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Originally Posted by Duke2.6
The rate of flame propagation does not vary significantly between gasolines with different octane ratings as long as there is no detonation, and the same applies to peak temperature of the flame front, which is where NOx forms. "Premium gas burns slower" is a myth!
To the thread originator - I don't think there is a "problem" with your converters, but it will help in diagnosis if you would post the entire test results for both 15 and 25 MPH including the 02 content.
Duke
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I don't want to hijack the thread, but your comments got my attention. Some time ago, I had classes in college that were tought by the same prof: thermo and internal combustion engines. I remember considerable time spent on flame front propogation, heat transfer, etc. It was stated emphatically that octane is designed to "provide a slower flame front" to maximize power output and minimize detonation. Too low octane in some engines, usually higher compression and timing design, will assure detonation. It can get so hot in some engines that you can get autogenous ignition - same as diesel - when the piston is compressing the fuel/air in the cylinder. It's been a few years since those classes, but I'm guessing the Laws of Thermo have not been thrown out.
Also, here is some written info:
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WHAT IS DETONATION?
Detonation (also called "spark knock") is an erratic form of combustion that can cause head gasket failure as well as other engine damage. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to autoignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.
Mild or occasional detonation can occur in almost any engine and usually causes no harm. But prolonged or heavy detonation can be very damaging. So if you hear knocking or pinging when accelerating or lugging your engine, you probably have a detonation problem.
A DOZEN WAYS TO PREVENT DETONATION
1. Try a higher octane fuel. The octane rating of a given grade of gasoline is a measure of its detonation resistance. The higher the octane number, the better able the fuel is to resist detonation. Most engines in good condition will run fine on regular grade 87 octane fuel. But engines with high compression ratios (over 9:1), turbochargers, superchargers, or with accumulated carbon deposits in the combustion chamber may require 89 or higher octane fuel. How a vehicle is used can also affect its octane requirements. If a vehicle is used for towing or some other application where the engine is forced to work hard under load, a higher octane fuel may be necessary to prevent detonation.
If switching to a higher octane fuel fails to eliminate a persistent detonation problem, it probably means something else is amiss. Anything that increases normal combustion temperatures or pressures, leans out the air/fuel mixture, or causes the engine to run hotter than normal can cause detonation.
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This is not meant as a challenge to you, rather asking for more information or explanation.