octane baby! (longish but possibly good reading)

[pats16v at columbus.rr.com (Erik)]

There is theory that I have been working with for a while (working third
shift I have ALOT of time), and it goes a little like this:

If the given Octane level is to resist Detonation (i.e. Hot spots in a
cyliner) essentially what you are doing is slowing down your Flame front to
a controlable level (Cooling Effect). That being said you are really getting
a slower and possibly a less efficient combustion process with the higher
Octane Rating fuel.

  So with a lower octane rating in reality the Flame Front is moving faster
and burning more quickly, so at that point in essence the motor is burning
more of the given fuel ingested into the motor during it particular
combustion process due to the faster flame front traveling across the
combstion area.

Example:  87 Octane fuel has less resistance to detonation than say 94
Octane, hence there will be a hotter flame traveling arond the cylinder and
conceivably (?) burning more of the air/fuel in the cylinder.

One thing to remember with this is that it's not the Octane in the fuel that
is making the power it is the cylinder pressure and camshaft profile. While
a large cam will bleed off some presure in the cylinder it is also allowing
a greater amout of Air/Fuel into the cylinder than a stock of very mild cam
profile.

This is why when a too low of Octane fuel is run a motor tends to run hot in
most cases and will detonate under load.  Add the high Octane fuel and
Viola! cooler cylinders, no detonation.  Detonation being hot spots in the
cylinder that pre-ignite the Air/Fuel mixture before the ignition does.

I get asked this question at least 10 times a day, "What Octane fuel should
I run?"  I answer  "I can't give you an honest answer.  There are far too
many variables to this, compression, cam profile, altitude, timing curve,
car weight, gearing, etc."  What I try to convey is "Run the lowest Octane
possible."  See definition above.  "If the higer octane slows down flame
front you'll get a less efficient burn which equals less power."  I have
been witness several dyno tests run where a motor has 87 run through it and
then 92 and the motors make less power BFC goes up and V/E goes down.  Why?
Because the motor didn't need or want the higher Octane fuel.

Before somebody say "HEY ASS what about Alch. and Nitro?"  Well those are
not conventional fuels.  Nitro is just totally nuts.  If you pour some on
the ground and throw a match on it, it won't burn.  Hit that same spot with
a hammer and BAM!  no more fingers, toes or hammer.  Alch.  has a lower
Specif. Gravity that Gasoline so you have to run a greater volume in order
to make power.  Also note Alch. gets used in VERY high Compression
applications, say 13-1 and higher.  And if you notice it has a supreme
cooling effect.  This is part of the reason Top Fuel and Funny Cars don't
need cooling systems, and can run for as long and as hard as they do.   Now
adays these two classes are running usually 75-80% Nitro and the diff in
Alch. This way they can keep the Nitro under control and still keep the
motor cool.


What was the original question again?  Oh yeah.  Higher Octane fuel does not
equal better mileage.  Tell you Housemates to do less burn outs, change the
oil, and replace the air filter, and run the lowest octane possible that the
motor will live on.  An efficient package is the key.  If you've ever
noticed according to Bently most VW's total ignition timing is 21-23 degrees
before TDC.  Even the best Small Block Chevy is typically more than 30
degrees.  VW has very efficient cylinder heads.  They don't need alot of
ignition lead to make power.  A 93-97 275 H.P. LT1 makes .785 Hp per C.I.
and basic 1.8L 100 H.P. VW makes .925 H.P. per C.I.  sounds pretty
convincing to me

This was just my .02, but what do I know?  Nothing about Weber carbs that's
for sure!  :)

Cheers,

Erik

----- Original Message ----- 
From: "Dan Smith" <sad_rocc@yahoo.com>
To: <scirocco-l@scirocco.org>
Sent: Sunday, September 14, 2003 6:27 PM
Subject: octane baby!


> Yeah, a corny play on a song title, but I couldn't resist. Anyways, CO has
octane ratings at the pump ranging from 85 to 94. With gas prices ranging
from $1.77 to 1.99 respectively, I want to see if I can settle this hot
debate among my housemates. Does higher octane gas equal better gas mileage?
And enough of a better mileage to make it cost effective? This would be for
my relatively stock 84 8v, which doesn't deserve high octane gas if it
doesn't make a difference mileage-wise. When responding, please try to keep
the formula equations to a minimum as high school chem was a long time ago
and I just squeeked by.
>
> TIA,
> Dan
>
>
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