<VV>Octane/Ping

[Padgett]

>  Re: Octane/Ping
<rant>
OY! Read my lips: if there is any difference in BTUs between regular and 
premium, it is premium that has less.

Engines run best and give maximum mpg on the *minimum* octane gasoline that 
allows complete combustion without detonation (ping).

Keep in mind that we are really talking a fairly narrow range of engine 
characteristics and an equally narrow range of octane numbers and for my 
purposes, at or close to sea level.

"Premium" gasolines have additives to slow down the flame propagation rate 
and raise the flash point. They do not add to the "power" of the gasoline 
but rather change its burn (you want an even burn of the mixture and for 
economy cruise you want it a lot leaner than stochiometric) properties.

Even burn in a SI engine is a steady and complete combustion of the mixture 
from a single flame front starting at the spark plug. If a second flame 
front starts from a hot spot or the exhaust valve (why you index plugs tby 
pointing the gap at the exhaust valve) nasty things happen when they 
collide (the knock or ping) but mainly because the mixture burns much 
faster (two flame fronts ~ twice as fast) than planned and max pressure 
occurs too early. (see previous posts).

Once you wrap your mind around the fact that inside the combustion chamber 
you want an even burn that takes a measureable amount of time and concludes 
at a particular point, everything gets much easier.

This is why my original observation was that a N/A engine with only 9:1 
compression and a 3 1/2" bore should not require over 89 PON (and probably 
87) to develop maximum power, Something I intend to prove once I find the 
right car.
</rant>

Padgett