Jeriatric
Thinks out loud
By Ben Strader "click: http://www.aetconline.com/speaker/ben-strader/
Ben Strader Said:
Air/Fuel Ratio Management For Racers, (understanding AFR racer or not)
A Three Part Series
When it comes to racing, there is never any shortage of hard work and chores to be done before the next event. Often, race teams are required to travel long distances during the week, prep the car, show up on the weekend ready to run, and then do it all over again the next week. This doesn’t leave much time for experimentation and trying out new concepts. That means most of the time, when racers find something that works okay they tend not to change it, even though there might be a better way. They simply can’t afford to risk missing an event or losing a race.
Often times a discussion arises about the best Air to Fuel ratio to use for various tracks and atmospheric conditions. I want to try and address a few of these questions in this series.
Here is a list of some common questions asked by racers and tuners:
1) What Air to Fuel Ratio gives the best power?
2) Does the Air to Fuel Ratio that produces the best power change as the altitude my car operates at changes?
3) Does the Air to Fuel Ratio that produces the best power change as the intake air temperature my car operates at changes?
To find answers to these questions I have spent years on the dynamometer testing various engine combinations, talking with other knowledgeable tuners, reading various publications on the subject, and even wrote a book about tuning Electronic Fuel Injected engines, but I found the most convincing answers to these questions in a document written in 1922 by Stanwood Sparrow of the “Bureau of Standards” for the “National Advisory Committee for Aeronautics” (NACA), called NACA Report #189 “Relation of Fuel-Air Ratios to Engine Performance”.
In this report, a government agency set out to answer these and many other questions about the effect of Air Fuel Ratios on engine performance over a wide range of parameters, and the evidence proves out many of the answers I am about to present to you for the above questions in this three part series.
PART I
What Air to Fuel Ratio gives the best power?
Many folks have tried to shed light on this subject based on single-case observations made in sloppily controlled test environments which show results of all sorts, and yet other, seemingly more knowledgeable sources, (such as the companies trying to sell Air-Fuel ratio meters) are constantly trying to convince us that while they cannot (for reasons of liability) tell us what the magic number is, we cannot possibly hope to achieve maximum engine performance without the help of one of their whiz-bang doo-hickeys!
Well, according to NACA report 189, a wide variety of engines were tested across a large range of Air/Fuel ratios and what they found was basically the following:
“In adjusting the carburetor to obtain maximum power, The following method was employed. First, the mixture was altered until approximately maximum power (for the chosen set of conditions) was obtained. As will be shown later, values of power within 1 percent of maximum are obtained over a wide range of fuel-air ratios. Hence, little difficulty was experienced in finding an Air/Fuel ratio to give approximately maximum power.”
The report goes on to state later that, “From the results to date it is concluded that ordinarily maximum power (at least in so far as aviation engines are concerned) is obtained with gasoline-air ratios of between 0.07 and 0.08 pounds of fuel per pound of air (12.5 to 14.5 pounds of air per pound of fuel).”
What all this means is that basically, if simply making lots of power is your only goal, nearly any Air/Fuel ratio can get you pretty close to the mark.
This corresponds quite closely to what my years’ of engine testing have show as well, and in fact, this is what we have been teaching at EFI University for almost four years, but what I find surprising is the number of supposedly “expert” tuners out there who are still arguing against this point, and pretending that what they do is a special brand of “Magic”.
My experience has been that typically the best engine tuners in the business are the first ones to say: “Ask me anything you like, I have nothing to hide.” Recently, I spoke with Shane Tecklenburg of FAST Motorsports in Huntington Beach, California, who is widely regarded to be one of the finest engine tuners in the USA, and he had this to say: “Nothing I do is black magic. Everything is based on simple laws of physics that anyone can learn with a little effort, so there is no reason for me not to answer a racer’s question about engine tuning, even if he is a competitor.” I have also spoken to a number of other well known tuners who have had quite the opposite attitude and tried to make it seem as if they knew some special trick or held the golden nugget of knowledge that, if shared with others would seriously jeopardize their standing. Most of the time, when I find a tuner with this attitude, it means they don’t actually know the answers and fear they might reveal this ugly fact if they say too much.
