Direct correlation between Timing and AFR ??
Thread Starter
Melting Slicks
Joined: Mar 2003
Posts: 2,472
Likes: 0
From: Toronto ONTARIO
Direct correlation between Timing and AFR ??
Out of curiousity, is there a general or direct correlation between the two ? I know that richer afrs can have more advanced timing, but how much ? Moving from 12.5 afr to 13.0, how much would timing be reduced ?
Burning Brakes
Joined: May 1999
Posts: 760
Likes: 1
From: Greenwich, CT
Um, not to sound negative, but I think you are way off on the wrong track here.
1. Spark timing is NOT something you play around with to make the AFRs change.
2. AFRs are NOT something you want to guage from a ~5 second dyno test - unless you
only drive your car for one minute at a time (dedicated drag-racing car).. Street or road
race cars are a far different story.
Automotive engineering is much too complex to lend itself to learning on Bulletin Boards,
but there are references around that can tip you off about where to find further reading.
Below are two excerpts from an article written by GM's design engineer for the Northstar
engine program. The whole article was referenced on the TGO site about a year ago, or you
can try a Google search for it.
The excerpts speak specifically to spark timing and AFRs - you may find them enlightening,
as they represent a glimpse of GM's recent thinking and practice on those subjects
(emphasis added by me, text is from GM):
"Engine Basics: Detonation and Pre-Ignition
Written by Allen W. Cline
Reprinted from Issue 54 of CONTACT! Magazine, published in January, 2000
All high output engines are prone to destructive tendencies as a result of over boost, misfueling,
mis-tuning and inadequate cooling. The engine community pushes ever nearer to the limits of
power output. As they often learn cylinder chamber combustion processes can quickly gravitate
to engine failure. This article defines two types of engine failures, detonation and pre-ignition,
that are as insidious in nature to users as they are hard to recognize and detect. This discussion
is intended only as a primer about these combustion processes since whole books have been
devoted to the subject.
First, let us review normal combustion. It is the burning of a fuel and air mixture charge in the
combustion chamber. It should burn in a steady, even fashion across the chamber, originating
at the spark plug and progressing across the chamber in a three dimensional fashion. Similar to
a pebble in a glass smooth pond with the ripples spreading out, the flame front should progress
in an orderly fashion. The burn moves all the way across the chamber and , quenches (cools)
against the walls and the piston crown. The burn should be complete with no remaining fuel-air
mixture. Note that the mixture does not "explode" but burns in an orderly fashion.
There is another factor that engineers look for to quantify combustion. It is called "location of
peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP
should occur at 14 degrees after top dead center. Depending on the chamber design and the
burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example)
the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees
after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The
piston can only go up and down so fast. If you peak the pressure too soon or too late in the
cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine.
I introduce LPP now to illustrate the idea that there is a characteristic pressure buildup
(compression and combustion) and decay (piston downward movement and exhaust valve
opening) during the combustion process that can be considered "normal" if it is smooth,
controlled and its peak occurs at 14 degrees ATDC.
Our enlarged definition of normal combustion now says that the charge/bum is initiated with
the spark plug, a nice even burn moves across the chamber, combustion is completed and peak
pressure occurs at at 14 ATDC.
Confusion and a lot of questions exist as to detonation and pre-ignition. Sometimes you hear
mistaken terms like "pre-detonation". Detonation is one phenomenon that is abnormal
combustion. Pre-ignition is another phenomenon that is abnormal combustion. The two, as we
will talk about, are somewhat related but are two distinctly different phenomenon and can
induce distinctly different failure modes."
Second excerpt from the same source:
"Engine developers run very cold spark plugs to avoid the risk of getting into pre-ignition during
engine mapping of air/fuel and spark advance. Production engine calibration requires that we
have much hotter spark plugs for cold startability and fouling resistance. To avoid pre-ignition we
then compensate by making sure the fuel/air calibration is rich enough to keep the spark plugs
cool at high loads and at high temperatures, so that they don't induce pre-ignition.
Consider the Northstar engine. If you do a full throttle 0-60 blast, the engine will likely run up to
6000 RPM at a 11.5:1 or 12:1 air fuel ratio. But under sustained load, at about 20 seconds, that
air fuel ratio is richened up by the PCM to about 10:1. That is done to keep the spark plugs cool,
as well as the piston crowns cool. That richness is necessary if you are running under continuous
WOT load. A slight penalty in horsepower and fuel economy is the result. To get the maximum
acceleration out of the engine, you can actually lean it out, but under full load, it has to go back
to rich. Higher specific output engines are much more sensitive to pre-ignition damage because
they are turning more RPM, they are generating a lot more heat and they are burning more fuel.
Plugs have a tendency to get hot at that high specific output and reaction time to damage is
minimal.
A carburetor set up for a drag racer would never work on a NASCAR or stock car engine because
it would overheat and cause pre-ignition. But on the drag strip for 8 or 10 seconds, pre-ignition
never has time to occur, so dragsters can get away with it. Differences in tuning for those two
different types of engine applications are dramatic. That's why a drag race engine would make a
poor choice for an aircraft engine."
HTH
1. Spark timing is NOT something you play around with to make the AFRs change.
2. AFRs are NOT something you want to guage from a ~5 second dyno test - unless you
only drive your car for one minute at a time (dedicated drag-racing car).. Street or road
race cars are a far different story.
Automotive engineering is much too complex to lend itself to learning on Bulletin Boards,
but there are references around that can tip you off about where to find further reading.
