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Re: Regular or Ethel ???

On Sat, 28 Mar 1998, James Marriott wrote:

>Not _always_ the case. Keep in mind:  the higher the octane, the lower
>the (volumetric) energy content. For example, my non-knock '87 4kq will
>get worse mileage with 92 octane (R+M/2 method) than with 87.

I thought they're suppose to be the same. The only difference is their
cumbustion rate -- the higher octane ones burns slightly slower than the
one with lower octane rating.

So, assuming engine (etc.) is in tip-top shape, an engine requiring 87 is
fed a higher octane (and assuming it cannot change spark timing) will
extract less power from the high-octane gas -- that is, the engine is not
able to get the right amount (not more, nor less) energy from the
high-octane gasoline.

Conversely, if an engine requiring high-octane gas is fed a low-octane gas
(still assuming it cannot change the timing), it will ping.

Theoritically, if both engines are able to adjust their timing, both will
get the exact same amount of energy out of same amount of (both types)
gasoline... Assuming the engine is an ideal engine (which no human can

In real-world, both (adaptable) engines will have slightly higher output
with the higher-octane gasoline because in case of high-octane gasoline,
the air-gas mixture is usually ignited slighty ahead of lo-oct gas in the
engine rev (where piston is beginning to come down toward cranck shaft)
cycle. The result is the high-octane gas will have slighty longer time to
transfer the cumbustion energy into mechanical energy than the low-octane
gas. Both produces the same energy, but hi-oct has better way of transfer
it to engine power.

True or false?

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