Discussion: What Air/Fuel Ratios are safe and why?
Discussion: What Air/Fuel Ratios are safe and why?
As some of you have probably seen, myself and others tuners have taken part in some non-healthy discussion on AFR's and their effect on knock probablilty, ignition timing, and of course, power and safety.
It's important to add that this thread is to continue dialogue on these topics, but in a HEALTHY format and not to become flame wars or pissing contests between tuners and/or vendors alike. It's also important to ONLY post if you have significant contribution to the thread or legitimate questions.
This thread's conception is to have a meeting of the tuners/vendors on this board to discuss and investigate different theories and methods with regards to AFR's and their effects on ignition timing, power and safety, for all who are intrested to see and understand. Please keep in mind that comments made in this thread are opinions of the people posting, some are based in fact, others based on theories in regards to internal combustion engine science, forced induction(specifically turbocharged) and other related topics.
You are free to copy/paste any info you might find relevant to support your argument, as long as you cite the source.
At the end of this discussion, hopefully everybody will have a much better understanding of what all this means for you and your engine, especially for the Do-It-Yourself tuners on these and other forums.
Enjoy!
CJ
It's important to add that this thread is to continue dialogue on these topics, but in a HEALTHY format and not to become flame wars or pissing contests between tuners and/or vendors alike. It's also important to ONLY post if you have significant contribution to the thread or legitimate questions.
This thread's conception is to have a meeting of the tuners/vendors on this board to discuss and investigate different theories and methods with regards to AFR's and their effects on ignition timing, power and safety, for all who are intrested to see and understand. Please keep in mind that comments made in this thread are opinions of the people posting, some are based in fact, others based on theories in regards to internal combustion engine science, forced induction(specifically turbocharged) and other related topics.
You are free to copy/paste any info you might find relevant to support your argument, as long as you cite the source.
At the end of this discussion, hopefully everybody will have a much better understanding of what all this means for you and your engine, especially for the Do-It-Yourself tuners on these and other forums.
Enjoy!
CJ
Originally Posted by iTune
It's a known fact that a lower mixture density burns slower than the reverse. The speed of the flame front resulting from the ignition sequence, is dependant on the the density of the mixture and AFR's amoung other obvious factors. i will leave out the other factors because we are not debating them. Factors like fuel octane, temps ect. are not what we are discussing, since we are not targeting a specific condition, rather, a general condition. we are talking about AFRs, all else considered.
richening up the Air/Fuel mixture results in a slower flame front speed, thus results in a slower burn. This effect allows you to move the peak pressure later in the combustion process, which allows for more ignition advance, which creates more torque, because the peak pressure, is later in crank angle.
The faster the burn rate and the closer the peak cylinder pressure is to TDC, the more likely conditions will be right for detonation.
If you lean an engine out more, you will have to retard ignition timing to create the same conditions and to keep detonation in check. IF you lean an engine out too much, and are forced to retard ignition timing a lot, your EGT's could be higher than ideal.
Of course, we all know that using leaner AFR's and less ignition advance will make a little more power, but is it actually safer? Is the few extra ponies gained actualy worth tuning on the ragged edge? My personal opinion is that richer mixtures do result in less power, but are safer.
I DON'T combat detonation with richer AFR's, i combat detonation with ignition timing. I'm not like some tuners that feel as though it's OK to throw a bunch of ignition timing at a engine and if it knocks, well just add more fuel. I tune for conservative AFR's and what ever the engine wants for ignition timing, i give it.
richening up the Air/Fuel mixture results in a slower flame front speed, thus results in a slower burn. This effect allows you to move the peak pressure later in the combustion process, which allows for more ignition advance, which creates more torque, because the peak pressure, is later in crank angle.
The faster the burn rate and the closer the peak cylinder pressure is to TDC, the more likely conditions will be right for detonation.
If you lean an engine out more, you will have to retard ignition timing to create the same conditions and to keep detonation in check. IF you lean an engine out too much, and are forced to retard ignition timing a lot, your EGT's could be higher than ideal.
Of course, we all know that using leaner AFR's and less ignition advance will make a little more power, but is it actually safer? Is the few extra ponies gained actualy worth tuning on the ragged edge? My personal opinion is that richer mixtures do result in less power, but are safer.
I DON'T combat detonation with richer AFR's, i combat detonation with ignition timing. I'm not like some tuners that feel as though it's OK to throw a bunch of ignition timing at a engine and if it knocks, well just add more fuel. I tune for conservative AFR's and what ever the engine wants for ignition timing, i give it.
Originally Posted by iTune
I think it's important to understand when and why you want to slow the burn rate. There are some incidents where the burn rate is not out of control, therefore there would be no reason to slow it further. But, to make it clear, a leaner AFR can eventually become unstable in some conditions. These conditions can be detonation rich enviroments if a lean condition is allowed to continue. The extent of what is considered lean is whats in question.
