fatboy7

Octane ratings aren't a measure of how fast the fuel burns.... or how easy it is to ignite with spark.... it is only a measure of resistance to detonation period.

Now, with that being said, usually higher octane fuels have more aromatics (circular carbon structures) than the equivalent lower octane versions. Octane rating , in and of its self is a misnomre as the fuel may or may not have lots of octane. The rating is determined by a test that uses octane (given a performance number of 100) and heptane (given a performance number of 0) in a special engine. When the fuel under question detonates with the same consistancy as prescribed mixture of the above substances, the percentage of octane used is the octane rating. (simply put)

So why does the rotary make more power on lower octane fuels than on higher ones? The answer is complex, but is related to the rate of burn of the specific chemicals in the fuel, which are more abundant in lower octane fuels.

Truth be told, genrex, is somewhat more correct. In the Typical lower octane fuels, the flame actually travels slower than the flame front in higer octane fuels. This makes sense if you understand detonation. Faster burning gas means that the flame front gets to the end gasses before they have time to become pressurized and overheat, thus exploding. This is one of the ways detonation, but the other is probably more noticable: fuel composition. Here is the kicker: Different refineries make different fuels but have the same octane rating. This is why you can't and shouldn't draw a hard line here. Temperature and humitiy also plays a huge roll, ie:

In the winter here in UT along the mountains, we often get a thick haze of pollution due to an inversion. To combat the situation, in my county gas stations are required to sell gasohol (a mix of typical gas, and alcohol). This gasohol has to pass the same tests for octane that the normal summer gas passes to be sold as regular, premium, etc. Obviously this change in the composition of the gas can and does have a huge affect on performance, all while showing the same Octane as before. While not every one has alcohol added to their gas, the composition of gas changes seasonally, so that your car will start and run more optimally (hopefully) with the correct gas, Volatility for instance is increased in winter so that less gas will be required for the car to start.

Ok, so back to the topic of the tread, looking at the "typical" gas (summer gas at sea level, at perfect conditions, etc.). Because of the elliptical orbit of the rotor, and the obvious differences in the combustion chamber vs. a piston engine, the point in the cycle (degrees ATDC) where a even pressure(obviously not what really happens) creates the greatest torque is later on our engines than piston engines. I'll call this the function of torque@ given pressure the mechanical gain. So the mechanical gain peak is later in the cycle. Thus a slower burning gas will have its peak pressure slightly later, which is more effeciently used by the delay in mechanical gain, so it makes more torque. This has a limit though, because as the gas burns the rotor continues to rotate, and the chamber volume starts to increase more rapidly, so the peak pressure will be lower. This is the reason behind spark timing tuning, and with proper spark timing many differing fuels can give comparable results. This means we are probably fighting over a 1-2% increase with the perfect fuel vs. what is available. Sheesh, the tuning on our various cars differs more than that. Anyway.... long post, hopefully useful to someone

browney

Thanks fatboy7 that was a great post!