Lone Ranger

Thought I'd experiment with putting 89 in Cobbs just in case GM was a little aggressive on spark advance in the ECM.

Well, turns out it ran better on 87. Unprecise proof that the GM factory tune on the 2.2 is pretty dialed-in for 87 reg unleaded. As we know, using a higher octane in an engine tuned for 87 can actually cause a slight performance hit, similar to slight retardation in timing.

Now I need to burn off this tank of 89.

Experiment was a success, though, because it loosely proved what I suspected-- running mid-grade or higher in the 2.2 won't help at all and will even simulate a slight reduction in spark advance.

HonestBlues

Then again...you may have bought some bad 89 octane.Quality of fuel can vary widely from station to station.How many tankfuls did you run through it?

sweetmama

We have only run 87 since we got the HHR and have had very good mileage. That is until recently and we changed stations and have had a drop in mileage. Not sure if it is the different gas or what?

nfboy

I posted a link to some info on octane here...post #8

https://www.chevyhhr.net/forums/2-4l-performance-tech-14/what-octane-use-hhr-10163/

Lone Ranger

octane is basically how fast the fuel ignites. A higher octane rating means it ignites slower. Lower octane ignites faster. Thus using an octane higher than what is optimal for your engine's spark programming (spark advance throughout the rpm range, usually vs. MAP value at a given rpm) can do the same thing as reducing (retarding) the timing (spark advance). Retarding spark advance when you don't need to (i.e. when there's no knock or "ping") will cause a decrease in performance.

It all depends on how the spark advance tables are programmed, so there probably is room to increase spark advance throughout the timing tables in the ECM to take advantage of higher octane fuel.

SandyBeach

sweetmama, it could be the fact that we're using "summer blend" now. I thought I read that it will reduce mpg. Has your station recently added ethanol to their blend? That will negatively affect your mpgs also.

Goose

Actually my experience has been that the winter blend is when you will take the mpg hit...at least in my case it has been that way



Goose