Strangeness after using lower grade of fuel...

[RASelkirk]

Hi All,

I've always used 93 in my RAM. Last week I had to fill my mower jugs, and being lazy, decided to compromise and use 91 for everything instead of setting up the pump twice. Within 3 days of minimal driving, my OA mileage indicator dropped from 13.8 to 13.3. No change in driving habit. Is there logic behind this?

TIA!

Russ

 

[KC Cryptkeeper]

Higher octane burns more efficient so better mileage.

 

[bigdodge]

The only thing higher octane fuel is good for is ignition at a higher pressure/temperature. That's why high performance engines run high octane fuel, they run high compression and lower octane fuel will ignite from the pressure/temperature before the spark plug. That is "spark knock"

.5 change in mpg could be caused by anything. Weight, wind, farting in the wrong direction, a plump one nighter.....
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[RASelkirk]

So there's no magic "gizmo" that reads/adjusts/diddles with the ECM for fuel grade? I can't see OA mileage dropping that much in less than 20 miles of driving. Not a name-brand fuel (local HEB market chain), but all pumps there have a huge turnover.

Russ

 

[buddy guy]

would adjust timing to retard a bit

 

[indept]

would adjust timing to retard a bit

Or advance based on knock sensor input.

With that the recommended fuel is 89 octane and 87 is acceptable. From what I've read on our engines, unless you have a custom tune 93 is a waste of money, it gives no added benefit.

 

[RASelkirk]

Yeah, maybe I should run a tank of 87 and get the mileage down around 11. Running it would prolly show something one way or the other. Pretty much just wondering if anyone else has seen this happen?

Russ