(Hildegrin) wrote:

Higher octane allows you to use higher boost pressures. It doesn't
create more boost, it just allows you to "overboost" the engine at
lower alts. Thus at rated alt and above, increased octane had no real
effect (it may have reduced power by a tiny amount, because the fuel
has a lower calorifc value, I think).


Yes, this is exactly right...some think that the higher the
Octane Rating the more "powerful" the fuel when actually high
Octane fuel is less 'powerful' that low Octane fuel. You get the
extra power because you can increase the Manifold Air Pressure
(boost) without causing DETONATION. This is the whole reason
behind high octane useage. Heavy detonation will trash an engine
in short order so you must prevent it.

Water injection also results in higher engine power in a slightly
different manner. Without water the fuel controls supply more
fuel than the engine needs, this very rich mixture cools the
charge in the cylinders lowering the pressure and so prevents
detonation. With water, the fuel mixture is automatically leaned
considerably by the 'de-richment valve' therefore producing more
power.
--

-Gord.