Commandos and fuel octane rating

worntorn said:
I just ride my bike up the big hill full bore and see what happens. The results are very repeatable.
Sometimes dyno results aren't!

High Octane fuels have a bit less energy per unit of fuel.
The phrase high Octane has come to mean high powered but really it's just the opposite.
You made the motor into a high compression, high powered thing so now you'll need to use some slightly lower powered gasoline to make it work!
Al made a suggestion that Octane ratings are bullshit. I don't think they are.
Some here might recall some of the high performance v8s from the 60s. We had a Factory 11.0 to 1 cr 429 engine in a 69 T bird. If you were a silly cheap teenager and tried to run it on regular, the pinging/pinking sounds were incredible.
On high Octane fuel there was none of that.
As has been stated earlier, there is no reason to use fuel with a higher Octane rating than needed. You will lose a bit of power in running extra high Octane fuel as it slows the burn, that is how it prevents pre-ignition.

Glen
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If you slightly retarded the ignition or jetted slightly richer, the pinging might have disappeared without much power loss. The suggestion might be that higher compression ratios get more energy out of the fuel. However at higher compression ratios, you probably use more fuel, for not much gain.
I apologise for mentioning race tuning, however it is much the same game as tuning for road use. The main difference might be sensitivity to changing weather conditions. When I rode on public roads, I sometimes used Avgas at about 11 to 1 compression. But ordinary petrol would probably have made my bike perform just as well. I was too lousy to buy jets and needles even for single carb. Every one of my Triumph 650s had race cams. They were fun to ride, but probably nowhere near as fast as they could have been. I was pretty ignorant.
With any fuel, the optimum is always balance between comp. ratio, ignition advance and jetting. Normally, the first three variables are set and we adjust the jetting. It is possible to go another way - get the jetting correct by using an oxygen probe on a dyno at an arbitrary ignition advance, then advance the ignition timing to get maximum torque. When that is done, the difficulty in getting the jetting changes fine enough, is overcome. Ignition advance is not always changed step wise, sometimes it is altered by a lever on the handle bar. A distributor can usually be turned, to change the ignition advance.
 
The energy content of the fuel is determined by it's chemical composition. I think that for MotoGP, the fuel is specified. Elf fuel is probably mainly hydrocarbon. Aromatics can provide more energy. In the 1930s Mercedes race cars used a blend of methanol, benzene and acetone. I once used it, it is very good but very poisonous. - BP JA and BP JB were about 60% benzene - we used to be able to buy them in the 60s. You can get leukaemia that way. These days people are freaky about the benzene content in ordinary petrol. Toluene is much safer.
 
Another word for pinking/pinging/knock/det(onation) is pre-ignition.
I understand this to mean ignition due to (I guess) a combination of excessive heat and load resulting in over-compression, and is independent of the ignition system, so you could unplug the coils if you wanted, it would still keep on pinking.
The state of the art is to detect the onset before it happens, and then make the necessary adjustments. I don't think my manual A/R K2F is going to cut it, somehow
 
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B+Bogus said:
Another word for pinking/pinging/knock/det(onation) is pre-ignition.
I understand this to mean ignition due to (I guess) a combination of excessive heat and load resulting in over-compression, and is independent of the ignition system, so you could unplug the coils if you wanted, it would still keep on pinking.
The state of the art is to detect the onset before it happens, and then make the necessary adjustments. I don't think my manual A/R K2F is going to cut it, somehow
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The issue with many K2F mags is the timing on one side may be 10 degrees different to the other side due to worn cam lobes and or poor assembly. An excellent way to melt the top of a piston
 
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