Long term penalty for using lower octane?

Hi Group,

I'm wondering what the long term penalty is for using lower octane fuel than called for.

If the owner's maunual calls for, say, 91 and 87 is used my understanding for the short term is:

The ECM will retard the timing based on the knock sensor output.

The retarded timing will cause the engine to run less efficiently and gas mileage may suffer.

Now, if 87 octane was used exclusively in place of 91, would the engine suffer any damage over the long term?

Thanks, Gary

Reply to
G Mulcaster
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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).

Reply to
Dyno

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.

Reply to
Mike Walsh

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.

Reply to
Don Stauffer in Minnesota

WRONG!!! That won't happen.

WRONG!!!!!

Reply to
Mike

Reply to
Mike Walsh

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.)

Reply to
Steve

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.

Reply to
Noozer

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

Reply to
G Mulcaster

But WHY would you want to?

Reply to
Noozer

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

Reply to
Ray

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

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Reply to
TE Chea

Sometimes it's all that is available.

--scott

Reply to
Scott Dorsey

Excellent info. Thanks. Gary

Reply to
G Mulcaster

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