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Premium Vs. Regular for Older Cars

I just acquired a 1992 Mercedes Benz 300E from my grandparents (yay!) and it calls for premium. I notice on the “Premium Vs. Regular” portion of this site, it states that it doesn’t really matter whether you use regular instead of premium in newer cars. What about older cars like my 1992? I don’t think it has that knocking sensor mentioned on that portion of the site.

If it has fuel injection, it has a knock sensor. I have a 1988 Toyota Supra, and it has a knock sensor. The Turbo version has two. It calls for premium, and that’s what it gets. 265,000 miles on her, and she still purrs (growls when you step on the loud pedal).

If the owner’s manual calls for premium, use premium. The knock sensor will retard the timing to reduce the knocking and pinging with regular, but it will also accelerate the formation of carbon deposits and may reduce your gas mileage and increase your emissions. This will eventually cause parts to go bad much earlier and may require more repairs than sticking with premium, which are expensive for Mercedes, even with independent mechanics. The parts alone are more expensive than other brands.

Premium is only a couple of dollars more per tank. Cheap insurance. Trust me.

Now, you get to Benz over at the parts counter AND the gas pump!

Premium fuel is not “better” fuel, but rather different fuel. It is a shame that some marketer came up with the idea of calling it “Premium” as if it was by nature “better.” All premium means is that it will burn slower in the engine. Some engines will work better with premium and will say so in the car’s owner’s manual Some engines NEED Premium to prevent engine damage (see owner’s manual and if it says USE premium do so.)

Using regular when the owner’s manuals indicates that premium is “required” can damage the engine and may reduce mileage.

As with most of these kind of questions, the answer varies from engine/car to another.

READ the OWNER's Manual!

As BustedKnuckles stated, if the car in question has fuel injection, then it has a knock sensor.

However, after almost two decades, I would question whether the knock sensor is still working properly. Even if it is working today, it could cease working tomorrow.

19 years is a long time, and I would not want to place any bets on the electronic systems of this car functioning correctly, on a consistent basis. When you consider the likely cost of repairing engine damage on a Benz engine as a result of improper combustion, I would recommend using the octane specified in the Owner’s Manual.

Follow the owner’s manual. You’ll be spending a pile of money to maintain this car already. You don’t want to add engine damage to the list.

Engine “octane” rating is quite simply a measure of a particular fuel’s resistance to self-ignition under pressure. Quite simply, if your engine’s compression ratio is 10.5 or less to 1 and naturally aspirated, you do not need to run anything over 87 octane, or 85 octane above 4000 ft above seal level.

The addition of turbochargers or fancy ram induction intakes changes things. Now you’re jamming MORE air (and thus more oxygen) into the cylinder to be compressed in a hot container with a highly volatile and flammable chemical. Even if you have one of these whiz-bang high-compression engines or turbocharged engines, the knock sensor (basically just a microphone) will feed back to the ECM to retard ignition timing in the event of a knock until the knock goes away. Put better gas in it, and yes, you’ll have a SLIGHT improvement in performance when really gunning the engine but certainly not a terribly noticeable improvement for daily driving.

Corrosion and moisture concerns aside, ethanol does improve octane rating of fuel. Running pure alcohol allows for INSANELY high compression ratios like the methanol racers back in the days of open-cockpit Ducattis, Alfa Romeos, and Dusenbergs. Methanol will take some pretty severe compression before it auto ignites, hence the stupid power figures alcohol race engines can put out.

The age of the car has nothing to do with whether regular or premium should be used. The compression ratio of the engine and other factors determine the octane of the gasoline that is required. If the owner’s manual says premium, then that is what you need to put in the gasoline tank.
I’m a senior citizen. I require premium beer. Don’t give me the cheap stuff just because I am old. I’ll detonate and do other kind of nasty stuff. Your Mercedes is a senior citizen in car years. Treat it with respect.

Triedaq: Yeah, and MGD is the “Champagne of Beers.” The Mercedes straight six 300 engine didn’t move UP to a 10:1 compression ratio until '93, I’m pretty sure his is running at a 9.5:1 ratio.