Below are two excerpts from an article written by GM's design engineer for the Northstar
engine program. The whole article was referenced on the TGO site about a year ago, or you
can try a Google search for it.
The excerpts speak specifically to spark timing and AFRs - you may find them enlightening,
as they represent a glimpse of GM's recent thinking and practice on those subjects
(emphasis added by me, text is from GM):
"Engine Basics: Detonation and Pre-Ignition
Written by Allen W. Cline
Reprinted from Issue 54 of CONTACT! Magazine, published in January, 2000
All high output engines are prone to destructive tendencies as a result of over boost, misfueling,
mis-tuning and inadequate cooling. The engine community pushes ever nearer to the limits of
power output. As they often learn cylinder chamber combustion processes can quickly gravitate
to engine failure. This article defines two types of engine failures, detonation and pre-ignition,
that are as insidious in nature to users as they are hard to recognize and detect. This discussion
is intended only as a primer about these combustion processes since whole books have been
devoted to the subject.
First, let us review normal combustion. It is the burning of a fuel and air mixture charge in the
combustion chamber. It should burn in a steady, even fashion across the chamber, originating
at the spark plug and progressing across the chamber in a three dimensional fashion. Similar to
a pebble in a glass smooth pond with the ripples spreading out, the flame front should progress
in an orderly fashion. The burn moves all the way across the chamber and , quenches (cools)
against the walls and the piston crown. The burn should be complete with no remaining fuel-air
mixture. Note that the mixture does not "explode" but burns in an orderly fashion.
There is another factor that engineers look for to quantify combustion. It is called "location of
peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP
should occur at 14 degrees after top dead center. Depending on the chamber design and the
burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example)
the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees
after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The
piston can only go up and down so fast. If you peak the pressure too soon or too late in the
cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine.
I introduce LPP now to illustrate the idea that there is a characteristic pressure buildup
(compression and combustion) and decay (piston downward movement and exhaust valve
opening) during the combustion process that can be considered "normal" if it is smooth,
controlled and its peak occurs at 14 degrees ATDC.
Our enlarged definition of normal combustion now says that the charge/bum is initiated with
the spark plug, a nice even burn moves across the chamber, combustion is completed and peak
pressure occurs at at 14 ATDC.
Confusion and a lot of questions exist as to detonation and pre-ignition. Sometimes you hear
mistaken terms like "pre-detonation". Detonation is one phenomenon that is abnormal
combustion. Pre-ignition is another phenomenon that is abnormal combustion. The two, as we
will talk about, are somewhat related but are two distinctly different phenomenon and can
induce distinctly different failure modes."
Second excerpt from the same source:
"Engine developers run very cold spark plugs to avoid the risk of getting into pre-ignition during
engine mapping of air/fuel and spark advance. Production engine calibration requires that we
have much hotter spark plugs for cold startability and fouling resistance. To avoid pre-ignition we
then compensate by making sure the fuel/air calibration is rich enough to keep the spark plugs
cool at high loads and at high temperatures, so that they don't induce pre-ignition.
Consider the Northstar engine. If you do a full throttle 0-60 blast, the engine will likely run up to
6000 RPM at a 11.5:1 or 12:1 air fuel ratio. But under sustained load, at about 20 seconds, that
air fuel ratio is richened up by the PCM to about 10:1. That is done to keep the spark plugs cool,
as well as the piston crowns cool. That richness is necessary if you are running under continuous
WOT load. A slight penalty in horsepower and fuel economy is the result. To get the maximum
acceleration out of the engine, you can actually lean it out, but under full load, it has to go back
to rich. Higher specific output engines are much more sensitive to pre-ignition damage because
they are turning more RPM, they are generating a lot more heat and they are burning more fuel.
Plugs have a tendency to get hot at that high specific output and reaction time to damage is
minimal.
A carburetor set up for a drag racer would never work on a NASCAR or stock car engine because
it would overheat and cause pre-ignition. But on the drag strip for 8 or 10 seconds, pre-ignition
never has time to occur, so dragsters can get away with it. Differences in tuning for those two
different types of engine applications are dramatic. That's why a drag race engine would make a
poor choice for an aircraft engine."
HTH
Thread Starter
Melting Slicks
Joined: Mar 2003
Posts: 2,472
Likes: 0
From: Toronto ONTARIO
Thank you !
I've been reading non stop for the last 2 days. Maybe my post made me sound like an idiot but Im not trying to change afrs with timing. Reason I asked is because Im learning that tuning to 13.1 etc isn't very good for long periods of WOT, just as your quoted article says. So since my ecm is fully adjustable by me, the dfi gen 7 I'm deciding to pull back on leaning the motor across the board from 13.05 to about 12.8. i was wondering if that would generally require a slight increase in timing and if so How much. I havn't had time to adjust any on a dyno and I will soon, I was just shooting int he dark and hoping I hit something
Again i appreciate the response i love to read !
I've been reading non stop for the last 2 days. Maybe my post made me sound like an idiot but Im not trying to change afrs with timing. Reason I asked is because Im learning that tuning to 13.1 etc isn't very good for long periods of WOT, just as your quoted article says. So since my ecm is fully adjustable by me, the dfi gen 7 I'm deciding to pull back on leaning the motor across the board from 13.05 to about 12.8. i was wondering if that would generally require a slight increase in timing and if so How much. I havn't had time to adjust any on a dyno and I will soon, I was just shooting int he dark and hoping I hit something
Again i appreciate the response i love to read !
