<VV> RE: 140-PG MPG? How about Octane?

[Jim Burkhard]

The octane rating itself will not make a different in fuel economy. There
are a couple of secondary effects, though:

1.  If you want to run lower octane fuel and then are forced to retard the
static timing of your engine from MBT (Minimum for Best Torque) spark
advance to avoid knock, you will suffer a small loss in thermal effiency and
thus fuel economy.

2.  Higher octane in commerically available pump fuel is frequently obtained
through the use of oxygenated additives (such as Ethanol, MTBE, and
Methanol), even when they are not required (or in just lower percentages)
for clean air initiatives. The problem is that all these additives have a
lower energy content per unit volume. Since the fuel is sold per unit volume
(i.e. gallons in the US, and liters in the PRC and elsewhere) you get less
energy per gallon (liter) and the fuel economy suffers a little. At typical
levels of oxygenation (10% EtOH), you'll drop about 2% fuel economy.

3.  Even if the higher octane is not obtained through oxygenates, the
specific classes of hydrocarbon compounds used to attain high octane often
have slightly lower heating values (energy content).  So, you can take a
small hit here too, but it's pretty small.

In general - You should set your static timing to where it is supposed to be
(per the factory if the engine is stock, or dyno test to find the optimum if
you've substantially modified things) and then run the lowest octane fuel
you can without having spark knock.  If you are at optimum MBT timing,
runnning higher octane fuel than you need to avoid spark knock doesn't buy
you anything. 

Jim Burkhard

> -----Original Message-----
> From: virtualvairs-admin@corvair.org 
> [mailto:virtualvairs-admin@corvair.org] On Behalf Of Henri 
> van Wandelen, Jr.
> Sent: Sunday, April 25, 2004 12:39 PM
> To: virtualvairs@corvair.org
> Subject: Re: <VV> 140-PG MPG? How about Octane?
> 
> 
> Happy Sunday,
> 
> Has anyone noticed a difference in Corvair gas mileage based 
> on the octane you're using?  I've just begun to experiment.
> 
> Henri van Wandelen, Jr.
> 
> CORSA / Silicon Valley Vairs / '65 Monza 110/PG 17+ MPG
> 
> =====
> henri@alum.calberkeley.org
> http://www.concentric.net/~henri8
> 
> 
> 	
> 		
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