Ive played with higher octane fuel in all my cars at one time or the other.
Ranger:
Its been plagued with off idle and running knock since new. Doesn't matter what fuel you run in it. If it wants to knock,,, so be it.
Accord:
It likes 89 and 91. The car settles down. Pulls the a/c better. Timing advances per the SG. It tends to have a very mild ping at times during the hottest of the summer. On any fuel you put in it.
Civic:
With its 9.5:1 compression ratio its on the very very low side of high compression by todays standards in a gasser. Lord only knows what Honda has put into the ECU to help with controlling ping and knock in the R-18. They did put in oil sprayers to spray oil on the inside dome of the piston to control heat as part of the anti knock package.
The Civic responds to 89/91 very well. The SG shows timing advance. And its easer to hold speed during a climb. Pulls the a/c better. And is a bit more snappy. All off idle and running knock and ping are gone.
But in the long run. I haven't seen any real increase in mpg from using higher octane fuels in the Accord or Civic. But hold on. There is a twist concerning the Civic.
When running 89/91 in the Civic. I was doing it in the summer months. In the cool of a summer morning I was shocked to see 42 mile work commute segments in the mid to high 50 mpg area. But only see low to mid 40 mpg segments in the heat of the day of 100F or better and no a/c. I was stunned to be only seeing 42 mpg for the trip home! After 7 or 8 tanks of this I went back to 87. My morning numbers went down to 48 to 50 mpg and my afternoon numbers went up to 46 to 48 mpg. No WAI in use at the time.
This brings up another point of interest. Well at least for me. It seems that the Civic hates hot temps as much as it does cooler temps. Anything below 50f and you better have a WAI and be in winter mode. At 70f the WAI better be off and just breathing air from under the hood. At above 90f I better have the factory CAI on or mpg suffers something awful.
So with temps starting to be lower this time of year. I'm going to start running 89 or 91 in the civic again and try to keep the intake air temps to under 100f and see how things go. I'm sure this car has 60 mpg tanks in it. But the question is? Does the driver have them in him?
I know that 89/91 hurts mpg in hot air temps. And I know that high air temps hurt over all mpg in my Civic. I know higher octane fuel helps in cooler temps. And I also know the WAI helps in cooler temps.
So where is the balance point?
psy
The knock reading on my datalogger kept spiking everytime I touched the throttle. You'd think a motor with 7.8:1 compression ratio would be OK cruising on the freeway with cheap gas? Nope! Perhaps it's because the timing map is too aggressive for cheap gas.
Linear Mode
