KOOLY2

I am currently putting 87 octane fuel in my '92 400E and it runs fine or seems to at least. I also go to the cheapest fuel stations like Beacon and Ultramar. Rarely I will put 91 octane in but only when the prices drop low enough.

I talked to my mechanic about it and he said that carbon buildup could be a problem because I have the newer engine with four valves per cylinder. The car idles and runs smoothly, has plenty of power; so I just don't see why it supposedly needs 91 octane.

Does anyone know of the effects of what I'm going?

Also is Shell gas better than Beacon?

Jackd

If your car performs well with the type pf gasoline you are using, you have absolutely nothing to gain by going to a higher octane gasoline. If there is no pre-ignition (pinging)., going to high octane would be a waste of money.
You mechanics suggesting carbon built-up with regular grade gasoline is wrong. He is probably a good mechanic but does not know much about gasoline.
Beacon and Ultramar are both the same company (Total). I worked for them for 26 years.
Gasolines from all petroleum companies are all the same. They have to meet minimum specs. and you can rest assured they don't excees those specs, which costs more.
In addition, you really never know the source of the gasoline you buy. With thousands of exchange agreements between the various supplies, you can end-up buying Exxon gasoline in a Shell service station and Ultramar gasoline in an Shell station.
JackD

bamberger_1

I would fully support was JackD was posting. All is completly correct. There Octane numbers are describing the ingnitability of the gasoline, and this is the point where you need to be careful. The motor management (timing etc) is based on these Octane figures. In case of lower octane figures the ignition sets in earlier than with higher octane (given by the electronics). If your engine is set to high octane, you are however using low octane than the optimum ignition time set is not met and the firing of the cylinders comes too late and the forces on the piston heads causing the pinging JackD is mentioning, and pinging is bad for the crackshaft bearings etc. The problem is that the V8 is such a quiet and smooth engine, that you would not necessarily hear the pinging noise, which accures during higher speeds, since the wind noises would probably cover the pinging.
Having such a great car I would not sacrifice it due to a few pennies per gallon, especially not at this age of the car, because well kept it will last much longer.
Just calculate the MPG over your personal yearly milage, and you'll find out its not worth the risk.