Date:         Sun, 27 May 2007 15:36:55 -0400
Reply-To:     Jim Akiba <syncrolist@BOSTIG.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Jim Akiba <syncrolist@BOSTIG.COM>
Subject:      Re: Octane smoctane
Comments: To: chrisrvanagon <chrisrvanagon@COMCAST.NET>
In-Reply-To:  <001201c79fdc$2195b7b0$6500a8c0@vash>
Content-Type: text/plain; charset="us-ascii"

I'd expect to see little to no change in measuring 87 vs 91 mpg.. the
reported difference is still smaller than your reasonable margin of error in
figuring it out, which means that it's just too close to call for started,
but for weenies sake...

Here's my understanding, and I'm sure there are much better ways to explain
it. There are four factors we're talking about(excluding mechanical/internal
engine changes because it's out of scope for this):

w) Spark advance

x) Fuel burn rate

y) Engine combustion pressures/temps

z) Pre-ignition/knock threshhold

You want y to be as high as possible without causing the engine to exceed z.
To a point, this will give you the best volumetric efficiency(read power
potential).

If you increase w or x, Z goes lower, Y higher
If you decrease w or x, Z goes higher, Y lower
If you increase W and decrease X, Z and Y go higher together


Opinion:
If you increase Z and Y together, you'll gain marginal mpg if you demand the
same amount of power, but your power potential increase will be higher than
your mpg potential increase. If mpg is your goal, then increasing Y by means
of less fuel and not more W, in conjunction with decreasing X to increase Z
is the best way. But again that's out of scope since the boxer's engine
management is not capable of doing this easily and safely.

If you do not have good control over W, and don't know Y and Z, playing with
X is dangerous.

If you have good control over W, know Y and Z, playing with X can yield more
power, but is usually close enough from the factory to not be worthwhile in
normal driving conditions.

SO the real trick is always to maximize the potential of the fuel being
used. Simply upping the octane won't do it unless the engine management
system automatically tries to increase spark advance(which the boxer does
not)  so switching to higher octane alone is a waste of money, and running
the lowest octane fuel you can will always yield the best volumetric
efficiency, this is to say nothing of cost efficiency, but usually the gains
in mpg potential will not offset the increased cost for higher octane fuel.



Not sure if that helps or makes it more confusing... maybe I added high
octane fuel to the fire, which is of little value ha

Jim Akiba