Date:         Thu, 27 Apr 1995 13:50:43 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         wabbott@townshend.Corp.Megatest.COM (William Abbott)
Subject:      Octane, Porsche heads

	Octane is C8H18:

        H H H H H H H H
        | | | | | | | |
      H-C-C-C-C-C-C-C-C-H
        | | | | | | | | 
        H H H H H H H H

	The octane rating of fuel in the USA is equal to the
ACTUAL percentage of octane in the product, plus the "Research Octane
Number" or "RON", divided by two. In short, the average of the actual
% octane and the RON. The RON is measured using a standard engine and
the number given is the based on the performance of the gas being tested:
The RON number is what the octane 'seems' to be. Usually this is going to
be greater than the actual octane, through the addtion of various
additives, the most well known being the late, sometimes lamented,
tetraethyllead.

	The effect of higher octane is to allow the compression
to be raised higher, or the spark advanced further, or both. Both
result in greater yielded power, so I'm uncertain about the power-density
stuff as it applies to the real world. A high compression, sharply advanced
engine burning high octane fuel develops a LOT more power than a low
compression engine with a conservative advance. During the 40s and 50s,
airplane engines were built that took 'aviation gasoline', stuff that
came in grades as high as 130 octane- 1/3 better than 100% octane!
It could have been a vast conspiracy, but this worked for both water
and air-cooled engines, on all sides during the war. 

	I didn't know there was lower power density in higher octane
gasoline. I get very consistant milage with SU-2000, 92 octane, so will
try a couple of tanks of 89 and then 87 for comparison, in my Corrado.

	I will not burn 87 in an aircooled. Pinging you hear is extreme
detonation. Detonation you don't hear can still melt engines. I've seen melted
engines, and I never want to own one.

	Doing the math: 
	Fuel cost per mile: 14 mpg = $0.07/mile, 20 mpg = $0.04/mile.
Now some of us may be getting $0.15/mile total cost of ownership, in
which case, with 14 mpg, half your cost of ownership, more or less,
is fuel. So a 20% difference in fuel cost is a 10% difference in
cost of ownership, assuming the different fuel has no effect on
maintenance. At 20 mpg, a $0.15/mile total cost is less than 1/3
fuel driven and 20% more expensive fuel will result in a 6% increase
in cost of ownership. 
	Anyone out there have real cost-of-ownership figures? I suspect
that the average is higher than $0.15/mile!


	Someone had what they think are Porsche heads, part number
616.xxx.xxx... that looks like a Porsche number. Do they have
splayed exhaust valves- not parallel with the intake valves, and
square section exhaust ports? Then its a 356/912 engine of some kind. 
914 and 912E engines are 1.7, 1.8 and 2.0 litre type 4's with injection.
They have type 4 heads- symetric valve-covers, push-rod-tubes that
can be removed without removing the head from the engine, etc. I'd
expect the type 4 head to have a 4xx.xxx.xxx part number.

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