James Robinson wrote:
> Thomas & Sons Cycle Shop wrote:
snip
> A second consideration is fuel efficiency, since the anti-knock sensors
> will retard the timing on an engine, when lower octane fuel is used in
> an engine designed for higher octane fuel. The side effect is that fuel
> efficiency will drop. That means that people might be thinking they are
> saving money by using lower cost fuel, when they really aren't.
> Further, the engine will lose horsepower, which is part of the reason it
> was designed for high compression in the first place.
>

Not necessarily. In fact usually not the case. The octane requirement
for spark ignition engines is a funcion of operating load. That is, as
load increases, octane requirement also increase. That's the load
effect. Then there is the speed effect. Generally, octane requirement
DECREASES as engine speed increases (for engines with standard valve
timing).

What this all means is that at highway or around town cruising speeds
engines are operating at fairly light loads. Here premium fuel octane is
not needed. Even the "premium recommended" engines will be able to run
optimal ignition timing.

Only only during heavy accels or WOT operation will the fuel octane
affect performance. But, since most people spend a low fraction of their
driving time at high loads the net effect on fuel economy will be very
small.