[Diy_efi] The Hunt effect

Tue Oct 4 15:24:26 UTC 2005

But if higher octane fuel takes more energy to light off, then the ignition point would happen later than it should causing a power loss. Yes??? No??? I'm thinking out loud. Or would the ECM take these factors into account and advance the timing to compensate? This being the case, older non-controlled motors would show a loss unless the timing were manually adjusted. As slow as the older ECM's were (80's), would they be able to adjust fast enough to compensate? (By slow, I mean compared to todays comm speeds.)  Ski

John Gross <jogross3 at hotmail.com> wrote:
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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