The simple fact is, ten years ago, before the age of $300 wide bands for everyone, nobody even knew what their Air Fuel Ratios were. The rule of thumb was to change the jets one size for every one-thousand feet of elevation, and that was just the way it was. We looked down the tailpipes and at our spark plugs for various color patterns, and even that wasn’t an exact science.
Most racers would have been horrified if they actually saw what the A/F ratios were doing in their engines during a run, but because the engine still performed well, no one cared. What has changed the industry so dramatically in recent years is the advent of the low-cost wide-band Air/Fuel ratio meters. Suddenly, everyone could afford access to this tool to gain priceless insight into their engine’s performance, and then “numbers game” began.
It is not uncommon to go to the racetrack these days and find any number of racers with their laptops plugged into their cars trying to get that last tenth of an A/F point in line. I’ve heard guys say “yesterday she was running a 12.8 A/F ratio, and today it seems to be running about 12.7 and that’s just too rich!” I wonder if either their dyno, or their E.T.’s would support that. If what the NACA report says is true, then I suppose it begs the question, “What is to be gained by agonizing over minute changes in A/F ratios”? Isn’t there some other chassis or tire component that would be better served by spending this time tweaking them instead? What good is ultimate power if it can’t reach the ground?
I’m not suggesting that we abandon this great new technology and throw away our wide-bands just yet. I simply want to help folks get back to the reality of what it is we are trying to accomplish: Getting the maximum performance from the engine… not getting bogged down in the data. Let’s all take a step back, close our eyes, take a deep breath and remember, the only numbers that really matter are not the ones on the wide-band, but the ones that say the letters “E.T.” next to them! Good luck out there folks!
Coming up in Part II:
Does the Air to Fuel Ratio that produces the best power change as the altitude that my car operates at changes? Tune in next time to find out!
Ben Strader Said:
Hello again everyone! It’s time for another installment of our three part series called “Air/Fuel Ratio Management For Racers”!
In our last issue, we discussed ways to evaluate the correct ratio of air and fuel for your engine when trying to make maximum power. We used a paper written in the 1920’s that contains evidence, which still holds true today! The thing is, making max power is one of the easier tasks involved in mapping an EFI equipped engine. The hard part is getting the consistency, and reliability out of the engine that will win races!
So, once you’ve tuned your engine on a dyno at one location, what happens if you go racing somewhere else? Will the altitude and air density changes dramatically affect the engine, and if so, should you be thinking about using a different air/fuel ratio to maintain the best power? Let’s take a look!
PART II
When I find the right Air Fuel Ratio for maximum power, will that number change when I race at tracks of various altitudes?
When we first start thinking about a solution to this particular problem, we must begin with a strong understanding of what happens to the engine when we change altitudes.
First, and foremost, it is extremely important for us to recognize that as we gain altitude, the air gets both thinner and colder. We say that the “density” of the air is less.
Density of the air can be described as the “weight per unit of volume”. What I mean is this: If we have two empty one-gallon milk jugs sitting on a table and we fill one all the way to the top with goose feathers and one all the way to the top with sand, it should be obvious to most folks that the gallon of sand will weigh more, even though both containers have the same unit of volume: one gallon.
So, the air we breathe, much like the air our engines draw in, gets less dense as we go up in altitude, but the volume of air inside the engine does not change. We say that the “Volumetric Efficiency” of our engine stays the same!
So, if the engine’s Volumetric efficiency is the same at any altitude, then why does the engine make less power when we go up?
Because the quantity of air we are concerned about when supplying fuel to the engine is called the “Mass” of air. The “Mass” is determined by multiplying the volume of air you have by what it weighs.
So, M = V x D, where:
M = Mass in Lbs/ minute
V = Volume of air in CFM
D = Density in Lbs/ Cubic Ft
So, if our engine is flowing 650 CFM for example, then at sea level where one cubic foot of air weighs about .076 pounds we would have a Mass of about 49.4 Lbs of air per minute.
Ex: 650 * .076 = 49.4
However, if we take the engine up to the top of a mountain where one cubic foot of air might only weigh .064 pounds, we would only net 42.18 Lbs of air per minute!
Ex: 650 * .064 = 42.18
That’s about a 15% reduction in power at the SAME volumetric efficiency!
So, the question becomes then, since I’m making 15% less power, should I change the A/F ratio to be 15% leaner?
The answer: NO! Leave it the same.
