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Old 08-15-2005, 05:09 PM
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Join Date: Sep 2003
Location: Florida
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Premium to Regular

I have read in Newspaper that cars will run on regular as well as premium, article stated that computers on engines can adjust for anit Knocking. Quoted a Mercedes Dealer in New Jersy getting better mileage on regular. I have a 1988 300SE and would like to know if this is true and applicable to my car.

Thanks, Brian

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Old 08-15-2005, 05:21 PM
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Your car does not have a knock sensor. Cars with knock sensors and associated technology will indeed retard the timing when knock due to low grade fuel is sensed.

That said, yes such a car will indeed tolerate regular fuel without hurting the engine, but it will hurt fuel mileage. Now that gas prices are up, the difference between the price of regular and premium as a percentage is less. When regular was $1.10 a gallon, premium was $1.30 a gallon making premium about 18% more expensive. With regular now at $2.57 and premium at $2.77, premium is only about 7.7% more expensive.

In a high compression engine such as many late model MB's, running regular instead of premium would be false economy.

Back to your car. Since there is no knock sensor, you could do damage to your engine.

You should take comfort in the fact that your high compression engine is more fuel efficient than if it were the same engine, but with lower compression ratio allowing the use of regular fuel.

BTW, adjusted for inflation we still have cheaper fuel than in many years past. 1981 took a bigger percentage of our paychecks to buy fuel than it does today.

Have a great day,
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Old 08-15-2005, 05:33 PM
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Join Date: Jun 2002
Location: Monrovia, CA
Posts: 496

Your car, like my 86 300E, doesn't have any type of "knock" sensor to detect preignition knock and retard the timing to prevent it. You really do need to run premium in the older cars. You may experience some knocking even with premium on hot days especially under low to moderate rpm/high load situations. Avoid this as much as possible as preignition (basically, the "explosion" of the fuel/air mixture in the combustion chamber rather than the controlled rapid burn that it should be) is very hard on the engine components. It's been known to hammer or melt the aluminum piston crown until it blows a hole right though it, with rather disastrous results, as you can imagine. It might even be a good idea to run an octane booster since they require 92 octane minimum and the lowest (at least here in CA.) seems to be 91 octane.

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Old 08-15-2005, 11:18 PM
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Join Date: Oct 2002
Location: Near Raleigh, NC
Posts: 580
My wife (had to be) borrowed my car 2 weeks ago (it was 103 here in NC).
It was low in gas, so she decided to put 87 (I had told her a million times not to do that, only 93).
By the time she got home the engine sounded like a diesel due to detonation.
I was livid. I told her she could ruin the engine just to save 2 dollars.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
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Old 08-15-2005, 11:21 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,270
Even though your car does not have a knock sensor, you may be able to run lower octane fuel under many conditions, especially in cooler weather. Fuel mileage and power likely will not suffer. Most who make this change, especially on knock sensor equipped cars, find no difference in either, but this is purely anecdotal evidence and reports vary.

This subject has been beat to death. Suggest you do a search.


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