Long term penalty for using lower octane?

The ECM will only retard timing when the knock sensor actually detects knock. So, for light-load operation like just cruising down the highway, timing probably won't be altered at all. Only under high-load will you see much of an effect.

And since the typical duration of high-load operation is relatively brief, it's highly unlikely that you will cause any permanent damage.

This assumes the knock detection system is working correctly of coarse. This also assumes that when ignition is retarded, the ensuing elevated exhaust temperatures do not exceed the safe limit for the catalyst (a high speed/load issue).

I have had several SHO Taurus' that recommended premium fuel and never ran premium fuel. I was willing to trade the performance hit for lower fuel costs. Since most of my driving was freeway cruising I didn't see any point in wasting octane just cruising down the freeway. I never had any engine trouble (wore out as set of front tires in 12k miles though).

And the fuel mixture will be enriched (to cool the mixture and prevent pinging).

The rich mixture will foul spark plugs and fill the combustion chambers with carbon.

But under light load one is unlikely to suffer ping much anyway. Detonation is usually the result of high cylinder pressures which means high VE /open throttle.

Most likely damage from detonation is piston damage. While my Neon manual recommends premium (it is the RT version), after my initial tests with premium and regular I never ran premium again. The difference in gas milage was less than the probable error in the statistics. The car now has 125,000 miles. I recently ran milage on several tankfuls. I get exactly the same milage as when the car was new, so I assume the rings and pistons are still in pretty good shape.

WRONG!!! That won't happen.

WRONG!!!!!

That depends ENTIRELY on whether or not the engine managment computer can retard the timing enough to prevent detonation. If it can't, detonation will occur with all the long-term damage it causes (hammered bearings, possible piston damage, possible cylinder head and head gasket damage, etc.)

If your car can compensate for lower octane, you will end up with lower milage, so you aren't saving any money by using lower octane gas.

If your car can't compensate for lower octane, then you'll end up damaging the engine.

Just use the octane that the manufacturer suggests.

Thanks for the responses everyone.

Sounds like there would be no long term damage to the engine provided the knock sensing system was working properly.

Regards, Gary

But WHY would you want to?

depends on the car. I accidentally put 87 in my Trans Am once, and it pinged so bad I had to hit the next gas station and buy some octane boost. It NEEDS 91 or else. The price you pay for high performance. Long term use of 87 in this car would result in pistons that looked like swiss cheese.

Frankly, if your car specs 91, you're playing with fire by not using it.

Ray

| >I'm wondering what the long term penalty is

Sometimes it's all that is available.

--scott

Excellent info. Thanks. Gary