unnecessary octane = carbon?

No truth to it at all, Ryan.

I remember like it was yesterday how gas station attendants would warn customers against selecting the highest octane on the blender pump because it could "burn your valves"

At one time perhaps.

Older higher octane fuel blends tended to be higher in olefins that might not completely burn and could leave some carbon deposits. All reformulated gas these days are restricted in the amount of certain components, including olefins. There are also detergency requirements. Even if the olefins tend to create carbon, it's not a problem if the detergent level is high enough to prevent buildup.

Geeze we just saw this one in the chrysler newsgroup again.

The latest theory constructed around this idea is that the knock sensor doesen't hear the engine knocking so it keeps enrichening the mixture until it just begins to hear knocking, since it never will due to the high octane it makes the engine run too rich.

We have one regular poster who believes high octaine gas causes fouling fervently, most people just give up and ignore these posts.

Ted

when the computer enriches the mixture to prevent pre-ignition and ping. A higher octane gasoline will eliminate the problem.

On Tue, 30 Aug 2005 17:41:07 -0400, Mike Walsh wrote:

when the computer enriches the mixture to prevent pre-ignition and ping. A higher octane gasoline will eliminate the problem.

The ECM does not increase the fuel to reduce detonation. That would result in increased hydrocarbon emissions. The ECM retards the ignition timing and, in some cases, increases the EGR flow to combat detonation or ping. The octane rating indicates the fuel's resistance to autoignition or detonation. The major, but not only, determining factor for the requirement for increased octane is the effective compression ratio of the engine. All manufacturers recommend the correct octane for their vehicles. If there is a little ping, there is no problem and is, usually, expected where an engine is optimally tuned. Just because you hear an occasional, ping does not mean you engine needs or will perform better with a higher octane fuel. In many cases a bit of ping is, in fact desireable as it will many times dislodge carbon before it becomes a problem. All gasolines produce carbon as a part of the combustion process. It is, therefore, necessary that it somehow be maintained at an acceptable level. Occasional light ping can get this job done. That is also why many techs and engineers recommend that the engine be "blown out" occasionally by operating at full throttle for a few seconds. That is also why a vehicle that has always been "babied" or always just used around town can turn out to be a mechanical train wreck when a regular driver gets into it for a while. Best performance and fuel economy are almost always found with a good quality gasoline of the vehicle manufacturers recomendation. If the engine persist in excessive detonation or ping when driven normally, it should be checked for proper operation of the EGR system, proper state of tune and correct ignition timing. If these items are in spec, it may be time for a decarbonising which can be accomplished in many cases by simply "running it through th gears" a couple of times. You almost never see a performance engine with a carbon problem. In some cases a fuel additive can do this. There are several commercial carbon removal processes.

If these fail, it may be time to pop the heads and mechanically clean it. The bottom line is that, if the manufacturer recommends a particular octane fuel, it is most likely what that engine needs. Anything more is overkill and detrimental to you wallet if not the engine. Using a fuel with a higher than recommended octane rating is only a coverup for a problem that either does not exist or needs attention to be properly corrected.

Regards Lugnut

its all that sulfer and additives....

This is the same kind of misunderstanding that folks talk about with the high volatility racing fuel- methanol. Or the high energy jet fuel, etc. If something has high performance it must be because the fuel must be providing more energy, right? Size of engine, its makeup, power to weight ratio, all that is too hard to think about- performance must mean high energy, high volatility, high danger, fuel.

It was as hilarious then as now.

If you were going to use the knock sensor instead of the O2 sensors to calibrate the mixture, wouldn't you want to LEAN the mixture until it started knocking, and then slightly richen it from that point? How would you ever get knocking by continuing to enrich a mixture which is already not knocking?

Good point I hadn't really even bothered thinking about the logic behind it since my BS detector had already gone off.

Ted