I did a search about this subject but couldn't find any threads on it, so I figured I'd ask here:

Is there any appreciable difference between running premium versus regular gas? Are the effects on fuel mileage? On performance? On engine life? I know there is a cost difference (about 30 cents a gallon in my area), but I am wondering if the cost difference is justified.

I ask because the fuel filler door says Premium Recommended. I know some vehicles recommend it but do just fine on regular, but I also know a rotary is a different beast from a piston engine.

Thanks in advance!

 

Yes, engine life.

An engine that is dying and has low compression on ALL 6 faces of the engine and is already performing terribly in power and mileage will be safe to run on 87.

An engine with even one face at good compression, 87 octane will blow your seals from detonation. 89 CAN work for high altitudes, hot temperatures, but it isn't recommended. 91+ is really required.


Mileage and power are the same as long as the octane is high enough to be safe for the engine.

 

My last motor ran great on 87 octane.
Then I got a compression test, 5.3 5.2 5.1 front rotor and 4.7 4.6 4.4 rear rotor @ 251 rpm.. which kinda explains why

 

Okay. So the moral is to run premium unless the engine is dying. Got it.
I was thinking about running a tank each of premium, mid-grade, and regular, but I'll nix that idea now.

Thank you.

 

Keep in mind that you could have a dying engine with 7s in the front rotor and 5s in the rear rotor. Running 87 would blow the seals in the front rotor and shred what you had left in the engine. All you have to have is 1 face anywhere near proper compression and you are playing Russian roulette with octane.

 

search "octane"

 

i've run these cars on 87-89-and 91, and if its just going down the freeway i'll put 87 in it, and it doesn't seem to make a difference, but for anything else i actually get better mileage with the 91.

in fact i almost started a thread about it, 2mpg in the city is a HUGE difference.

i think if you factor in the MPG into the fuel cost, the 2mpg probably makes the 91 more cost effective

 

I'd guess that the difference is almost certainly because the ECU is yanking timing trying to save your engine with 87.

 

i think so too. effect is more pronounced in city driving, where there are accelerations

 

So, it'll cost me a little more in gas to run premium, but I figured it out to be less than $20 a month versus regular, which isn't much...

As the saying goes, nothing is too good for my girl.

 

$20 a month or $2,500 tomorrow for a half decent rebuild. Pick your evil

 

xero, that was my line of thinking, too. $20, I can handle. $2500, not so much.

 

I always use 89 Chevron with techron. The techron helps clean the fuel system as well. Never had serious issues so far using 89 octane...

 

 

Vegas is higher altitude and hotter temps (compared to temperate at sea level), so thinner air from both reduces the dynamic compression a bit and makes 89 safer to use.

 

Read what's down here...

 

For the record since 2008 (my RX-8 purchase) I use nothing else but Aussie High Premium 98 (RON) which is the equivalent to your US 91-92 (I think) for both my Mazda's, RX-8 and MX-5 (yes it is a banger).

I just wont use our 91 or 95 (RON).

It is a fact that for todays banger engines the Premium Fuel or higher grade 95 or 98 (RON) than normal fuels which makers recommend may cost you more but results in higher MPG...or Kilometers per litre.

End result it is actually cheaper to use the more expensive Premium Fuel..

Recently an Aussie test (2013) with Kia's proved this to be correct, all the same new cars running on different grades of petroleum fuel (gasoline), plus one diesel engined model running on diesel, only the diesel engined model achieved a higher KPL result.

Now as far as a rotary goes, well a normal Renny (10:1) is asked by Mazda to run on Premium fuel, however as RIWWP says if you have poor compressions you may get more than you wish for, but frankly if it is that bad a lower grade gas will not save your Renny..