[Diy_efi] The Hunt effect

Tue Oct 4 14:58:04 UTC 2005

You're correct about what the octane rating means, i.e. higher flash point.
However, it will still burn completely if you run high octane fuel in a low
compression engine.  A few common misconceptions about octane - the flame
front speed does NOT change with higher octane, and it's actual burn
properties do not change on the global scale (when you look at the free
radicals formed at various points during the actual combustion process, yes
there is a difference, but in the grand scheme of things, it's the same).
There are a few reasons why some high octane fuels make more power that low
octane fuels on certain engines.  First off, if the engine has a high enough
CR that it cannot attain MBT timing on lower octane, you would find a power
increase with higher octane from the simple fact that the ECU can put full
ignition timing in the motor.  This would be found ONLY under WOT
conditions, not part throttle.  The second reason you might find a power
gain from higher octane fuels would be fuel-dependant.  Some high octane
fuels use oxygenates (i.e. ethanol) to attain the higher octane rating.  Any
oxygenate, assuming the proper lambda can be maintained, will produce more
power because you have now introduced more oxygen into the motor than
before, with the corresponding amount of fuel to burn it..you just added
energy to the engine.  This power gain will be felt EVERYWHERE in the load
band, but will be accompanied by a loss in economy.  One other thing about
part throttle load.if you look at the timing map at part throttle on just
about any engine, you'll see the timing advance (on a wedge headed motor
like a pushrod V8) in the 45-50 deg BTDC range.  This is because the dynamic
compression at part throttle is so low that you need to start the burn that
early to try to burn everything for emissions purposes.  In these low
dynamic compression situations, the octane rating in the fuel is not going
to make any difference whatsoever from a burn standpoint.  There isn't
enough heat and pressure to preignite 87 octane in an 11:1 motor.  Long
story short, from everything I have ever seen in the lab, on dynos, etc, the
only explanation I can see for the changing fuel economy in these cases
under part throttle cruising mode is if the ECU is making changes to the
calibration based on fuel detected (which of course would be model
dependant), or there are other factors in your experiment which are
influencing the results which you have not taken into account.

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of Bret Levandowski
Sent: Tuesday, October 04, 2005 10:40 AM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] The Hunt effect

Octane rating is the fuel's resistance to detonation under heat and pressure
with no 'true' ignition source. I believe it equates to a higher flash
point. So, running high octane in a lower compression motor that requires
low octane could actually lower MPG as the lower comp. would prevent the
mixture form lighting off completely. In the case of the caddy, using lower
octane would cause detonation which would be picked up by the knock sensor
retarding the ignition (which I believe he stated). This would then require
more fuel to reach the same 'power' point reducing MPG.  Ski

Dustin Lof <bubblesjrtwo at yahoo.com> wrote: 

I have never heard anyone say more octane produces
better mileage, I was always told to burn, and to
reccomend to my customers the lowest octane it wont
ping on....? what is the ethanol content in the 87
v/s 89. Around here it is 10% corn ethanol in
everything but the 91 and your not suposed to burn the
91 unless your driving a collector car or a snowmobile
and most snowmobiles are jetted to run on 10% now too.
I am just wondering if the alchol content is what is
effecting the mileage. I know its eating up a lot of
fuel pressure regulators. There is also the r*m/2
factor which is how octane is rated the r factor is
what the guys in white coats in a lab say the octane
rating is the m is how it actually performs in a
single cylinder test motor. If one of those numbers
is skewed, for example crappy gas doped up with an
additive package it will make your mileage compairson
way off. it is all way more scientific than all this
but that is all I remember from the one day we talked
about this in college so please dont ask me to explain
in any more detail. just my $0.02---Dustin Lof 

p.s. last week Minnesota mandated all diesel be 2%
bio-diesel. Made from soybeans.
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