94 740IL - will 87 octane hurt performance?

Had about a quarter of a tank left and went to fill up today. Put in 6 gallons of 87 octane before I realized my mistake, and then put in 8 gallons
of 93 octane. Drove the car home (8 miles) with no problems. Should I expect to hear knocking or pinging? Not sure if my "combined" 91 octane or so will cause problems.
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You will not have any problems. Simple slip of the mind once in a blue moon.
Damage to the engine or exhaust system is unlikely. Many people will claim that damage is likely with continued use of low octane fuel, but that probably is an urban myth. I have not seen one validated example of engine, transmission or exhaust damage from the continuous use of low octane fuel in a high octane fuel designed engine. I have not yet read an owners manual that said "Continued use of low octane fuel will damage the engine", anyone read something like that in an owners manual?
That said, there are some things that do happen. With a low octane fuel, then engine is likely to ping, or pre-detonate under load. The Engine Control Unit will sense this through the ping sensor and will retard the timing. The retarded timing will result in the engine running richer than it was designed to do and your fuel consumption will probably go up. Possibly, the engine may get dirty carbon deposits inside. This might be an issue over 100,000 's of kilometres, hard to say. The exhaust gases will be hotter, this may make the catalytic converter run better.

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Richard Tomkins wrote:

Huh? That's a good one. How does it run richer because the timing has been retarded? It will not run richer, it will be ingited later in the compression stroke.
Perhaps you meant that the exghaust will be richer? The (supposed) hazard is that if the fuel charge is ignioted later it will not complete burning before the exhaust valves are opened and we will discharge raw burning fuel into the exhaust possibly damaging the cat.
This theory makes no sense to me. The whole reason the ECU is retarding the ignition in the first place is because the fuel charge is burning too fast. That is what low octane fuels do, they burn too fast. The explosion of the too quickly burning fuel hits the piston before it has a chance to reach TDC.
-Fred W
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snipped-for-privacy@whawhawha.tv wrote:

You didn't say how many gallons of 93 you pumped in, but even if it is 50/50 you would effectively have 90 octane. I use 89 octane in my '94 540i often with no problems or perfromance reduction. You will not hear pinging on lower grade gas anyway because the engine control computer will sense the pinging before you can hear it and retard the ignition to eliminate it. What you would sense first, if the fuel octane is too low, is reduced performance and lower mileage.
Remember, the pump octane rating is a minimum for that pump. It can be considerably higher.
-Fred W
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wrote:

"Put in 6 gallons of 87 octane before I realized my mistake, and then put in 8 gallons of 93 octane"
sure he did. 6 of 87, then 8 of 93.
Your still right though. he is still at or above 90-91 octane, so he is ok.
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Trey wrote:

Weird. I'd have sworn he said how much 87 he pumped (6 gallons) but then only reported filling it the rest of the way with 93. But it sure does say 6 and 8 right there...
So it would appear that he ended up with 90.42857143 octane in the tank.
;-)
-Fred W
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wrote:

Well, we are not sure home much fuel he had in the tank to start with. But at any rate, its still high enough to not make trouble. :-)
Question though. Why do BMWs require 91+ octane? Looking at the Jap cars, they are running the same compression ratios, and putting out more HP with the same displacement, while running on 87. Or does the factory run the dino with 91 octane, get the HP numbers, then put in the manual "can run on 87, but use 89 for improved performance"?? This whole three grades of gas thing is really lame.
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Most Japanese engines have higher rpm limits, which increases HP. For that matter, most BMW aftermarket engine computer mods add 200-400 rpm to gain HP.
Recent BMW's seem to have more aggressive factory tuning, however.
Floyd
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