Old man at the Benz dealership says he’s been driving the same car for a long time and usually only puts in 87 octane in sprinkled with 91 every few times. What problems could this cause? Do I have to put in 91?
Disclaimer: I’ve owned (paid off) the car for 6 years and do put in 91, but trying to cut costs.
Don’t worry it’s just the engine…
Reduced performance and economy. You’ll usually get better mpg with the recommended fuel so its best to keep using it. You might have some other small issues but it takes years of using the lower number to actually cause issues.
If it says REQUIRES 91, then you will have misfires, trouble starting, drastically reduced power and economy, and engine problems after a while
Everyone’s right about the minimum octane requirement as stated by the factory to mitigate knocking (bad) or a pull back in timing (resulting in no damage but lower performance).
However, there’s another advantage of using the highest octane fuel, which is cleaning additives. Fuels are pretty much generic, but additives are proprietary. The higher octane fuels generally have more of these proprietary additives, so using them can better ensure cleanliness within your engine.
You can also run a bottle of injector cleaner through it every few thousand miles and save the prices at the pump
If you want the additives in all octanes use TOP TIER FUEL.
100% agree. Here’s a link to Top Tier fuels.
If you’re trying to cut costs, you could sell the car and get something more economical.
The old man is on the right track with the sprinkling. But the issues will be like everyone else is saying with the pinging and the efficiency issues but will take longer for it to be an “issue” that needs to be fixed now
Read the manual. Do what it REQUIRES.
Over time, in a high-compression engine (over about 10:1), using a lower octane fuel than it’s engineered for will result in knocking (detonation), formation of hot-spots, and eventual preignition.
In an air-cooled or in a non-interference engine (lower compression, both, usually under 10:1) there is absolutely no benefit to running higher octane fuel.
The higher the octane number, the more tolerant of higher pressures before ignition.
Do you like gambling? Detonation is the term for uncontrolled fuel ignition, and is the possible result of running lower octane than required. The car can somewhat mitigate the issue by trying to adjust the spark timing, but if you’re lucky and the fuel ignites spontaneously in multiple locations you can get pressure spikes in the 10’s of thousands of psi. If they happen in the right place little pieces of piston / cylinder wall start getting blown off, then maybe they land somewhere they shouldn’t. Just buy the correct gas.
My 1998 Mercedes ML320 takes 91. The few times I tried 89, a check engine light comes on. I did not read the code but I’m sure it’s a knock sensor denotation code. It goes away after I refuel with 91.
It won’t have as much power and may knock
Nothing serious, it’ll just pull timing under more intense load. If not being driven hard, I don’t think there’s any evidence that it hurts the engine.
People who are saying “just follow the manual” are right, you should, but they aren’t providing any technical analysis and should generally be ignored.
but they aren’t providing any technical analysis and should generally be ignored.
why should they be ignored? they dont have to explain why, the “manual” says what is needed, so saying “follow the manual” is sufficient
No one has to do anything, genius.
It’s a false economy, you will get better gas mileage using premium gasoline in a car that’s engineered to use it.
Really nothing. I have an old VW tuned for 93 on the ECU but I can still run 91 and 89 the ECU will compensate for it and I’ll lose a bit of performance to prevent knock.
Modern cars have knock sensors that will adjust the timing on the fly, but there is a limit of how far it can adjust. If the timing is fully retarded and the engine still knocks damage could occur. Retarding the timing also retards engine performance. Around here (LA) to go from regular to midgrade is .10/gallon and another.10 to go to premium. So you are saving .20/ gallon. Assuming a 20 gallon fill up you are saving $4.00. So save 4 bucks and you won’t get all of the performance you paid for. So if $4.00 is going to change your lifestyle may I suggest you sell your Benz (Audi, Porsche, Vette, etc.) and buy a Corolla. They run on 87.
If the car is designed for/recommends 91 octane and is not modified, use 91 octane. Presuming this is a relatively modern car:
Using a higher octane than recommended is wasteful and you won’t see any benefit as the car can’t “take advantage” of the higher fuel grade.
Using a lower octane can cause poor/reduced performance, reduced fuel mileage, and increased deposits inside the engine due to an incomplete/in burnt fuel mixture as the vehicles computer will likely sense the lower grade fuel (through knock sensors) and adjust the ignition timing to prevent major engine issues.
I’ve also personally tested the savings on a few vehicles I’ve owned that require 91. Switching to a lower grade fuel only results in a few dollars savings per tank, but also got notably reduced MPG, negating any savings. In a 20 gallon tank, you’re usually only saving $6-$10, depending on the cost difference between 87-91.