fuel octane

[Ted K.]

I am a Corvete guy with 5 Vettes but have a fuel question concerning a Mercedes 450 which requires 93 octane fuel. I tend not to utilize the high octane because of the price except on a trip. I understand that lower octane is supposed to cause ignition ping and in long run do damage to the engine. My approach is to use higher octane when on road trip and lower octane when around town.
What are your thoughts? The reason I am asking on this forum as there are a lot of knowledgeable, truthful members and don't have to put up with all of the wise guys on other forums.
I use 93 or AV gas in my Vettes.
Thanks

[Joe L.]

Re: fuel octane

I am a Corvete guy with 5 Vettes but have a fuel question concerning a Mercedes 450 which requires 93 octane fuel. I tend not to utilize the high octane because of the price except on a trip. I understand that lower octane is supposed to cause ignition ping and in long run do damage to the engine. My approach is to use higher octane when on road trip and lower octane when around town.
What are your thoughts? The reason I am asking on this forum as there are a lot of knowledgeable, truthful members and don't have to put up with all of the wise guys on other forums.
I use 93 or AV gas in my Vettes.
Thanks

Ted------

My recommendation is this: if the owner's manual states that premium fuel is REQUIRED then I would use premium fuel at all times. If it states premium fuel is RECOMMENDED then, if engine performance is satisfactory on regular grade, then I would use regular grade all the time.

[Mark E.]

Re: fuel octane

Manufacturers specify premium fuel primarily to achieve the claimed EPA fuel mileage on the window sticker. Increasing claimed fuel mileage, even by marginal amounts, is a game manufacturers play to the extreme.

If regular fuel is used in these cars, there's a small fuel mileage penalty because the engine management system retards ignition timing real time to prevent pre-ignition. So no damage is done running regular; there's just a small fuel economy and performance penalty. Of course, this applies to "modern" computer managed cars with knock sensors... which includes my 30 year old '93 LT1.

I've been running regular fuel in our "use premium only" cars for decades. The fuel mileage penalty isn't measurable, and performance isn't noticeably different either. But there's a huge savings in fuel cost- 40 to 80 cents per gallon.

Also, premium high-octane fuel isn't better nor does it keep your engine cleaner. More octane does not mean a better quality fuel. It just means it requires more heat to ignite. For high quality fuels, look up the "Top Tier" brands and use their regular 87 octane for a happy engine.

[Duke W.]

Re: fuel octane

He said he has a Mercedes "450", but didn't state the year or whether it's a SE, SEL, or SL. The 450 models (4.5L SOHC V-8) ran from the mid/late seventies to the mid eighties and none have knock sensors that can pull timing or default to a less aggressive spark advance map.

OP: If you can't detect audible detonation then you can probably get away with 87 PON especially if you have a light foot and don't load up the engine too much at low revs. Detonation is more likely as ambient temperature and pressure rises.

Frankly the amount you save isn't much considering the risk. I've talked to some who have bad hearing and probably can't hear detonation.

Another alternative is to use "mid-grade". California gas station have two tanks, 91 PON and 87 PON, and the pump offers 89, which just blends the two in a 50-50 ratio. I expect most states are similar.

I have a '88 190E 2.6 special order five-speed manual transmission that calls for 91 PON, but I can run 87 even though I modified the spark advance map to be more aggressive that noticeably improved low end torque and reduced around town fuel consumption by about 10 percent. I occasionally get some transient detonation on up shifts, especially if the ambient temperature is more than 70 degrees F and the coolant temp is more than 80C, but I only drive it during good weather in the winter months when temperatures are usually less than 70F. Around town I usually shift at about 2000 and it will pull well from 1200 revs in fifth.

An automatic is tougher to load up at low revs because the engine will always rev to converter stall speed when a throttle is applied at below converter stall speed, which is typically 1800-2000 revs. If I'm on it in first and second gears I have to watch the tach and shift in time to not hit the 6650 rev limiter. The Germans build great inline sixes!

A lot of older cars that lack detonation sensors might run on lower octane depending on driving conditions/habits ambient and coolant temperature and the spark advance map. It's a matter of experimentation and knowing how to recognize detonation.

Duke

[Keith M.]

Re: fuel octane

My daily drivers have gotten mid grade for decades. Don't need more and don't like cheapest anything. My collector cars and numerous small engines all get zero ethanol.

[Terry D.]

Re: fuel octane

An episode of Engine Masters tested fuel octane on a 383 crate motor putting out over 400hp. The diference between 92 and 89 was very small under 5hp if I remember correctly. If might have more of an affect on very high compression engines. I switched to 89 in both my 383 stock crate engines ( 405 and 425hp) and have noticed no difference except in my credit card bill!

[Mark E.]

Re: fuel octane

He said he has a Mercedes "450", but didn't state the year or whether it's a SE, SEL, or SL. The 450 models (4.5L SOHC V-8) ran from the mid/late seventies to the mid eighties and none have knock sensors that can pull timing or default to a less aggressive spark advance map.

...

Another alternative is to use "mid-grade". California gas station have two tanks, 91 PON and 87 PON, and the pump offers 89, which just blends the two in a 50-50 ratio. I expect most states are similar.

Duke

Good catch regarding age of OP's car. My prior comments assume the car has an engine management system with knock sensors.

A comment about "mid-grade" fuel... in North Texas there's typically 87, 89, and 93 octane. Mid-grade is only 2 points higher than regular, yet its price is midway between regular and premium, and sometimes priced closer to premium. This is an expensive convenience that's very profitable for the oil industry. If "mid-grade" is right for you, consider alternating between regular and premium... much more bang for the buck.

[Owen L.]

Re: fuel octane

A comment about "mid-grade" fuel... in North Texas there's typically 87, 89, and 93 octane. Mid-grade is only 2 points higher than regular, yet its price is midway between regular and premium, and sometimes priced closer to premium. This is an expensive convenience that's very profitable for the oil industry. If "mid-grade" is right for you, consider alternating between regular and premium... much more bang for the buck.

[Ted K.]

Re: fuel octane

Good info thanks. The car is a Mercedes GLS 450 2019. I will be comfortable using 87 octane exclusively.