We are talking about a “ratio” of air to fuel, so since the air is 15% less, we will want the fuel to be 15% less as well, which would still net the same Air to Fuel Ratio!
This means we can build some automatic compensation tables into our ECU settings to detect a change in altitude or barometric pressure and add or subtract fueling as necessary. Wouldn’t that make life so much easier?
If we reference our previous document, “NACA Report #189 “Relation of Fuel-Air Ratios to Engine Performance” to try and solidify the above statements we’ll see what our old buddy Stanwood Sparrow has to say.
On page 111 of the report, we find following:
These tests were done at various air pressures from sea level all the way to 30,000 feet in elevation, and the results show that it is desirable to maintain the same ratio of air to fuel at all altitudes.
Of course, what we are talking about here is only the A/F ratio that produces maximum power. This is not to say that better fuel economy might not be possible by a reduction in A/F ratio due to the fact that since we are making less overall power, we may not need as much fuel for cylinder cooling, and could find some savings there. Then again… we are talking about RACING, not grocery shopping, so lighten up a little and get out there and have fun!
There are always a lot of variables in engine tuning that we could continue to converse about until the cows come home, but the only thing left to answer in our three part series is whether or not we would need a different A/F ratio when the intake air temperatures get hotter or colder.
Tune in next time for an in depth description of how air temperatures affect air density, as well as how they affect the way our engine runs! See you there.
Ben Strader Said:
Hello again everyone! It’s time for the final installment of our three part series called “Air/Fuel Ratio Management For Racers”!
In our last issue, we discussed how the engine reacts if we tune it on a dyno or at a track in one location and then take the engine to a totally different altitude or location. We found a government study put out in the 1920’s shows that the same air to fuel ratio would be required of the engine at any reasonable altitude. The engine made less power overall due to the lack of air density, but the ratio of air to fuel did not need to be changed because of this. Knowing this, the only thing a racer needs to do is make sure to maintain the same air fuel ratio at the track that they found to work when they were on the dyno!
In this final article of the series, we wanted to ask the question: “What happens to my engine at various inlet air temperatures, and how does this affect my choice of air fuel ratios?”
Let’s take one last look at our favorite document, “NACA report 189” to see if we can find the answer!
PART III
When I find the right Air Fuel Ratio for maximum power, will that number change when I race at tracks with different temperatures?.
If you’ll remember back to the last article, we used a mathematical formula to calculate the mass of air that went something like this:
Mass = V*D
Where:
V = the CFM of air the engine was breathing,
and
D = the density, (or weight) of one cubic foot of air.
There are primarily two things that affect the density of air. One is the air pressure, and the other is the air temperature.
We can use the following formula to determine how much one cubic foot of air weighs:
Density = 2.7 P/T
Where:
P = PSI (absolute)
And
T = Temperature in degrees Rankine (Degrees F + 460)
If we use the standard temperatures and pressures at sea level, we will find that one cubic foot of air weighs around .076 Lbs.
Ex: 2.7 [14.7/(60 + 460)] = .076
Now, if we simply plug in different values for various altitudes or temperatures, we can find out how much change in air density we have and then add or subtract fuel from the engine accordingly to maintain the same air to fuel ratio.
Take a look:
Lets say we are up in the mountains, and the barometric pressure is down to around 12 psi absolute, (which is around 24.4 inches of mercury, or about 82 Kpa), and the outside temperatures are about 40 degrees F.
Using the above formula, we see that:
D = 2.7 P/T
D = 2.7 [12/(40 + 460)]
D = .0648 Lbs per cubic foot
So, .0648 / .076 = .85 or about 85% of the original air density at sea level!
That means in order to keep the same air to fuel ratios, we would need to subtract about 15% of the fuel we were previously giving the engine!
We can very easily program a table into the engine computer to automatically measure the intake air temperatures, and then add or subtract fuel to maintain a constant air fuel ratio at all temperatures.
The question is though, do we need a different air fuel ratio when the air gets very hot, or very cold?
Well, to find the answer, we must once again visit “NACA Report 189”.
On pages 111 and 112 we see this following statements, (which are paraphrased here):
“An analysis of a large number of tests covering an inlet temperature range of –20 C to +40 C has shown maximum power to be obtained with approximately the same air fuel ratios at each temperature.”