Quote:
Originally Posted by iTune
The faster the burn rate and the closer the peak cylinder pressure is to TDC, the more likely conditions will be right for detonation
Quote:
Originally Posted by iTune
richening up the Air/Fuel mixture results in a slower flame front speed, thus results in a slower burn. This effect allows you to move the peak pressure later in the combustion process, which allows for more ignition advance, which creates more torque, because the peak pressure, is later in crank angle.
Think of why we use high octane race fuels. By using a slower burning, less erratic combustion fuel, we can run leaner AFR's and more ignition advance, which results in more power. This also allows us to run more boost safely. Fuels are measured by knock resistance. The higher the octane, the slower more controlled the fuel burns. This raises the detonation threshold of the fuel.
Quote:
Originally Posted by iTune
If you lean an engine out more, you will have to retard ignition timing to create the same conditions and to keep detonation in check.
The key is to make peak pressure, later in crank angle, where there is more leverage on the crank. This creates more torque. Slowing the burn process, allows us to use more ignition advance, which will put the peak pressure, again later in crank angle.
Like stated above, if you lean an engine out more, you WILL have to retard ignition timing a bit to control detonation. This speeds up the burn process, but puts ignition advance later. Both will put peak pressure later in crank angle, although a leaner mixture will make a bit more power, because it can actually be tuned to put peak pressure even later in crank angle.
There is no question that a leaner AFR will make more power, which means the engine is obviously more effecient. But at what point do you reach the point of diminishing returns? Where you sacrifice power for reliability? Running a leaner AFR leaves little room for error. Unfortunatly, we don't live in a perfect world and conditions are always changing. ECU's do have correction factors, they must be set-up properly in order to work properly. But, there are other unforseen factors that can happen. Especially when dealing with turbocharged, high powered engines. Where the margin or error is greatly reduced.
CJ
Quote:
Originally Posted by iTune
The faster the burn rate and the closer the peak cylinder pressure is to TDC, the more likely conditions will be right for detonation
Quote:
Originally Posted by iTune
richening up the Air/Fuel mixture results in a slower flame front speed, thus results in a slower burn. This effect allows you to move the peak pressure later in the combustion process, which allows for more ignition advance, which creates more torque, because the peak pressure, is later in crank angle.
Think of why we use high octane race fuels. By using a slower burning, less erratic combustion fuel, we can run leaner AFR's and more ignition advance, which results in more power. This also allows us to run more boost safely. Fuels are measured by knock resistance. The higher the octane, the slower more controlled the fuel burns. This raises the detonation threshold of the fuel.
Quote:
Originally Posted by iTune
If you lean an engine out more, you will have to retard ignition timing to create the same conditions and to keep detonation in check.
The key is to make peak pressure, later in crank angle, where there is more leverage on the crank. This creates more torque. Slowing the burn process, allows us to use more ignition advance, which will put the peak pressure, again later in crank angle.
Like stated above, if you lean an engine out more, you WILL have to retard ignition timing a bit to control detonation. This speeds up the burn process, but puts ignition advance later. Both will put peak pressure later in crank angle, although a leaner mixture will make a bit more power, because it can actually be tuned to put peak pressure even later in crank angle.
There is no question that a leaner AFR will make more power, which means the engine is obviously more effecient. But at what point do you reach the point of diminishing returns? Where you sacrifice power for reliability? Running a leaner AFR leaves little room for error. Unfortunatly, we don't live in a perfect world and conditions are always changing. ECU's do have correction factors, they must be set-up properly in order to work properly. But, there are other unforseen factors that can happen. Especially when dealing with turbocharged, high powered engines. Where the margin or error is greatly reduced.
CJ
CJ
Last edited by iTune; Apr 25, 2007 at 05:55 PM.
Evolved Member
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I'm not sure how much traction you'll get on this as I suspect the top tuners won't jump on and say what AFR's they target.
Here is thread from the Lancer Register which has a top UK tuner "the dentist" talking about AFR's.
http://www.lancerregister.com/showth...hreadid=144081
Here is thread from the Lancer Register which has a top UK tuner "the dentist" talking about AFR's.
http://www.lancerregister.com/showth...hreadid=144081
I'm not sure how much traction you'll get on this as I suspect the top tuners won't jump on and say what AFR's they target.
Here is thread from the Lancer Register which has a top UK tuner "the dentist" talking about AFR's.
http://www.lancerregister.com/showth...hreadid=144081
Here is thread from the Lancer Register which has a top UK tuner "the dentist" talking about AFR's.
http://www.lancerregister.com/showth...hreadid=144081
CJ
Newbie
Joined: Aug 2005
Posts: 54
Likes: 0
From: MN
you want to run afrs that satisfy a number of conditions (timing,power and egt)
turbos are mainly limited by knock and egt. so you want to run at an afr that rich enough for reasonable egt and yet lean enough to make power but not knock.
10.0 to 12.5 is a real good place to start and will vary for every combo.
turbos are mainly limited by knock and egt. so you want to run at an afr that rich enough for reasonable egt and yet lean enough to make power but not knock.
10.0 to 12.5 is a real good place to start and will vary for every combo.
