Re: Super unleaded
Higher octane is only of use to prevent detonation and pre-ignition, and the additives discourage the unintentional ignition of the air-fuel mixture.
On stock motors, there's no reason why it should be of any significant benefit, although on older, worn ones, it could potentially help to control the flame front, minimising uneven combustion caused by hot carbon deposits and so on.
However, its primary function is to prevent pre-ignition (pinking) on high compression motors, where the reduced combustion volume, and the higher gas temperatures generated by compressing the fuel air mixture (Boyles Law?) can cause the mixture to explode before the spark, leading to holed pistons and so on. Ideal combusion assumes that the mixture does not ignite until the spark, and the flame front advances uniformly from the point of ignition. This gives you the highest efficiency, and the highest power, as most of the fuel is burnt during the power stroke, in a controlled fashion. Detonation, where ignition occurs at several positions in the combustion chamber, reduces both power and efficiency, as the uncontrolled advance of flame is more likely to leave unburnt pockets of fuel, and combustion efficiency is reduced. Ideally, you want all the fuel to burn uniformly as the piston moves down, but detonation can result in too much combustion, too early. If I remember rightly (and that's unlikely at my age) the ideal combustion chamber would be completely spherical, and the spark would occur at the centre of the sphere, which is clearly impossible using any technology available today, but explains the hemispherical piston crowns and head recesses on many performance engines.
BTW: adding Redex to fuel may be harmless, but adding 2-stroke isn't, and is more likely to cause carbon deposition in the combustion chamber, that will ultimately lead to detonation. Also, adding oil to fuel reduces the amount of fuel, so the air/fuel ratio is reduced, and the motor can run weak. This used to be a big problem with petroil 2-strokes, where well-meaning owners would add extra oil, thinking they were improving engine life, only to have their Villiers/James/Bantam, etc. seize solid, due to overheating caused by weak mixture. Can't see a dribble in the tank having much effect on a Harley's mixture, but probably does more harm than good?
Mind you, a spoon or two of Castrol R can make your exhaust smell like the road racers of the sixties...
