[Diy_efi] Timing Advance Curve?
A6intruder
A6intruder at adelphia.net
Thu Dec 19 16:28:28 GMT 2002
OK Mike, Thrashing received. Next time I'll only post if I have my list of
SAE papers as references. But since I've entered into an interesting area
of discussion I'll continue my thought process because I obviously didn't
convey my complete thought. And yes you're right I haven't read every post
ever made on this list, AND, this list isn't my only source of learning.
My beginning statement:
>Octane has everything to do with the speed of burn. Higher octane actually
>controls ping by slowing down the burn.
I actually have read about the octane, studied combustion thermodynamics
going through college, and no I'm sure I don't have your level of expertise.
However, my understanding of the typical combustion event is that as the
flame front starts, pressures and temperatures start to rise. As temps and
pressure rise this allows the flame front to move faster and the goal is to
complete the burn in an orderly manner before the temp/pressure conditions
allow the fuel to go high order, detonate. The net effect of a higher
octane fuel is to delay that critical temp/pressure point long enough to
avoid detonation. My understanding has been (this is why I like to continue
the learning process even in my old age) that high octane fuels achieve this
by slowing the combustion process. This sounds counter productive but since
the speed of the flame front increases drastically as temp-pressure builds,
actually forming a shock wave as it approaches the detonation level, slower
combustion actually delays the build up of the shock wave ever so slightly
effectively allowing the flame front to finish it's travel throughout the
mixture before hitting the critical temp-pressure point. I think, and you
might actually agree with this, that the octane rating system is fairly
imperfect system of inferring a pre-ignition and detonation resistance.
Second part of my original statement: This is why a very low HP/cubic
>inch engine doesn't necessarily gain power with higher octane.
My point here is that many commoners I know take a normal street car, we'll
use a 1994 Mustang GT with purely stock 215 HP motor, and put premium in the
car and say it gives them better performance. The stock tune on that car
needed 89 octane to keep from pinging under any thrashing based on my 65,000
of use in purely stock trim. Since this engine control system does not have
knock sensors, it maintains whatever spark table it has and does not on its
own make use of the advantage premium offers. As you authoritatively state
the only power difference between the two fuels in this case would be based
their formulation and actual BTU released in combustion, probably not
measurable by the "seat of the pants". Without changing timing one could
not expect any significant change of power by just changing fuel. That was
the point of that statement.
Thank you,
Daniel R. Nicoson
Equipment Exchange Company
Phone: (814) 774-0888
Fax: (814) 774-0880
-----Original Message-----
From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org]On Behalf
Of Mike
Sent: Thursday, December 19, 2002 9:10 AM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] Timing Advance Curve?
At 08:35 AM 12/19/02 -0500, you wrote:
>Octane has everything to do with the speed of burn. Higher octane actually
>controls ping by slowing down the burn. This is why a very low HP/cubic
>inch engine doesn't necessarily gain power with higher octane.
Daniel,
You are wrong on all points, I take it you either havent been on this
list for long or havent read anything about octane, what it is or how
its measured...
Referring to your three sentences above, denoted as a to c:-
a. Octane has negligible effect on speed of burn,
ability to resist spontaneous ignition due to rising temperature
or pressure has negligible relationship to the speed of burn,
these are separate issues. It is possible to contrive a
mixture of fuels to achieve higher octane whilst also reducing
the burn rate but it is also possible to do the exact opposite.
Think about the fuels used in F1, highest octane and faster
burn then our engines, how else could they operate with reasonable
power outputs at 15,000 rpm.
b. Higher octane doesnt 'control' ping, it *only* means its less
likely to ping ie more resitant to ping but has negligible
relationship to speed of burn.
c. Any HP/ci engine may not have any imporvement in power out if
higher octane is used *because* higher octane fuels may not
necessarily have more energy than lower octane fuels. However,
if an engine is retuned for higher octane - such as advancing
the timing or raising the boost then its possible to both
improve economy and raise power output as required. So bear in
mind there are four separate issues which 'might' overlap,
and these are:
- Octane rating
- Speed of burn
- Energy content
- Volatility
Its possible with the large variety of fuel components available
(ie. Flammible petrochemicals) that a particular combination can
be reached for a particular aim - but it in no way means there
is an automatic causal relationship between octane and speed of burn,
none, zero - zilch !
I'm sure others on this list can articulate this better than can, I
dont know where you got your information from but its rather one
dimensional and makes woefully invalid generalisations which can
lead people astray - the issue is not at all as simple as you
suggest.
Eg. Here in Australia (amoung others) we have these fuels:-
1. Unleaded, 92 octane (sometimes reaches 94)
2. Premium, 96 octane, same energy as 1, same effective speed
of burn - or actually a little faster as a porsch tuner has
suggested to me.
3. BP Ultimate, 98 octane *and* a little higher energy than
1 or 2 plus a detergent/friction modifier of sorts but BP
arent saying precisely. Recommended for high performance
engines where higher speed of burn is more likely to occur.
There are others but I think you get the picture, have a search
for research octane number (RON) and motor octane number (MON).
rgds
mike
>Daniel R. Nicoson
>Equipment Exchange Company
>Phone: (814) 774-0888
>Fax: (814) 774-0880
>
>-----Original Message-----
From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org]On Behalf
>Of Mike
>Sent: Thursday, December 19, 2002 5:26 AM
>To: diy_efi at diy-efi.org
>Subject: RE: [Diy_efi] Timing Advance Curve?
>
>At 11:23 AM 18/12/2002 -0800, you wrote:
>>--- "Geddes, Brian J" <brian.j.geddes at intel.com>
>>wrote:
>>Well, if you are talking in that sense, simply up the
>>octane to cover the extra compression. However, you
>>can run into a point of diminishing returns as far as
>>power output at higher rpms with the slower burn...
>
>Huh ?
>
>WHat has octane to do with the speed of the fuel burn ?
>
>Isnt octane a measure of the fuels ability to resist knocking ?
>
>Mike
>
>
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