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Car Forum / Mercedes-Benz Cars / April 2005Abuse?
I have never put anything but premium octane in my 90 300CE per the owner's manual. Would it harm the engine to put in a lower octane? I have been reading up on this and am not quite convinced that it would be safe. Thoughts? High octane gasoline ignites at higher temperatures than "regular". That means that "regular" will ignite at lower cylinder temps. than premium and so preignite or cause "pinging" - especially in hot weather. However, the engine's knock sensors will retard the engine's ignition timing to reduce the preignition and that retardation somewhat reduces engine power and efficiency (fuel economy). Nothing terrible will happen if you use mid grade fuel - try a few gallons on an empty tank - refill with premium if you're not satisfied. Remember, as outdoor temps. rise there's a greater need for premium fuel. > High octane gasoline ignites at higher temperatures than "regular". > That means that "regular" will ignite at lower cylinder temps. than [quoted text clipped - 7 lines] > > Remember, as outdoor temps. rise there's a greater need for premium fuel. Actually there is no need whatsoever for premium fuel unless an engine has a high enough compression ratio to require it. Check the owners manual for your vehicle and see what is recommended. There is no advantage using premium in a car that doesn't need it. It's a waste in several ways. > Remember, as outdoor temps. rise there's a greater need for premium fuel. Do the same issues exist with diesel? Never heard of high octane diesel... but fuel composition does (or should!) vary seasonally. DAS For direct contact replace nospam with schmetterling --- >> Remember, as outdoor temps. rise there's a greater need for premium > fuel. > > Do the same issues exist with diesel? Not at all, there's no risk of a diesel having preignition (if its injection pump is correctly timed) for the diesel injects its fuel at the correct instant, there's no fuel to preignite before that spray. Better to speak in terms of octane numbers instead of adjectives to avoid misunderstandings. What octane is the OP using now and what is recommended for car by manufacturer? Why deviate from recommendation? How much is saved in a year? Worth the risk? DAS For direct contact replace nospam with schmetterling --- > High octane gasoline ignites at higher temperatures than "regular". That > means that "regular" will ignite at lower cylinder temps. than premium and [quoted text clipped - 7 lines] > > Remember, as outdoor temps. rise there's a greater need for premium fuel. I refilled with midgrade (89) and notice a definite decrease in power and increase in knocking. It is tolerable, but I like the "feel " of 93 octane much better. Thanks for the tip. > High octane gasoline ignites at higher temperatures than > "regular". That means that "regular" will ignite at lower [quoted text clipped - 11 lines] > Remember, as outdoor temps. rise there's a greater need > for premium fuel. What do you think the cylinder temp is after a few minutes of running? Ambient temp is irrelevant with regards to octane. >I refilled with midgrade (89) and notice a definite decrease in power >and increase in knocking. It is tolerable, but I like the "feel " of 93 >octane much better. Thanks for the >tip. >> High octane gasoline ignites at higher temperatures than >> "regular". That means that "regular" will ignite at lower [quoted text clipped - 11 lines] >> Remember, as outdoor temps. rise there's a greater need >> for premium fuel.  Signature----------------------------------------------------------- Fred Emmerich Fred_Emmerich@comcast.net ----------------------------------------------------------- > What do you think the cylinder temp is after a few minutes of running? > Ambient temp is irrelevant with regards to octane. Intellectually I would tend to agree with this argument, but experience teaches otherwise... A hotter car, whether caused by idling or higher ambient temps will definitely be more prone to preignition and hence pinging... Huh? Ambient temp is the air temp coming into your combustion chamber. Unless you have some magic climate-controlled intake system, ambient temp can vary by over 100 degrees in North America, and many other locations - which absolutely affects the temp of the air/fuel mix as it's being compressed. Sorry. I've driven too many vehicles that could get by with a lower octane rating in winter, but suffered pinging when summer came and needed a higher octane rating. For the record, ambient pressure (altitude) and humidity also affect the combustion process. Conrad >> What do you think the cylinder temp is after a few minutes of running? >> Ambient temp is irrelevant with regards to octane. [quoted text clipped - 4 lines] > A hotter car, whether caused by idling or higher ambient temps will > definitely be more prone to preignition and hence pinging... Conrad has got it right nothing worse than the denotation BANG in a cylinder as it is VERY HOT. it is an explosion not a rapid burning as it is suppose be. kind of like taking a ball peen hammer to the pistion. the reason the spark plugs fire before top dead center is it takes time for the proper fuel air mixture to burn . in older <and new> cars retarded the spark because of the dumping of fuel & air in the wide open carb during the tire spinning age of your life. SUNOCO dial pumps were great. from 190 to the 260. GOOD STUFF! the case, minus a few cans! Actually higher octane fuel does not ignite at a higher temperature, it simply burns more rapidly. Here's how pre-ignition happens. With lower octane fuel, hence a slower burning fuel, the air/fuel mixture is ignited at the spark plug. As the flame front moves across the top of the compression chamber, pressure begins to build which drives the piston down in the cylinder. If the octane rating is too low the slower burning mixture begins compressing the air/fuel mixture on the other side of the compression chamber. Since the slower moving flame front can't move any faster it reaches a point where the remaining portion of the air/fuel mixture is compressed too tightly and ignites simply from very high compression....like a diesel. This secondary explosion is the ping you hear. The problem is if you continue to drive under these conditions you will begin burning away either the compression rings on the "far side" of