This would indicate that one would always want the same air fuel ratio, regardless of the inlet temperatures. However, the report goes on to state the following:
“The volatility of the fuel is in reality the determining factor in this question. A constant fuel air ratio is desirable only so long as a change in air temperatures does not appreciably change the relative quality of the mixtures supplied to the various cylinders or the amount of fuel that has been vaporized at the time the compression stroke is completed.”
Essentially, what they are saying is that if the intake temperatures are so hot or cold that they cause the fuel to be ignited prematurely, causing detonation, or cause the fuel to remain in a more liquefied, un-vaporized state, which would make it not ignite so easily then the need for a richer or leaner air fuel ratio might exist.
Overall, what we learned from this is that if the fuel being used is fairly stable, and the temperatures encountered while racing are not extreme, then a constant air fuel ratio is desirable across a wide range of air temperatures. If however, the temperatures your engine will see are extreme, then there is a possibility that a change in air fuel ratios might be warranted.
However, most ECU manufacturers have understood this for some time, and nearly all give you one or more tables to create a method for adding or subtracting fuel as the inlet temperatures increase or decrease.
Hopefully, this series of articles has given you a small amount of insight into understanding the engine’s requirements when it comes to selecting, and maintaining a given air fuel ratio.
Only thorough testing and some trial and error will tell you what is exactly right for your engine, but perhaps with a better understanding of the factors involved we can shorten the time spent tuning, and increase the time spent racing and enjoying your vehicles! See you at the track!
FOUND HERE.
http://www.chevytalk.org/fusionbb/showtopic.php?tid/214216/
Ben Strader Said:
Air/Fuel Ratio Management For Racers, (understanding AFR racer or not)
A Three Part Series
When it comes to racing, there is never any shortage of hard work and chores to be done before the next event. Often, race teams are required to travel long distances during the week, prep the car, show up on the weekend ready to run, and then do it all over again the next week. This doesn’t leave much time for experimentation and trying out new concepts. That means most of the time, when racers find something that works okay they tend not to change it, even though there might be a better way. They simply can’t afford to risk missing an event or losing a race.
Often times a discussion arises about the best Air to Fuel ratio to use for various tracks and atmospheric conditions. I want to try and address a few of these questions in this series.
Here is a list of some common questions asked by racers and tuners:
1) What Air to Fuel Ratio gives the best power?
2) Does the Air to Fuel Ratio that produces the best power change as the altitude my car operates at changes?
3) Does the Air to Fuel Ratio that produces the best power change as the intake air temperature my car operates at changes?
To find answers to these questions I have spent years on the dynamometer testing various engine combinations, talking with other knowledgeable tuners, reading various publications on the subject, and even wrote a book about tuning Electronic Fuel Injected engines, but I found the most convincing answers to these questions in a document written in 1922 by Stanwood Sparrow of the “Bureau of Standards” for the “National Advisory Committee for Aeronautics” (NACA), called NACA Report #189 “Relation of Fuel-Air Ratios to Engine Performance”.
In this report, a government agency set out to answer these and many other questions about the effect of Air Fuel Ratios on engine performance over a wide range of parameters, and the evidence proves out many of the answers I am about to present to you for the above questions in this three part series.
PART I
What Air to Fuel Ratio gives the best power?
Many folks have tried to shed light on this subject based on single-case observations made in sloppily controlled test environments which show results of all sorts, and yet other, seemingly more knowledgeable sources, (such as the companies trying to sell Air-Fuel ratio meters) are constantly trying to convince us that while they cannot (for reasons of liability) tell us what the magic number is, we cannot possibly hope to achieve maximum engine performance without the help of one of their whiz-bang doo-hickeys!
Well, according to NACA report 189, a wide variety of engines were tested across a large range of Air/Fuel ratios and what they found was basically the following:
“In adjusting the carburetor to obtain maximum power, The following method was employed. First, the mixture was altered until approximately maximum power (for the chosen set of conditions) was obtained. As will be shown later, values of power within 1 percent of maximum are obtained over a wide range of fuel-air ratios. Hence, little difficulty was experienced in finding an Air/Fuel ratio to give approximately maximum power.”
The report goes on to state later that, “From the results to date it is concluded that ordinarily maximum power (at least in so far as aviation engines are concerned) is obtained with gasoline-air ratios of between 0.07 and 0.08 pounds of fuel per pound of air (12.5 to 14.5 pounds of air per pound of fuel).”
