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Old 07-31-2003, 10:58 AM
MBR MBR is offline
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Join Date: May 2003
Posts: 3
Question Fuel Requirement Question

I have an 97' E320(Straight 6 cyl) and a 99 ML320 (V6). The Owners manual indicates they both need 91 octane fuel. Locally we have 87, 89 & 93 Octane. I have been putting 93(we dont even have 91 here) in them and they both run fine. My question is, do I REALLY NEED to put in the 93, or does anyone think I can get away with putting in only the 89 or even the 87. Obvioulsy, if money was no object I would not even be posting this question, but if I REALLY dont need this extra octane and expense I would rather not use the higher octane. I have not tried using the lower octane yet, as I was going to see what others had experienced here before trying it. Could/would using the lower octane cause damage in anyway? I understand I may lose a bit of power, but probably not enough to notice and if there was pinging or obvious loss of power I would go back to the higher octane. We use the cars for mostly around-town driving. Thanks in advance for your help! Mark
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Old 07-31-2003, 11:13 AM
CEC CEC is offline
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Join Date: Dec 2001
Location: GA
Posts: 48
The best suggestion is to use the search function. This topic has been covered many times here on the forum.

1985 230TE 5 speed
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Old 07-31-2003, 11:27 AM
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Join Date: Mar 2003
Location: Southern California
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Modern engines have knock sensor(s) and at least two spark advance maps. Since MB engines have fairly high compression ratios, premium fuel is required to run the most aggressive spark timing map for best knock-free performance.

If the knock sensor detects persistent detonation, it will default to the less agressive advance map and this is what will happen if you run regular unleaded (87 PON) fuel. The knock sensor will detect light detonation that you probably can't hear.

Fifteen years ago most OEM owners' manuals had dire warnings of engine damage if lower than recommended octane fuel was used. Now they usually just recommend, say, 91, and say that if you use a lower octane you may notice a loss of performance. This is because of the less aggressive timing when lower octane fuel is used, but you won't harm the engine.

I've recommended a number of owners of several makes who were using the recommended premium fuel to try 87 octane fuel and none of them even noticed a performance difference. I got tired of hearing them whine about having to buy premium fuel when gasoline prices were high.

To satisfy yourself that using lower octane fuel will not cause detonation try the following experiment. Let the fuel get close to empty and add five gallons of regular unleaded. Drive the car and listen for detonation and determine if it has a noticeable loss of performance. If you detect significant detonation (which you probably won't) fill it up with premium. If not, when it gets close to empty again add five more gallons of regular unleaded and continue the test. By the time you consume the second five gallons of regular unleaded you should know if the engine knocks or you experience an unacceptable loss of performance. If you find no adverse affects, fill it up with regular and continue to use it.

The only difference between regular and premium fuel is octane rating. All modern gasolines have detergent additives to keep your fuel system, injectors, and valves clean.

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