Young_Gun

Well, I was tight on cash recently so I was waiting as long as possible to fill up.

I normally run 89 octane - middle grade - and thats all I have ever put in it since I got it in late October

The car is still running with all stock computer settings.

When I finally decided to get gas, it was late and night and the end of the week. I pulled into a gas station - and they were out of everything but 91 octane - so I filled up with that.

The car runs like a new machine. My throttle response has gotten noticeably better, I'm getting better MPG's, and the engine sounds like its running cleaner.

But I can't afford to fill up on 91 every time.

My friend said that when I go back to 89 octane, it could cause some problems? Can anyone verify?

Thanks

Avalanch3

Well honestly I don't really see how using 91 octane on a stock CAI and stock tune can improve anything. The car is tuned to run 87 octane and 87 octane only. I mean it won't hurt the switch from 89 to 91 to 89, but if I were you I'd stick to the cheaper 87 gas if you can't afford 91.

If you really want to use 89, order a tune for 89 octane (with stock CAI or aftermarket)

Young_Gun

Then maybe when I bought it the guy had it tuned to run on better than 87? It is - without a doubt - running on a "soccer mom" tune - but when I put in 91, there was a certain difference in engine sound, somewhat better throttle response, and a highly noticeable increase in MPGs

Nuke

Could be the placebo affect. My car runs better after I wash it.

If your car was tuned for 91, it would likely ping with 89. The PCM in the '05 thru '09s (with the exception of the Bullit) can't compensate enough for higher octane which is why running a higher octane fuel than what you're tuned for is useless.

It is possible that the previous owner had it tuned. I guess you'll have to experiment. But that'll take quite a few cycles of running 87 vs 91 (at least that's how I'd do it).

You mention better gas mileage. Unless you've already run about 4-6 tanks of fuel with 91 octane, there's no way that you can determine better gas mileage. There are so many variables related to fuel mileage that 1 or 2 tanks just doesn't mean squat.

blkside

Going back to 89 will not do anything to your engine. Going back to 87 will however put money back in your wallet. Changing gas will not improve anything in your vehicle to a noticeable effect without getting the ecu tuned for it.
And how does an engine sound like its running cleaner?

Young_Gun

Its a bit quieter

JimC

More than likely your car is running better - after all it is much lighter without all the extra cash in your wallet!

It has been tested - unless you tune your car for the higher octane the only thing going with the higher octane does is make your wallet lighter. It is a placebo effect like mentioned; my car always feels better after I wash and wax it.

kevinmalec

Maybe the 89 gas you had in it was crappy, hence making the 91 feel a lot better. Gas prices are ridiculous right now and I'm sticking with my 87 custom tune, which feels a heck of a lot better than stock 87.

Avalanch3

Here in cali, 91 octane is near 3.80+

I can't bear giving up my 91 tune though and going back to the dreaded stock tune ._.

Young_Gun

I would LOVE 3.80 91 octane

Ehhh. Gas is usually cheaper along the coasts and the north and south west