Normally aspirated, turbo, or supercharged it doesn't matter. All these motors will make more power with an AFR between 12-13 on gasoline. The timing map is then used to maximize torque.
http://www.innovatemotorsports.com/resources/rich.php
If you look at the fastest and most successfully cars at the end of the season they all have a clean vapor trail behind them as they have their AFR's in the sweet spot, mid 12's. Low swirling heads that have a layered AFR density need to run artificially rich to combat lean pockets. Good swirling heads like a modern 4 valve per cylinder can achieve mid 12's. A perfect cylinder head will run stoichiometric AFR's even under full load as perfect combustion of air and fuel is achieved.
http://www.innovatemotorsports.com/resources/rich.php
If you look at the fastest and most successfully cars at the end of the season they all have a clean vapor trail behind them as they have their AFR's in the sweet spot, mid 12's. Low swirling heads that have a layered AFR density need to run artificially rich to combat lean pockets. Good swirling heads like a modern 4 valve per cylinder can achieve mid 12's. A perfect cylinder head will run stoichiometric AFR's even under full load as perfect combustion of air and fuel is achieved.
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I have - and more than one, although it doesn't usually happen right away. Excessive EGT are generated by something or another being excessive. The extra heat transferred to metal parts accelerates oxidation and fatigue (simple chemistry), and typically results when combustion takes place very late in the piston's travel.
As for AFR, fortunately, a substantial 'fudge factor' exists (unlike ignition mapping along the ragged edge). It can be debated to death, but in reality, all one need to where street tuning is concerned is be patient enough to determine RBT (rich best torque), which is the AFR beyond which running a richer mixture results in a significant rate of torque loss. So long as one stays just on the lean side of RBT, the difference within the range of LBT (lean best torque) and RBT is relatively nominal.
As for AFR, fortunately, a substantial 'fudge factor' exists (unlike ignition mapping along the ragged edge). It can be debated to death, but in reality, all one need to where street tuning is concerned is be patient enough to determine RBT (rich best torque), which is the AFR beyond which running a richer mixture results in a significant rate of torque loss. So long as one stays just on the lean side of RBT, the difference within the range of LBT (lean best torque) and RBT is relatively nominal.
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Joined: Sep 2005
Posts: 241
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From: Chicago, IL
EGT is a direct relation to the a/f. If its two lean the egt's will be alot higher. Richer they will be lower.There is other factors that play into this such as intake temp, fuel and what not. But the egts are just a tool to help the tuner/ driver to what is going on inside the motor.
EGT is a direct relation to the a/f. If its two lean the egt's will be alot higher. Richer they will be lower.There is other factors that play into this such as intake temp, fuel and what not. But the egts are just a tool to help the tuner/ driver to what is going on inside the motor.
EGT's are a direct relation to not only AFR's, but more imortantly ignition timing as well. Ignition timing that excessivly retarded will raise EGTs. Also, backpressure also plays a role in EGT's as well.
CJ
Last edited by iTune; Apr 25, 2007 at 05:05 PM.
CJ
Honda was one of the first to mass produce a lean burn motor in the civic CVCC. Later they EFI'd it in the civic VX. They could cruise at 22:1 AFR with less emissions and cool as a cucumber exhaust gasses.
Last edited by dan l; Apr 25, 2007 at 05:06 PM.
If you richen things up you can also increase EGT's because once again you are burning fuel in the manifold. EGT guages are simply measuring the still burning mixture which in relation means that since the heat it outside the combustion chamber less heat is inside it meaning a cooler combustion chamber.
I've had my manifold and turbine housing glowing red from a clogged cat and knock sensor that pulled 15 degrees of timing while I was racing the car for 15 mins on a back road. A quality turbo should not and will not fail because of too much EGT's. It was so hot the parts were semi transparent.
Actually exhaust valves are fine. When you reduce ignition timing you increase EGT's because you burn more fuel outside of the combustion chamber. The energy (heat) is moved to the exhaust manifold and the exhaust valve sees less of the heat from the burn energy.
If you richen things up you can also increase EGT's because once again you are burning fuel in the manifold. EGT guages are simply measuring the still burning mixture which in relation means that since the heat it outside the combustion chamber less heat is inside it meaning a cooler combustion chamber.
I've had my manifold and turbine housing glowing red from a clogged cat and knock sensor that pulled 15 degrees of timing while I was racing the car for 15 mins on a back road. A quality turbo should not and will not fail because of too much EGT's. It was so hot the parts were semi transparent.
If you richen things up you can also increase EGT's because once again you are burning fuel in the manifold. EGT guages are simply measuring the still burning mixture which in relation means that since the heat it outside the combustion chamber less heat is inside it meaning a cooler combustion chamber.
I've had my manifold and turbine housing glowing red from a clogged cat and knock sensor that pulled 15 degrees of timing while I was racing the car for 15 mins on a back road. A quality turbo should not and will not fail because of too much EGT's. It was so hot the parts were semi transparent.
CJ