I’m still unconvinced, from an engineering perspective, as to why a naturally aspirated engine with below 10:1 compression would need premium gasoline, unless the ignition timing is very advanced to obtain more combustion pressure and power. Let’s look it up…

“The 103.980 engine (181 in.³ or 2960 cm³) produces 177 hp or 132 kW at 5700 rpm (with catalyst), 185 hp or 138 kW at 5700 rpm (without catalyst), and has a torque of 188 ft·lbf or 255 N·m at 4400 rpm (with catalyst), 191 ft·lbf or 260 N·m at 4400 rpm (without catalyst). It uses the CIS-E (Continuous Injection System - Electronic) and has a bore of 88.5 mm (3.48 in) and a stroke of 80.2 mm (3.16 in). The compression ratio is 9.2 : 1 with a redline of 6550 rpm in 1987 and 6200 rpm from 1988 on. The firing order is 1-5-3-6-2-4. Lubrication system is pressure circulation lubrication system. Amount of oil in engine is 6.0 litres and amount of coolant is 8.5 litres. Number of valves is 1 intake, 1 exhaust with V-shaped overhead configuration. Valve operation was 1 top camshaft. Camshaft drive was simple roller-type chain. Starter motor is electrical with 1.5 kW; since 01.88 was 1.7 kW. Ignition system is electronic ignition system. Recommended fuel octane for the M103 is 91 RON/MON (96 RON)(86 MON). This engine can be found in the W124 E-Class, W126 S-Class Mercedes-Benz R129, W463 G-Class and the R107.”

Well, there you have it. I have a 260E and she runs like a scalded dog on 87 octane. She requires 91 RON/86 MON, which in AKI terms (what we use here in the U.S.) is 90-91 octane. So, I stand corrected… you don’t need Premium, and Mid-grade doesn’t cut the mustard.

I still maintain, an engine with 9.2:1 compression (somewhat low) and naturally aspirated should get along just fine on 87, especially with a 10% ethanol blend.

There’s more to knock tendency than compression ratio.
Anything that increases the flame path (bigger bore, spark plug to one side, less turbulence/swirl) needs higher octane because of increased combustion time.

Modern engines generally use a compact shape combustion chamber with a centrally located spark plug.

This paper talks about some of the things done to reduce knock tendancy in a Toyota engine:

An operative EGR system also helps; a lot.

If it calls for premium, use premium. If you find that expensive, wait till the first service or repair. I would suggest selling the car if you don’t want to live with the much higher operating costs of a Mercedes.

My daily driver is a 1992 300E that I’ve owned for 13 years. The car manual and the gas filler door both say premium is required. If you use regular, the knock sensor will retard the timing and decrease your mileage to where you probably won’t be any further ahead. I tried this a few years ago for a few weeks and when I got the car smog tested it failed (and it was fully warmed up). After refilling my next tank with premium the car passed during the retest. Also, find a good independent mechanic to work on the car…this will save you a bundle on repairs.

I still don’t get the squabbling about premium if required vs regular if REQUIRED.

If you drive 15,000 miles/year you are talking about $10 extra per month if you achieve 25MPG to buy premium.

Using regular fuel you risk some engine damage if REQUIRED and also a performance/MPG loss too in some engines. Many times the $0.20 increase for premium buys you more MPG than what you “save” using regular.

Try it both ways for yourself and see.

Everyday, hundreds of thousands of cars labeled “Premium Fuel Only” get filled up with regular…Few if any of them ever suffer “serious engine damage” or ANY engine damage. The 5 octane points just don’t make that much difference and 9.5 to 1 compression engines can burn almost anything without damaging themselves…

It’s more about shelf space and pool margins…

[citation needed]

@ Caddyman

The dude who I saw throwing a fit in the service dept at the Ford dealership where I worked at one time when his warranty claim roundly rejected because he chose to run 87 octane in his Lightning pickup instead of the required 91 octane would probably disagree with you. :slight_smile:

Also when I worked at Nissan, we had alot of people complained that they were getting lousy fuel mileage in their V6 Altimas and Maximas and 350z’s. Turns out the VQ engines from around 2003-2006 really don’t like 87 octane (premium is recommended, but not required for those years) The ECU’s software is such that when it detects knocking (via the knock sensors) that it reduces the timing to such a degree that power is and fuel economy is reduced by about 15% (general consensus), that’s significant. We told the customers that 91 octane or better is recommended for optimum performance and fuel economy, and to try a couple tankfuls to and see if the fuel mileage increases, that fixed the problem every single time. The later VQ’s have some design and ECU changes that make them more tolerant to octane levels though.

You have to remember that today’s cars with electronic controls are able to be to tuned much more precisely that cars of 70’s and even the 80’s. There’s less wiggle room, cars can be tuned to get the very most out of any given fuel, which means that when the fuel is less than what the car is designed for, you have to rely on the failsafe (knock sensors) which aren’t 100% reliable. When those fail, you’re engine can be damaged even if the fuel you’re using is 5 points lower than what’s spec’d for the car.