higher octane petrol

[hocky1]

I used regular unleaded and my 1994 C180 goes alright though feels very under powered. Then I used premium unleaded and it feels better. Now I am using 98 octane premium and somehow, the engine feels odd, running a bit rough, power is there, but at higher speeds. At low speeds, it feels like the regular which is only 89 octane I reckon. The normal premium is about 94 here, any Aussies can correct me if I'm wrong. Sometimes I regret getting this car because the motor has very low power and I have to step hard on the gas. Then its got no 5th speed, so you keep stepping harder and harder to get along at 100kph or 110kph on the country highways. Whats making it worse is its winter now and somehow, the suspension which is unrelated feels a lot harder now than Summer??? . Maybe I should release some pressure or get the shockers up front inspected for total or close to total collapse!

Originally posted by hocky1
I used regular unleaded and my 1994 C180 goes alright though feels very under powered. Then I used premium unleaded and it feels better. Now I am using 98 octane premium and somehow, the engine feels odd, running a bit rough, power is there, but at higher speeds. At low speeds, it feels like the regular which is only 89 octane I reckon. The normal premium is about 94 here, any Aussies can correct me if I'm wrong. Sometimes I regret getting this car because the motor has very low power and I have to step hard on the gas. Then its got no 5th speed, so you keep stepping harder and harder to get along at 100kph or 110kph on the country highways. Whats making it worse is its winter now and somehow, the suspension which is unrelated feels a lot harder now than Summer??? . Maybe I should release some pressure or get the shockers up front inspected for total or close to total collapse!

which 98RON petrol are you using? I find optimax does a simular thing to my mothers car. BP ultima is much better.

[bjcsc]

I am no gas expert, but this is what I understand to be the case (and it's probably more than you wanted to know): You've have two things going on when you get higher grade fuels - higher octane rating and usually detergents and other additives that your engine might like. Either one can make your engine run a little better. However, running higher octane than necessary won't make it run worse, ie. your engine doesn't really know what the octane level is, unless it's too low. Engines do not get more power from higher octane fuels (Most people think they do) but instead they get it from the advance in timing that higher octane fuels can support without detonation and/or from higher compression ratios. Since your compression ratio can't vary itself, we'll say nothing more about it. Detonation needs to be defined, because it is often confused with pre-ignition. It is not the same thing as pre-ignition, but it is the same thing as knocking. When you increase the timing, the spark occurs earlier in the stroke. There is a point at which the advance will be too much and cause detonation. Detonation occurs when the mixture gets a spark and it is not sufficiently compressed enough to burn at a regular and controlled rate. Instead it flashes off, it detonates, burning much faster than it would have a few degrees back. This creates a full pressure condition in the cylinder, but the piston is not at the top of its stroke and the engine forces it through and against this wall of pressure. You hear a knock, or worse throw a rod. This is very hard on the involved piston/rod, etc..

A higher octane fuel is less volatile than a lower octane fuel, and thus more stable, allowing more advance without detonation. Similarly, a vehicle requiring higher octane fuel can be set up to run lower octane fuel by retarding the timing - and a loss of power will result from the loss of advance. Running lower octane fuel without retarding the timing may cause detonation depending on how close to the "line" the timing is set.

Another interesting observation is that 87 octane fuel produces more power per unit volume than 100 octane fuel (more volatile = more power). But you won't see that power in an engine because the 100 octane fuel will allow for both a higher compression ratio and more timing advance and thus way more power - but it's not technically attributable to the octane level of the fuel.

So how does this relate to your question?
1. The higher octane fuel is not causing your engine to run worse.
2. Ignition timing and the octane level of your fuel are related, as above. I know nothing about a C180, but if the timing is adjustable, it or some part of the system that controls it, could be your problem. If it is not adjustable and you are using fuel below the minimum octane requirement you could be causing the problems by doing so.
3. If you're having trouble reaching 100kph, you've got some kind of serious problem. But depending on how computer and emissions controlled a C180 is, it could be any number of things - none of which I can even comment on. If I were you, I'd take it in for service ASAP and hope that it is a timing issue.

[hocky1]

Tonight I just had the chance to smoke a few cars including Holdens, Fords, Volvos and a few others! I just stepped hard on the acc and even floored the button to get away faster. Some people were amused or shocked how this 1799cc little Benz could smoke them, even for the first 100 metres was good enough, lol. Must have been the 98 octane petrol I have been using lately! The car is usually very slow off the mark. Today I also had the kind wife to ride with me and she is only about 50kg, I am 85kg and the C180 is 1350kg.

For many years now I've been using 91 octane fuel in my '91 300E because that was the best I could get in the US states I've lived in (Minnesota and Arizona). These fuels include various additives to control emissions in those states and apparently they reduce the octane rating.

When I take trips, I've been able to obtain fuel with octane ratings as high as 94. When I do, I get much better fuel economy.

I believe this is because I am getting fuel with less crap in it.

[Hocky]

I'm back to 94 Regular unleaded and the car feels great. Also my pocket feels better too. The 98 gave me a feeling of pressure to go at lights under gear. We don't normally free the auto gears at lights. Its not so comfortable to do it in a Benz due to the noise of the gear change and the slight jerk. In my Camry, you hear and feel nothing when you shift to free at the lights. Thats why I always do this in my Camry esp when trains are going left and right, sometimes there are 4 trains and it releases your tensions at the wheel(usually need to pull the parking brake lever too). I may have the idle speed reduced to a lower rate like 600 or 650 if it doesn't result in the engine stalling. Now its more like 900 or 850. What is the recommended level???