type of gas
Professional
Joined: Aug 2007
Posts: 158
Likes: 1
From: St. Paul, MN
RE: type of gas
Higher octane fuel is less volatile, and thus burns slower. The only time you would need this is when your engine is running a high compression ratio, a particular advance curve, or an air/fuel ratio that would require it. In the case of the Hemi, if they say use 89, that is what the factory tune and compression ratio are calibrated for. The only good reason to run 91 would be if you are running an aftermarket performance tune, which advances the timing to the point where you would get detonation, or have performance suffer when knock sensors retard the timing to avoid said detonation.
With older built cars, the compression ratio is usually high and power gains are big enough by running more advance, so the premium is worth it. Newer electronically managed systems don't benefit quite as much, as the controls just throttle back the timing a little if there is detonation. Most people that buy 91 octane don't need it. Anyone getting a 2mpg increase from it was either running the wrong octane all along, or more likely, got it from a source that doesn't blend or oxygenate their 91 octane fuels.
With older built cars, the compression ratio is usually high and power gains are big enough by running more advance, so the premium is worth it. Newer electronically managed systems don't benefit quite as much, as the controls just throttle back the timing a little if there is detonation. Most people that buy 91 octane don't need it. Anyone getting a 2mpg increase from it was either running the wrong octane all along, or more likely, got it from a source that doesn't blend or oxygenate their 91 octane fuels.
Administrator
Joined: Jul 2007
Posts: 24,686
Likes: 21
From: South Georgia/East Florida
RE: type of gas
I run 93 in my Hemi, but I have it tuned for performance and have to run 91+. Always ran 89 before the tuner. My last truck was a chevy 5.3 and called for 87, it knocked on extreme up hills towing my boat with 87 and didn't with 89, so I started always using 89 in it.
My rule of thumb with my daily driver (4 cyl. econo type 5 speed) and many vehicles before it that called for 87 octane was I used 87 in a name brand, if I got gas at a cheapy no-name station (like on vacation & it was just off the exit ramp) I'd stick 89 in it. No real scientific reason, it's just what I've always done.
I also throw a bottle of some kind of fuel additive (detergent, octane boost, injector cleaner, etc.) in it every 3000 miles or so.
My rule of thumb with my daily driver (4 cyl. econo type 5 speed) and many vehicles before it that called for 87 octane was I used 87 in a name brand, if I got gas at a cheapy no-name station (like on vacation & it was just off the exit ramp) I'd stick 89 in it. No real scientific reason, it's just what I've always done.
I also throw a bottle of some kind of fuel additive (detergent, octane boost, injector cleaner, etc.) in it every 3000 miles or so.
Joined: Sep 2007
Posts: 39
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From:
RE: type of gas
I have a 4.7 liter in my 04 dodge ram quad cab. I was wondering will it make my engine last or run any better if I use 89 octane. Just wondering if it is anybetter for the engine. Sure this question has been beat to death just got this truck so I am trying to take good care of her.
