kb13 - 8/15/2004 10:57 PM
I spoke with a chemical engineer who did research on the subject and discovered the only difference between high octane and regular was additives. He said, unless you are experiencing knock, use the lowest octane. They just add additives to the same basic gas to increase the octane level.
For the most part that is a true statement. There is more than one way to make a higher octane fuel and not every company uses the same recipe. The octane rating is really a ratio of octane, the stable component under pressure, to heptane, the unstable component or
a mixture that gives you the equivalent performance of that ratio. One thing not really discussed in this thread is the fuel's sensitivity. A fuel with high sensivity would be one that behaves like a lower octane rating under stress. I imagine that this is more of a concern in areas that use RON instead of (R+M)/2. That's because a high RON fuel can have a low MON. That may be why the dealer recommended that Skipper use the 98 rating instead of 95 [based on some anecdotal experience].
Just because you don't have ping or knock, because the engine management system is able to compensate, doesn't mean that the fuel is burning efficiently. It won't burn efficiently and there will be deposits that could cause bigger problems later. This is a cost you won't see right away.
A recurring theme seems to be that using a lower than recommended octane will save you money. Now, how much are your really saving if you use the lower octane? If you live near me, you might see 87 at $2.11, 89 at $2.21, and 91 at $2.31. Most of our current MB's in the NA require 91. Let's forget about 87, it is too low. The difference in price between 89 and 91 is $.10/gallon or 4.5%. Based on some reports on the R170 forum, using 89 degrades fuel economy by ~3%. That gives a net savings of ~1.5%. If you purchased 20 gallons of 91, you would need 20.6 gallons of 89 to go as far. You would pay $46.20 for the 91 and you would pay $45.53 for the equivalent amount of 89. That would be a net savings of 67cents not the $2 you saw up front. Rather than trying to save a few cents with the lower octane, why not up your tire pressure a little and get a gas card, or equivalent, that gives you 5% back on your gas purchase. Plus you'll get better performance and you won't have to be concerned about those nasty deposits.