[NotQuiteCricket]

I drive a 1991 Escort and used to put in the lowest cost - but she knocked when accelerating. So, now I use midgrade, and she doesn't knock. I don't think that kocking is a good thing.

[Fixit]

As engines age their effective compression ratio increases due to carbon deposits in the cylinder & on the pistons.

Essentially, the built up crud inside the engine forces the fuel/air mixture inside each cylinder to get squished into a smaller volume, raising the compression ratio.

Start with an 8.5:1 compression ratio, subtract a small amount from each (Exaggerated, but I'll use .2) of the volume from each side to get 8.3:.8 = 10.3:1 new compression ratio.

The crud reduces starting volume too, but it is more important how much volume is missing when the piston is at the top of its stroke and the fuel/air mixture is at its densest.

[Smartaleck]

"I drive a 1991 Escort"

Try running a fuel cleaner through your engine. May take 3 tanks or so, but on an older car you most likely have a carbon build up in the cylinders.

The carbon gets hot and causes the lower octane fuels to pre-maturely fire causing the knock. Clean out the carbon and run the lower octane.

[ItsTheMediaStupid]

So, now I use midgrade, and she doesn't knock. I don't think that kocking is a good thing.

Buying mid grade runs counter to the purpose of an economy car doesn't it? Most likely your ignition timing is off, I would have it checked so that you can buy the lower price gas.

[bad company]

it may not be the gas it may be the timing is off. the higher grade gas will make the problem go away but probably didn't cure the real problem.