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.

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.)

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

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