the compression chamber or the top of the piston....or both. I've seen pistons with holes burned completely through the top of the piston. They don't produce much power at that point. One way to use a lower octane fuel in higher compression engines is to retard the distributor so the spark is introduced into the cylinders later than what the manufacturer calls for. This means the piston is moving downward in the cylinder further than normal as the flame front moves across the top of the piston. This relieves the pressure on the "far side" of the chamber and eliminates the "pinging" caused by pre-ignition. Usually retarding the timing 4-6 degrees will solve the problem. Then the question becomes, "do I save money doing this?" All depends. Retarding the spark lowers fuel economy, perhaps a drop of 10% or more. Trial and error and simple math might give you an answer. Want to completely eliminate pinging? Try some 110 octane aircraft fuel. Works great! Check the price first, though. Good luck! Higher Octane fuel does not burn "more rapidly". Octane is derived from the mix of fuels blended at the refinery after cracking. Some of the combustibles you get from refining are; methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together. Heptane ignites at a lower temperature than octane. (For temperature, read compression - the more compression, the higher the temp). Ideally, you want a blend of heptane and octane that is right at the ignition temp when the spark goes off. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). Sometime in the 1940s, it was discovered that cheap gas could be used in high-compression engines by adding combustion INHIBITORS - like tetra-ethyl lead. Combustion inhibitors definitely do NOT make the fuel burn more rapidly. They (get ready for it) INHIBIT combustion. While we've decided that spewing tons of lead into the environment is a bad thing, we still use other chemicals to achieve the combustion inhibiting effect that one gets for a given heptane/octane blend. There are a couple of levels of pinging - one is indeed caused by the proper and desired flame front compressing the remaining fuel- air mixture in the combustion chamber. But another name for pinging is pre-ignition. Under severe conditions, the mixture will actually detonate _before_ the spark. Causes for this are either severely underated octane in a high-compression engine, or an older engine that has a good buildup of carbon crud, the rough edges of which can serve as glowing hot spots to pre-ignite the fuel/air mix. Symptoms of the latter can include dieseling, or run-on, where the ignition is cut off, yet the engine continues to chug and puff on with no spark. With the prevelance of electrically controlled injection systems, dieseling is less heard of these days. Carbed engines would deliver fuel/air as long as the engine was turning, but injected engines stop the fuel when the ignition is cut off. In general, use the lowest octane you can get away with. The myth of Premium being somehow faster-burning when to make higher-octane fuels combustion inhibitors are typically added is still a prevalant piece of internal combustion folklore. Conrad P.S. Be nice to our air - av-gas still contains lead - use it if you gotta fly somewhere - not drive to burger king. > Actually higher octane fuel does not ignite at a higher temperature, it > simply burns more rapidly. Here's how pre-ignition happens. With lower [quoted text clipped - 25 lines] > though. > Good luck! The (Nutter) plates in the cracking towers pull off different types of distillants at different heights (temps). We're not talking about gasses here but rather direct petro products. In fact higher octane fuels (with additives) do support a faster moving flame front. I simply refer to this as a "more rapid" burn, or burn rate. The KISS process is more practical when trying to explain what takes place in these instances. The fact is the description is accurate. Pre-ignition is normally referred to as dieseling and gasoline engines will do this if the temp inside the cylinder is hot enough and something hanging around in the chamber is hot (glowing, usually) enough to keep the ole pistons moving. Nuff said on this thread. > Heptane ignites at a lower temperature than octane. (For temperature, > read compression - the more compression, the higher the temp). [quoted text clipped - 3 lines] > (or some other combination of fuels that has the same performance of > the 87/13 combination of octane/heptane). Modern refined gasoline generally contains between 70% and 90% isooctane (and is usually on the lower side). The octane *rating* posted on the pump only indicates the fuel's performance as compared to fuel with this percentage of isooctane. This means a fuel with an 89 octane rating might only be 75% isooctane, but it has enough anti-knock compounds added so that it performs as if it were 89% isooctane. This performance is proven in test engines. By the way, octane ratings are linear, so you could use half a tank of 89 and half a tank of 93 and be running 91 octane. In fact, some gas stations only store two tanks of fuel (their highest rating and their lowest rating) and then combine them at the pump to get the "middle" grade. Signature- RODNEY Good info. Thanks for the reply. >I have never put anything but premium octane in my 90 300CE >per the owner's manual. If you do try a lower octane fuel, make sure you check your milage on the different grades. My 78 Buick Estate Wagon with a 455 got more than 20 percent better milage on high octane fuel with less than a 20 percent increase in cost, FWIW... >> I have never put anything but premium octane in my 90 300CE per the >> owner's manual. [quoted text clipped - 3 lines] > than 20 percent better milage on high octane fuel with less than a 20 > percent increase in cost, FWIW... Good idea. This is another way to show whether your car "needs" premium. If the mileage stays the same on higher grades, you are wasting your $. I guess after the 20 percent gain the estate wagon got about 8 MPG :~) Marty >I guess after the 20 percent gain the estate wagon got about 8 MPG :~) Yeah, though raplacing the driver with someone less agressive got it up around 15MPG, so it wasn't _all_ fuel... Strangely, I get much higher milage when I drive like a little old lady in my E320 4-Matic wagon than when I drive "normally". 8*) |
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