I have spent the last few days tuning my car on some 94 octane fuel that a certain station near me sells. The car has felt awesome.

Well around lunch today I felt I had I the tune dialed in, I was also on my gas light. So I stopped and put some 92 Chevron in because I was too far away form my 94 pump. I then logged some pulls and was really surprised. Not only did the car feel slow, like it was pulling timing, but when I reviewed the logs the AFR was nearly a point lower.

I still had no real knock, and the timing was unchanged, but the car felt massively slower and record lower AFR's. Anyone else see this happen.

 

well u have two problems:

1) your timing will need to be retuned for 92oct, what works for 94 wont work for 92.

2) different fuels have different amounts of ethanol, this would have a slight affect on the AFR but probably not a full point. What are your idle trims like?

 

Could just be some really garbage gas.
It might not JUST be the octane difference you are dealing with.

Kinda like Buschurs Valero problems with people coming in on the interstate and having nothing but trouble VS the 93 he uses locally.

Personally - either tune very conservatively. Or tune on the highest octane that you can get all the time. Luckily I never stray far enough away I can't find Sunoco 93.

 

He switched from Cenex 94 which is at least E20. I have seen AFR go from 11.2 to 12.0 on that stuff no other changes. Its not bad gas, just requires a different tuning strategy. FWIW I have tuned 8's with the aforementioned 12 AFR and 18 degrees at redline (6* at peak torque) at 24psi with 0 knock including roll in/tip in.

 

Just to add what other's have said: AFR's will change with different fuels because of changes in specific gravity of the fuel, as well. Usually, between pump gases, this isn't an issue, so that most likely isn't your problem. My guess is the ethanol content difference, as many have said.


Eric

 

That's exactly what I was thinking, and I was hoping that you had some experience with that Cenex station as well. I thought to myself "Cenex must have pretty high alcohol content to have made my AFR's change so much". I even went by the pump last night to check for an alcohol content label at the pump and found nothing.

Now I need to decide on whether I'm going to base my tune around the Cenex 94, or just the standard get it anywhere 92...

 

It's gotta be the corn content.