Re: Gasoline grade BTUs per gallon?

On Mon, 29 May 2006 02:59:14 -0400, "K. Jones"
<shadetree1999@xxxxxxxxxxxxxxxxxx> wrote:


"Bill Ward" <bwardREMOVE@xxxxxxxxxxxxx> wrote in message
news:44776f92.19310714@xxxxxxxxxxxx
On Fri, 26 May 2006 13:48:08 -0400, "K. Jones"
<shadetree1999@xxxxxxxxxxxxxxxxxx> wrote:

<snip>

[Bill]
When you use higher octane fuel, the engine has slightly
greater efficiency, because the effective compression ratio
is higher. The trade off is fuel cost. It's usually about a
wash in $/mile, but you do get higher peak performance (max
power) with higher octane, if that's important.

[Kevin]
Bill, would it be more accurate to say an engines can "*be built* with
slightly greater efficiency by utilizing a higher effective compression
ratio"?
AFAIK, on a typical, un-modified, car engine, as delivered by the factory,
built to use say, 87 octane gasoline, you will see no "performance
improvement" by using "premium" (say 91 octane).....more likely less
performance. It's only on engines that have a higher effective
compression
ratio, that performance gains will be found.

I was using "effective compression ratio" to include the
effect of spark timing. The earlier the mixture is ignited
(to a point), the higher the eventual flame temperature, and
thus Carnot efficiency. If the octane is higher, _and_ the
ECU advances the spark, then you should in theory get a bit
more efficiency and peak HP.

I must admit I have never actually tested this theory
myself, but it seems plausible and is accepted by many
e.g.:

http://www.jandssafeguard.com/tech.html

Very true.
When I first entered the discussion, my comments were about Joe-blows
grocery getter, exactly the way he drove it off the dealers lot....I tried
to qualify that carefully with the disclaimer: "on a typical, un-modifed car
engine, as delivered by the factory"....other than that, you can throw my
comments out the window......AFAIK though, most factory mapping is pretty
conservative, and isn't designed for higher than "recommended" octane fuels.

And my original intention was to shed a little light on why
the theory might be plausible, not to try to explain basic
engine operation to a semi-pro racer who happens also to be
an engineer. (;->) That's what I like about this NG - I
start out to explain, and I wind up learning.

If my engine is designed for "regular" gasoline, I'll get slightly more
miles/gallon, and a higher "max power" from regular gasoline, than I will
from premium.

The following link agrees with you:
http://www.automedia.com/High/Octane/Fuel/ccr20050501ok/1

It says:
"Unless you're driving one of the very small percentages of
performance vehicles with an engine designed specifically to
utilize the properties of higher-octane premium gasoline,
there is no performance gain in using premium."

As in anything, there are a lot of "myths/lies/poorly understood concepts"
in the amateur/semi-pro racing circles (many are no better at the "sounds
louder therefore it feels faster seat-of-the-pants dyno" misconceptions than
a pack of teenage boys and thier first car) so I very rarely took any "words
of wisdom" from other racers unless it made sense to an engineering-driven
mind, and then I would test the theories out myself using hard data, as best
I could. It's generally accepted at the drag tracks by the more than
weekend-serious racers that the lower octane you can run ok, the better your
ET's and speed. I tried it, logged every pass, and in my experience, it's
been true (remember we're talking tenths/hundreds/thousandths of a second
over 1/4 mile)....must admit though, I'm not entirely certain why, I'd just
assumed the lower octane fuel must have a higher energy content, never took
it farther than that.

I also take experience seriously. BS doesn't win races.

(You hear a lot of the "weekenders" kibitzing about how they put "super" in,
then dump in "octane boost" pills or liquids, thinking they run faster on
mostly stock mills though).

I drive a little 2000 Civic Si, which says in the operators
manual:

"Use a premium unleaded gasoline with a pump octane rating
of 91 or higher.
If you are unable to find premium unleaded gasoline, you may
substitute an unleaded regular gasoline. The engine will
compensate for the lower octane, but you may notice a slight
decrease in power as a result."

Being cheap, I've used 87 octane only, with no problems at
all, but I never thought of the engine as "high performance"
per the above link.

So I guess it all depends on how you look at it - designed
for 91 octane with a degradation on 87, or designed for 87
with a boost on 91.

Thanks for the comments - I learn a lot in this NG.

Over the years I've gained a lot of respect for your comments, and have
learned many things regarding automobiles from comments you've tossed
out...and you make me research many things that I'd previously accepted as
"It's general knowledge that..."....and found that ("general knowledge") to
be wrong.....so I thought I'd pick your brains a little on the topic,
wondering if I missed some other explainations that I hadn't thought of, for
what I'd found.

Thanks for the compliments, but as you've no doubt figured
out by now, I'm no expert on racing. I've hung around
racers and occasionally designed a few little widgets for
them, but I stand in awe at what sheer determination and
perserverence can accomplish in shaving a few milliseconds
off a run. I've seen a lot of things work that I didn't
understand why. I'd be no good racing - I'd be spending all
my time solving puzzles instead of trying to win.

I'm afraid my brain is pretty slim pickin's on this subject,
but I would put an engine exhibiting the suspected "higher
energy" effect on a dyno, then sweep octane rating over a
range, tuning ignition advance at each octane for peak
power. The plot of power and fuel flow vs octane rating
should tell the story. Unless, of course, it's some complex
dynamic process involving acceleration and not just the peak
power. In racing, as you know, it usually isn't the simple
answer.

Have fun.

Regards,

Bill Ward


.

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