Drive it gingerly, NO REDLINE at all !! And fill with 93 octane as you use up the 87 octane. It will take a couple of tankfuls of premium before you will get your optimum engine performance safely back. If you really push the engine while using 87 octane, you may be in the market for a new engine. It has happened to RX owners before you !! Mazda tells you that you can use 87 octane in an emergency to keep owners from freaking out when they make a mistake at fill up, thinking they can't drive the car at all. I have done it myself, more than once over the years. The RX8 will continue to drive but you are not doing your engine and transmission any favors. If it didnt make any difference, as some would tell you, then Mazda would tell you to use 87 Octane , and save its RX8 owners loads of money. And the ECU will NOT adjust the spark for the lower octane. It will think you put in the minimum 91 octane like you are supposed to have done.