<VV> Economics Re: Regular Gas and Fuel Economy in a 110hpCorvair

[Jim Burkhard]

You are definitely wrong on count #1 on count #2, it is "depends"

Count#1:  Best highway fuel economy results from peak thermal 
eifficency. This occurs when you time the spark event right at MBT 
(minimum advance for best torque).  If you over-advance, the thermal 
efficiency will gradually drop off, but over-advancing typically isn't 
a concern since the engine will knock like hell first and let you know 
not to run there.  On the other side of the coin, If you *retard* the 
spark timing from MBT, efficiency drops off more quickly. You slow the 
combustion process and more combustion occurs later and later in the 
expansion (power) stroke. The slowed combustion means more heat gets 
transferred to the head and cylinder walls and out the exhaust and 
less is doing useful work on the piston. You lose performance and fuel 
economy AT THE SAME TIME (!) and get everything a lot hotter. Bad idea.

Count#2:  In a world of pure regular gasoline against pure premium 
gasoline, yes the premium gasoline often does have a very slightly 
lower volumetric enegry content on account of the specific 
hydrocarbons in teh blend.  The effect is pretty slight, and more 
importantly basically gets lost in the wash when both premium and 
regular fuels get adulterated with oxygenates such as ethanol, or 
until a few years ago MTBE.  When said oxygenates are added, the 
relative energy differences are very puny. Still, it makes sense to 
set the spark timing to where it should be and run the minimum octane 
needed to keep the car from knocking. There is no benefit to running a 
  typical 95hp Vair engine on premium (it doesn't need it). In 
contrast, it really dumb to take a 110hp Vair engine that typically 
*requires* premium and retard the spark to keep it running (hot, weak, 
and inefficiently) on regular.  Better to just buy a 95 engine...

QED:
a. It is best to run spark timing right at MBT (approximately by the 
factory setting if you don't know better). If you can't get there, get 
as close as you can without knocking.

b. Retarding the timing so that you can run on regular to save $ is a 
pretty dumb thing to do. If the engine was designed for premium, run 
the recommended timing with the recommended fuel. By retarding the 
spark advance, you lose power, hurt the thermal efficiency, and run 
the heads hotter which makes burned valves, dropped valveseats, 
pre-ignition and knock all the more likely.  Doing so in an emergency 
when you need to get home and can't find premium is one thing, but 
purposefully doing this to "save money" is a fool's errand.

Jim Burkhard

Shaun McGarvey wrote:
> I believe retarding timing gives better hwy mileage.
> I also believe that regular fuel gives better mileage.
> Am I wrong?