My car’s manual specifies mid-grade fuel. 89. However, my mechanic says that with 110,000 miles on it, it now makes no difference and I am just throwing good money down the drain. He wouldn’t lie to me, but he might be mistaken.
Search for “octane” on www.cartalk.com - this question is asked frequently of the brothers.
Here goes the explanation…
The octane rating of gasoline is an indication of its resistance to knocking in an engine. The lower the rating, the more likely it is to knock.
Knocking in engines is when the gasoline/air mixture ignites BEFORE the spark plug sets it off(predetonation). This is a bad thing for many reasons.
When the compression ratio in an engine or the temperature of an engine increases, the tendency for knock is increased as well. That’s why many high-performance engines such as the BMW M3 engine specify 91+ octane gasoline. They have much higher compression ratios than normal engines, and need the higher octane to prevent knock.
As cars get older, deposits and crud build up on the cylinder heads and pistons, effectively raising the compression ratio a little bit, generally raising the octane requirement.
There is a caveat though… most modern cars have a knock sensor that detects knock and retards the ignition timing to prevent it (it’s harmful). So you can run 87 octane in a new M3, but it’ll run relatively badly and without proper power, because the timing is retarded quite a bit.
I’d just stick to whatever your manual recommends, and if you get performance problems, move up an octane grade and see if they go away.
The carbon deposits can make detonation more likely, but it’s also possible that you may have lost compression by worn piston rings. A compression check can help determine this - although carbon deposits can cause pre-ignition by staying hot too, which a compression check won’t help with. I’d stick with mid-grade.
I agree with bump. If your mechanic is suggesting lower octane just because the car’s older, he is sadly ignorant.