I seem to be getting better gas mileage using 91 octane in my car? When I use 91 octane I normally get around 24-26mpg and when I use 87 octane, I typically get around 22-23mpg. Has any one else experienced this? I have read that it is recommended for you to use the lowest octane rating possible? Is it okay, for me to use 91 octane?

Mike

 

Always used 91 in my car...but never got 24-26mgs...I could only get 20-21 on the open road

 

I think you're trying to propagate an old rumor among piston heads.

There is absolutely NOTHING in higher octane that will give your car better gas mileage.

Sure, it's okay to put higher octane gas in your RX7. It isn't going to hurt anything but your wallet.

On the converse of that, putting LOWER octane offers ABSOLUTELY ZERO performance gain (OR LOSS!) over using a higher octane.

I guess you could say that putting higher octane gas in an NA rotary is like using "name brand" pepper (McCormik for example) over Albertsons Pepper. (the albertson's pepper being the lower octane). You won't see any real difference in common useage (other than draw on your wallet).

 

Yah, that is exactly what I am trying to accomplish?

This has been my observation, I get better gas mileage using 91 than when I use 87, thats all.

I just wanted some opinions and if others had experience the same thing?

Mike

 

My ranger spark knocks on 87, and gets worse gas mileage with 87. If I run 89 it runs fine and seems to get better gas mileage. I can tell no difference between 89 and 92 though. And in the rx7 it seems to get the same mileage no matter what octane it is.

 

Rumor? Not quite. Some of us with piston engined cars get much better gas mileage with higher octane fuel. I don't think the reasons apply with a first gen (The ECU in my CLK checks for knock and retards the timing until there is no knock. Aside from saving the slugs, this results in substantially lower fuel economy), but it is not hearsay. Lastly, my 89 GXL got much better fuel economy with 91 octane fuel. Of course, my GXL would knock on 87 in 100+ degree weather with the AC on too, which is a bit abnormal.

 

Typically,N/A rotaries have low enough compression that they dont need premium fuel.If the engine is in sound shape and runs fine on 87,then theres no advantage to using premium(performance,economy or otherwise)

If you have a turbo,super,high compression or high carbon buildup engine,then theres a much greater chance your going to detonate on 87 octane.When you detonate,you lose power and thus economy,not to mention hurting your engine.If premium can eliminate the knock,your ecomony will return,but at higher dollor cost to you.
If you want a high performance engine,you should pay for premium's "cusion" of protection against knock.
If you have a stock/mild/tired engine and are using premium to mask an underlying problem(carbon buildup,lack of maintenence,oil blowby),your just delaying the innevitable.

 

Marcus_F,

was that a turbo or non-turbo?

I fully understand that octane alone does not create more power. I understand that there is nothing added to the 91 octane that will increase gas mileage.

However, in my RX-7, I have noticed a difference between the two octane ratings.

I have not changed the plugs yet and don't appear to have been changed for a while. I know the plugs (if bad) would have an effect on gas mileage. Could this have an effect on the different octane ratings producing different results?

Mike

 

The car has 87,000 miles on it now, all stock. The car runs very efficient right now(it smogged great) and does not seem to show any signs of problems. I have installed after the smog check, a new air filter(k&n), fuel filter and I am trying to find time to install the new spark plugs(NGK's).

Mike

 

Non-turbo Mike. The Rx-7 pinged because of my upgraded ECU stupid HP trick. (I've done a number of stupis HP tricks) The theory behind the ECU upgrade was to advance the timing earlier, but not beyond the total advance factory specs. Pings like crazy below 3K. 91 octane seems to cure what ails it. I'm guessing the Mazda ignition engineers know a lot more about timing than I do.

 

My carbed 12A ran a lot better and did get better mileage on 98 octane. Fact.
Now that the car is fuel injected I find it runs best on 95 octane. Dont know if the octane has much to do with it, rather the quality that gives a better burn.

My thoughts Sue

 

Sue, what 'method' are you guys using in Australia?

I know it's probably different than here in the States

 

98 and 95 octane? crazy Aussies with their goofy octane ratings ;-). I read somewhere that RB won the IMSA series back in the early 80's on 80 octane fuel. I've always thought that a lower octane fuel would run better and give you more power in a NA rotary than running a higher octane, reason being the long and narrow combustion chamber hinders the flame front. my understanding of Octane in gas (albeit a very shallow view of it) is how fast it burns. The higher the octane, the slower it burns, making it harder to detonate with high octane fuels. a high octane fuel, in turn, would have a hard time burning completely in the rotor housing, thus giving you lower hp numbers, and since it has a hard time burning, it would hinder performance, and give you worse gas mileage. that's my two sense worth, but it's also 2:20 am and my mind isn't working all too well, so if this makes no sense, chalk it up to sleep deprivation and The Captain.

 

Octane is actually a thing,not just a rating.
In petroleum distillation,the type of product you get is determined by its carbon chain.Octane is actually a product thats a little above gasoline.
100 is Octane and 0 is Heptane. Modern automotive gasoline falls in the middle,around 80-90's.The higher the octane rating,the more compression impact is needed to spontaneously ignite the fuel.Heptane will ignite on its own at even the slightest amount of pressure,thus making it unsuitible for an auto fuel.Thats why higher octane rated gas is better for high compression and/or forced induction engines.The fuel is more stable and less likely to self-ignite before the sparkplug sets it off.In a low compression engine,theres less pressure on the fuel/air mixture,thus less reason to need high octane fuel.
Back in the old days,most flathead engines(which were very low compression) would run just fine on octane ratings down in the 20-30 range.No way you could get by on that today,but look at how much more power can now be pulled from the same displacement,by simply upping the compression ratio and increasing fuel stability.

And they say you cant learn anything watching TV.................

 

RON vs MON vs Average...

I think most places use Research Octane Number (RON)...
I don't think anyone uses Motor Octane Number traditionally.

In the states, it's averaged (RON + MON) / 2 to get the octane advertised at the pump.

 

Actually, the funny thing is, when I run my Audi on 93 (which the manufacturer recommends), I get 19-21 mpg. When I run it on 89, I get almost 25 mpg. Go figure.

 

^^^^^^

The exact opposite! But the Audi is a piston engine, that’s not fair?

Someone in this thread had posted if the engine was not running properly the car might run better on higher octane. This could be the case, I don’t know, my car seems to run very good?

Would old spark plugs make higher octane run more efficient in the rotary?

I will continue my little experiment and see if my results change?

Mike

 

Anything thats foreign to the combustion chamber,or severly worn out, can cause preignition in the engine,resulting in knock.Also,too much ignition advance is a sure way to knock,since you no longer have to worry about the glowing items in the chamber occasionally pre-lighting the mix.The premature spark will light it off early everytime and the engine will knock incessantly.
Running premium will mask the problem and return your economy because its less likely to pre-ignite when a glowing piece of carbon or a super worn out sparkplug tries to set it off.This is a bandaid cure of course,the real fix is to replace the worn items,clean the carbon out and/or rebuild the engine.
Worn oil rings/seals can contribute to detonation as well.When oil mist is introduced into the combustion,is basically contaminates the fuel charge.Being full of volitile combustion by products and carbon,it effectively lowers the fuel's octane level.Yet another reason to keep your oil changed and clean.