[jackalope]

I just found this thread researching gas mileage because I just got a 2005 V70 2.5t and am averaging 16mpg on my commute. The dealer probably put in 87 so I'm hoping the mileage improves with 92.

I know this is an old thread but I think it's worth bumping because of the misinformation. Someone like me searching for info could read it and think that using 87 octane is actually OK.

87 octane is the minimum according to the manual on these cars because the ECU will retard timing-- IE the spark will happen sooner to prevent detonation. That alone hurts gas mileage. Every engine fires the spark plug before the piston reaches the top of its travel, but the closer to the top it can fire, the better for efficiency. When timing is retarded, energy is wasted fighting the expanding gases in the cylinder. The manual recommends 91 octane so that's what you should use for best performance and efficiency, rather than depending on the ECU and knock sensor to compensate for the *wrong* octane.

 

[jackalope]

Your understanding of timing retardation and its impact on cylinder pressure / efficiency / knock is backwards. Timing is retarded to prevent knock but that means delaying ignition. The efficiency impact is due to sub-optimal timing of peak cylinder pressure relative to crank position more than anything else.

Wow, I've had it backward all these years. Thanks

Now I don't understand why an engine is more efficient with 91 octane if tuned for it...

 

[jackalope]

My S60 has been given nothing but Shell V Power since the first fill up after purchase.

I ran a few tanks of Shell regular and then a few mid-grade in my XC90 and while I'm sure someone will tell me it was my imagination, I did notice a big difference. Nothing but premium for her since then.

Volvo is the only brand of car where people who know better will "try" a lower octane than what the car requires!! Go on a BMW forum and talk about using a lower octane fuel and you'll probably get banned