THEBLACKSTANG

I decided to put 91 octane in the last time I filled up and I've noticed a fairly large difference in how smooth the engine runs. Is this normal? The previous owner installed a S&B intake (no tune required) and at first I thought that maybe they had gotten a tune and I was running the wrong octane. There are no check engine lights on the dash, and I would be having some serious issues if I was running the wrong octane right? Sorry for the newb question.

JimC

Running higher octane than the car is tuned for won't give you serious issues with the car.
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It is probably running smoother because your wallet is so much lighter though.

157dB

Higher Octaines without the proper timing
adjustments is just $$ thrown down the drain.

Nuke

Have you ever noticced how much better your car runs after you wash it, too?

I'm sure you believe that you felt a difference and maybe you really did. But I wouldn't jump to the conclusion that higher octane is what did it. It may be possible that the 87 octane you had was inferior.

Higher octane fuel than what you're tuned for supposedly has no positive impact with the exception of those S197's with the Bullitt tune (NO V6's) where the PCM can make minor timing corrections. Some have even claimed that chronic use of higher octane can cause issues (besides poverty) but for the life of me I can't recall what those are.

Keep experimenting and researching. The choice is yours but you'll find the concensus tell you that you ARE wasting your hard-earned cash.

157dB

The only thing the 91 has going for it is the additional additives
that 87 does not have. Maybe it will clean your injectors that
have had 89 run thru them for so long....

Nuke

And, BTW, running a higher octane than what you're tuned for will NOT cause you to throw a CEL/code. If you were tuned for a higher octane and were using a lower octane, you STILL wouldn't throw a CEL but you'd hear the engine pinging/knocking under most any load.

157dB

Then the knock sensor takes over and the timing is pulled back
to prevent knocking and the power goes out the window persay...

THEBLACKSTANG

This was my main concern, and I didn't want to do any damage to my engine. I know that without a tune I wasn't going to see any gains, which was why I thought this was weird.

and yes nuke, my car runs much better after a wash lol