89 or 91 octane - '91 300CE

[rgnprof]

Is there that much difference? I have always used 91, but it was $2.97 today (no ethanol), and $2.49 for 89 (10% Ethanol).

I know the manual says "premium", but is anyone else running 89?

Thanks, Ryan

[tjts1]

89 octane is usually blended at the pump as its going into your fuel tank using 91 an 87 octane. Since you never really know how well its being proportioned, I stick to 87 or 91.

[JohnM.]

The only difference between higher and lower octane is the gasoline's resistance to pre-ignition (detonation/pinging). Your engines compression ratio is directly related to what gasoline is required for safe operation. Will the car still run on lower octane? Yes. Will there possibly be detonation and will you run the risk of ruining valves, pistons, internal engine components over time? Yes. Plus I think the W124 has a very rudimentary knock detection and retardation system. I wouldn't chance it. Saving ~$5 per fill-up is not worth destroying your engine over.

[junqueyardjim]

Unless you are out on the big roads flogging that thing like a state police car. I have an M103, I run it all the time on 89 octane and never so much as a hint of a ping and I can and once in while do overload it, by not shifting quickly enough. Now I don't do that often, but just once in a while I forget that it won't shift by itself, as it is a five speed. My M103 likes the revs and I drive it hard, not to abuse it, but I don't think it will carbon up the way I drive it either. Mine gets about 60 miles a day on local metro roads, but no high speed interstate driving. And I think that sustained high speed interstate type driving is where I would be using the high octane gasoline.

[macdrone]

Well you can put 89 octane init, but my sticker and book says 92 or better. I am sure there was a reason like 10.5 to 1 compression which is stamped on my block but its completely up to you. Save money now or put what it asks for in it.