<VV> Re: Old Flame Speed

[Jim Burkhard]

Padgett wrote:
> Completely true but when I learned about gasoline fuels,  one of the 
> "fuel's characteristics" was that the higher the octane, the slower the 
> burn rate and was a byproduct of "resistance to self ignition"). Has 
> this changed ? 

It hasn't changed... it was never right.  Basically, it and the rule 
of thumb about advancing are old racer's wive's tales.  If you learned 
that at GMI, maybe your professor spent too much time bull$%^&*ing at 
the paddock and not enough time in the lab with his combustion 
analysis equipment and a bomb to measure burnrates. :-)  Hee hee... my 
wife went to GMI, so it's occasionally fun to pick on it.

True, different gasolines may have *slightly* different burn rates 
that are a function of speciation (which HCs they are made of) and 
additive content.  It's not that big a deal though, and you will find 
that it does not correlate all that well with octane rating.

 >Otherwise why would we advance the timing for hi octane
> and retard it for low ?

You don't really "advance" the timing for high octane fuel. Rather, 
you set it to what it should be in the first place based on combustion 
characteristics for the engine (influenced by engine speed, load, 
compression ratio, mixture motion, etc.).  This is called MBT timing 
(minimum advance for best torque).  With low octane fuel you can't 
reach MBT timing (if you try the engine knocks like mad), so you 
retard until you are out of knock and lose some thermal efficiency (= 
torque, power, and fuel economy) in the process.

hope that helped!

Jim Burkhard