In a quick search using "google" I found the following very
interesting FAQ about motor fuel, octane ratings, lead, and
additives:
http://repairfaq.org/filipg/AUTO/F_Gasoline.html
It assumes that most cars have engine management systems,
but does give details useful to us.
Interesting points include that indentically manufactured
engines may differ by as much as 7 "octane" units in their
actual fuel requirements. Also, modern fuels contain all
sorts of additives that do have good effects -- even for
our old non-managed engines. Perhaps this explains why
a properly tuned engine with all of the early-'70's
"emission control systems" removed will still pass state
tail-pipe emissions tests with flying colors. Different
brands of fuel are different, fuels with identical
octane rating may be quite different in energy content,
combustion temperature, and "flame speed"
But, the bottom line for an engine without an engine
management system is that a.) it needs to be tuned for
the particular fuel that it runs on, and b.) there is
an octane rating that will give most efficient operation.
This value will depend on many factors other than
compression ratio, and does tend to increase with engine
wear.
Douglas McKinnie
'72 B-GT
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