Since fuel has become SO high priced I went ahead and tried a tank or two of Regular 87 octane.

95' S420 (119 motor). I cannot tell one bit of difference in the way the engine sounds or performs. Mileage is still good (same as it was on 91 octane),
17+ in all city.
20+ on highway 70-85 mph

What components might be performing differently with regular grade fuel. I am real picky about how my car performs and I do all regular services myself.

How could I tell if anything is not OK. Every measurable engine parameter seems the same.
Engine temps,
mileage,
engine sounds,
idle characteristics and rpm,
oil pressure,
EVERYTHING !
I can't measure such stuff as cat. temps. however.
If there is any difference in power I can't tell it, although I have not tried any really high stress stuff like 1/4 mile times or top speed, But I don't know those with premium fuel anyway?

DanielW
95 S420, 82,000m (still on Starmark for 6 more mos.)
2005 Dodge Dakota 4.7V8
1987 FJ1200 Yamaha Cycle, 56,000m
2004 Acura TL, 9000m, (traded on the Dodge PU)(fast, but)
(I learned the hard way that I did NOT like FWD cars)
1987 300E, MB,210Km, (traded in on the S420)
1984 533 BMW, 220Km (Sold)
1979 320I BMW, 311Km (Sold)
1976 530 BMW, 170Km (Sold)

Yikes! That sounds great but don't void your warrenty!

__________________
2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)

Yeah, it's been beat to death. Your engine's management system will protect the engine from detonation. Most with "premium fuel" cars who try regular grade cannot detect any difference in performance or mileage. If you don't hear audible detonation there is nothing to worry about. The engine management system is a lot "smarter" and faster than any owner.

If you search for some recent posts by me I explained why in detail.

Duke

If the engine senses detonation, it will retard the timing to prevent detonation. If it does need to do this under high load conditions, the result of the retarded timing would be less energy being extracted from the fuel and thus a decrease in performance and efficiency.

__________________
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

Intake air temperature also influences the tendency to knock (that's one of the differences betwen RON and MON). So, it is possible that when it is hotter outside, you may notice a performance difference as the timing is retarded even further.

You are right, in your car you will see no significant change in:

Engine temp...
The knock sensor will change engine management parameters to stop the knock, producing somewhat less horsepower, but no additional heat load or change in coolant temperature.

Mileage
Most of the time, and especially cruising at a steady speed, you are using a fraction of the full horsepower, so mileage will be within normal ranges anyway.

idle characteristics and rpm
Engines almost NEVER detonate at idle because of octane insufficiency...

oil pressure
Detonation has NO effect on oil pressure until something breaks and you spring a leak.

engine sounds
Detonation detection and engine parameter adjustements are fast, and un-noticed, aren't they? MB has some very good engineers!

If there is any difference in power I can't tell it
No, and you won't unless you have an engine dynamometer with full instrumentation, and perform several test runs using both fuels, and climate conditions simulating the environments the engine could be expected to see nationwide, or even worldwide. The power difference may be small, and not show up to the observer in a vehicle. You may also want to perform durability runs with both fuels to see if there is a long-term advantage.

Do not think for a moment that any of the 'consumer-grade' instruments on a car dashboard will tell you anything near what the analytical gauges the manufacturer uses will tell you.

As discussed, your engine management will adjust settings to stop the harmful detonation, at a small sacrifice in 'performance.'

The Germans believe that if you buy performance, you should be able and willing to pay for it, both at the pump and in the service bay. They expect you to follow their recommendations, and that by doing so, you will receive all of the performance designed into the car.

It's up to you...

Best Regards,
Jim

Its beyond me why anyone would risk saving 2-3 dollars on a fill up, because gas went up. Your driving a vehicle designed for high octane. if you want to use 87 octane buy a Honda Civic. i know guys who bought 90,000$ Benz and then worry about gas prices? to each his own i guess

I ran a tank of Sunoco 100 octane pump gas in my 4cyl C230, and I tell ya, I think the engine ran ultra smooth. When the car was cold my slow warm cruise down the driveway seemed to not have the faint throttle pulses - hard to describe the feeling - I get with 91 octane.

Also when I turned the car off I didn't feel the same pulses from the crank coming to a halt that I got with 91 octane.

I need to do some more comparing and I didn't get to compare a high speed cruise, but my full throttle cruise has always been quite good. with this car.

Maybe all my imagination, but I will run a second test shortly. WIth the rise in gas to over 2.30 a gallon I feel not so stupid ( just mildly stupid ) paying 4.80/gallon for the Sunoco.

Ridiclous statement. Why not just throw money out the window?

__________________
Norm in NJ
Next oil change at 230,000miles