Fuel octane


#1

2015 Jeep Cherokee, 3.2L V6, 271 HP. The owner’s manual specifies 89 octane gas for this engine. After much research (the dealer had no clue) I discovered the compression ratio for this engine is 11.7:1. Seems to me this would call for 91 octane. Would there be any benefit in this case in using the top octane gasoline?


#2

Yes, the station will make more money . It also seems the people who wrote the manual just might actually know what they are talking about.


#3

Try it and see. I think any benefit you experience would be a placebo effect, or self-fulfilling prophecy, but it’s your call.


#4

Engines have much higher compression ratios than they did years ago, and still can use regular. All that computer modelling and computer controls pays off!

Mazda has 13:1 and still uses regular.


#5

The vehicle was certified to operate on 87 octane but 89 octane is recommended. If you do a lot of full throttle driving you may notice an improvement with premium fuel.

I use premium fuel in my old 4 cylinder Dodge when the weather is above 100 degrees, in hot weather and city driving the power drops off noticeably. The high temperature this week is 99 degrees.


#6

Thank you for your responses. @texases, I knew that about the Mazdas. I was called a liar on an internet chat when I said that, until I copied and pasted the engine specs from the Mazda website. @shanonia, I think you’re probably right. @Nevada_545, I started by just using 87 octane and didn’t noticed any problems, then switched to 89 octane when I found that tidbit in the owner’s manual (which is some 500 pages). I tried 91 octane once but didn’t notice any difference. @VOLVO_V70, you’re probably right as well about the authors of the owner’s manual and you’re definitely right about the service station owners.

Back in the old days, it was a lot easier to know when you needed to upgrade your fuel selection because you’d get engine knock. With all the computers they use nowadays, you’d probably have to use something like kerosene in order to get engine knock (if the engine would run at all).


#7

You didn’t state the type of use for this vehicle. Ten years ago I would drive full throttle up certain hills in the sand dunes with my V-6 CJ-5 to show off in front of the 4.0 liter Wrangler boys, they couldn’t clear the hill. If you are not engaging in any extreme activities 87 octane should be sufficient, you don’t need premium fuel to transport a cup of coffee to your place of work.


#8

GDI engine typically have very high compression yet still run on 87 octane. Apparently introducing the fuel directly into the combustion chamber creates an evaporative effect that cools things down allows for greater compression. Mazda’s Skyactive engines employ this. It’s also very effective in turbocharged engines, allowing for relatively high compression even with forced induction.