I switched to regular (87 octane) gas
 

I'm fed up with the price differential between regular and premium. Premium now costs 34% more than regular where I fill up. The argument about false economy by losing efficiency with regular doesn't hold up anymore. It made sense when premium was only 7% more than regular, but not anymore. My engine has a knock sensor, so I should be good. I now pay $21 per fillup instead of $28.

there was a recent newsstory on the lack of value in premium gas, My E320 seems fine on any grade..one issue with the EGR system is likely true with either grade..plugging the fresh air line to the throttle and the ports on the head..These can be cleaned and mine were in great shape..Well the head ports were carboned up but still the car was passing at three times cleaner than the legal standard..50 ppm when 220 is allowed, and that was BEFORE I rebuilt the head..

Wow 21USD for a full tank - **** - here my W201 can swallow 90 euros easy

Yea, what he said. Too many in the USA don't realize what it costs to drive a car in much of the rest of the world. ( And that most high end MB cars are delivered new here, I think it is something like 2/3 is US bound. )

As for the original post, are we talking about the 95 in the signature?

The only way to tell if regular / premium is a good value is to keep long term accurate fuel mileage records for both.

On my 97 SL320 with a M104, mid grade leads to sluggish performance so premium is needed as it is a heavy car.

On my 97 C280 ( Actually on permanent loan to Mom ) mid grade is just fine.

From what I recall, all automotive engines delivered to the US since 19?? must be capable of "operating without damage" on regular grade fuel.

Yes, these engines have a knock sensor but depending on how aggressive the ECU is with pulling back timing, it might be costing you more with regular. Rather than just pulling timing back a bit, and adapting as needed, it might just pull loads of timing out of it and leave it at that. The ECU on the 95 isn't as complex as the 97 + ECU.

On some cars like a GM product from the late 80's early 90's, timing pulled out in one big chunk in order to save the engine.

That said, I was scrolling through a STAR tester with a 98 V6 ECU attached and found a menu that allows adjustment for reduce fuel quality. I'm guessing it reverts to a different timing map rather than waiting for knock to occur, possibly adds fuel to cool the chambers and probably is more aggressive with EGR if it can be modulated.

Ok, I'll bite. How small is the fuel tank in this car? Gas here is in the neighborhood of $2.15/gal for regular, and I just dumped 16 gallons in my Honda this afternoon. Right at $35. The SL usually takes somewhere around 22 gallons. Unless your gas is stupidly cheap, or your tank is stupidly small, I'm guessing you're only doing a half tank or something here.

Keep an eye on your MPG's. With my Honda, I get ~30mpg on the highway with regular gas. With Premium gas I get ~35mpg highway. Regular is $2.15, Premium is $2.49 (as of this morning when I filled up). That makes the cost per mile 7.17¢/mile for Regular, and 7.11¢/mile for Premium. An insignificant difference in cost per mile. I do use Premium when I'm on the highway and travelling since the car idles smoother and feels like it has slightly more power. Cost is negligible between the two, as always Your Mileage May Vary.

Nearly €120 for my 300CE with the 90 litre tank.

You US guys don't know when you're well off!!!

RayH

I run my Street Triple on RUG most of the time, since that's what the manual says. One time I filled up on premium. No difference noted.

The Benz still gets diesel.

I haven't had a Mercedes with spark plugs in a long time, but my '95 E320 did have a measurable difference in fuel mileage between the recommended premium fuel and regular fuel, enough that it was cheaper to run premium. Same thing with my Land Rovers.

This was part of my calculation when looking at the (EPA) 28mpg hwy mileage on the new GLC on premium fuel and the new GLE Bluetec on diesel. Diesel is often more $$ than regular these days, but very rarely more than midgrade or premium.

For your M104 my speculation is that your engine will be fine, but I recommend a good additive to make the difference (preference is RedLine SI-1) in keeping injectors clean and valves clean. An engine that runs well (and tires, and no dragging brakes, and no unnecessary weight, and economical driving, etc.) will make more difference than the price delta from regular to premium. The M104 is a reasonably thrifty engine for its time and if it had overdrive and a lock-up TC ... I digress.

The change from premium to regular causes several things to change in an engine. It might be that the engine was only certified (EPA) on premium, it might be that lower octane forces retarding timing which increases exhaust temps and changes emissions, it might be that you will get worse mileage and less power, maybe something else (don't know).

In a modern engine all of the temperatures and compounds in the exhaust are carefully considered for engine life, power, and most critically emissions. My 2016 manual doesn't say that you can't run regular but "As a temporary measure, if the recommended fuel is not available you may also use regular unleaded gasoline with an octane rating of 87ADI/91 RON. This may reduce engine performance and increase fuel consumption. Avoid driving at full throttle and sudden acceleration. Never refuel using fuel with a lower AKI."

So more fuel through the cylinder and at a later ignition timing should lead to higher EGTs, and more emissions, as well as not reaching the EPA certified fuel mileage.

^^^ This was largely my point using the Honda as a point of reference (I've never had the SL on a long enough trip to do any real science with it). With a vehicle advanced enough to change engine timing based on a knock sensor, premium fuel can increase economy and performance.

The Honda I referenced is designed for 87 octane and it benefits from running 91 or 93. Other vehicles I have access to (2012 F-150, 2003 Explorer) are so primitive that there is no change whatsoever by changing fuel grades. Older vehicles (Older than Mid-90s) likely won't have any performance difference either, though if you hear pinging or rattling by changing to a lower grade, quit using it!

To the Euro guys thinking that $21 will buy you a full tank of gas, let's be real here:

The OP is located in Michigan. AAA says as of this morning that the average cost of 87 octane in his state is $2.15. The 1995 W124 had a fuel tank from 18.5 - 21 gallon capacity depending on model. Assuming he has an 18.5 gallon tank, filled it at the empty mark, he would add in roughly 16 gallons of fuel. 16 x $2.15 = $34.40. Granted, this isn't 90€, but it isn't $21 either. The OP isn't buying full tanks of fuel. For $21 to buy a full tank of gas, he'd have to be at $1.30 or lower per gallon which hasn't happened in a very long time in this country.

In my experience "going into town to do some shopping" costs about the same in fuel in the US and in Europe! It might sound a bit mad but (having to) travel that little bit further in the US kind of makes it all about equal.

I can get away with Regular in the W140 but the W221 will fart and burp to high heaven.

In the old days, putting low octane fuel in a car designed for premium would result in an audible knock, which was a reminder to the driver not to be penny wise and pound foolish. The ECU in any modern engine will use a knock detector to adjust timing. Using regular fuel will work, but the timing will end up being electronically retarded to avoid knock. The result may be obvious changes to fuel economy and performance, and not so obvious changes in EGT's which will affect long term durability. Emissions tend to remain acceptable, as the ECU uses the O2 sensor to maintain closed loop operation. If the manufacturer recommends premium, that's what you should use.

I'm pretty sure the compression of a M104 3.2 is 10 to 1 , pretty high for regular gas I'd say.

Most modern cars are in the 10:1 - 11:1 compression ratio and still use regular 87 octane fuel. My daily is 10.7:1 and was designed for "regular" gas.

Static compression ratio is one thing, and dynamic compression ratio is something else. Dynamic compression ratio is measured from the point the intake valve is closed, which is well past BDC. Cam timing is critical in determining the octane requirement for the car.

In plain English: knowing the static compression ratio of an engine isn't enough to know whether premium is required.