What all this means is that basically, if simply making lots of power is your only goal, nearly any Air/Fuel ratio can get you pretty close to the mark.
This corresponds quite closely to what my years’ of engine testing have show as well, and in fact, this is what we have been teaching at EFI University for almost four years, but what I find surprising is the number of supposedly “expert” tuners out there who are still arguing against this point, and pretending that what they do is a special brand of “Magic”.
My experience has been that typically the best engine tuners in the business are the first ones to say: “Ask me anything you like, I have nothing to hide.” Recently, I spoke with Shane Tecklenburg of FAST Motorsports in Huntington Beach, California, who is widely regarded to be one of the finest engine tuners in the USA, and he had this to say: “Nothing I do is black magic. Everything is based on simple laws of physics that anyone can learn with a little effort, so there is no reason for me not to answer a racer’s question about engine tuning, even if he is a competitor.” I have also spoken to a number of other well known tuners who have had quite the opposite attitude and tried to make it seem as if they knew some special trick or held the golden nugget of knowledge that, if shared with others would seriously jeopardize their standing. Most of the time, when I find a tuner with this attitude, it means they don’t actually know the answers and fear they might reveal this ugly fact if they say too much.
The simple fact is, ten years ago, before the age of $300 wide bands for everyone, nobody even knew what their Air Fuel Ratios were. The rule of thumb was to change the jets one size for every one-thousand feet of elevation, and that was just the way it was. We looked down the tailpipes and at our spark plugs for various color patterns, and even that wasn’t an exact science.
Most racers would have been horrified if they actually saw what the A/F ratios were doing in their engines during a run, but because the engine still performed well, no one cared. What has changed the industry so dramatically in recent years is the advent of the low-cost wide-band Air/Fuel ratio meters. Suddenly, everyone could afford access to this tool to gain priceless insight into their engine’s performance, and then “numbers game” began.
It is not uncommon to go to the racetrack these days and find any number of racers with their laptops plugged into their cars trying to get that last tenth of an A/F point in line. I’ve heard guys say “yesterday she was running a 12.8 A/F ratio, and today it seems to be running about 12.7 and that’s just too rich!” I wonder if either their dyno, or their E.T.’s would support that. If what the NACA report says is true, then I suppose it begs the question, “What is to be gained by agonizing over minute changes in A/F ratios”? Isn’t there some other chassis or tire component that would be better served by spending this time tweaking them instead? What good is ultimate power if it can’t reach the ground?
I’m not suggesting that we abandon this great new technology and throw away our wide-bands just yet. I simply want to help folks get back to the reality of what it is we are trying to accomplish: Getting the maximum performance from the engine… not getting bogged down in the data. Let’s all take a step back, close our eyes, take a deep breath and remember, the only numbers that really matter are not the ones on the wide-band, but the ones that say the letters “E.T.” next to them! Good luck out there folks!
Coming up in Part II:
Does the Air to Fuel Ratio that produces the best power change as the altitude that my car operates at changes? Tune in next time to find out!
Ben Strader Said:
Hello again everyone! It’s time for another installment of our three part series called “Air/Fuel Ratio Management For Racers”!
In our last issue, we discussed ways to evaluate the correct ratio of air and fuel for your engine when trying to make maximum power. We used a paper written in the 1920’s that contains evidence, which still holds true today! The thing is, making max power is one of the easier tasks involved in mapping an EFI equipped engine. The hard part is getting the consistency, and reliability out of the engine that will win races!
So, once you’ve tuned your engine on a dyno at one location, what happens if you go racing somewhere else? Will the altitude and air density changes dramatically affect the engine, and if so, should you be thinking about using a different air/fuel ratio to maintain the best power? Let’s take a look!
PART II
When I find the right Air Fuel Ratio for maximum power, will that number change when I race at tracks of various altitudes?
When we first start thinking about a solution to this particular problem, we must begin with a strong understanding of what happens to the engine when we change altitudes.
First, and foremost, it is extremely important for us to recognize that as we gain altitude, the air gets both thinner and colder. We say that the “density” of the air is less.
Density of the air can be described as the “weight per unit of volume”. What I mean is this: If we have two empty one-gallon milk jugs sitting on a table and we fill one all the way to the top with goose feathers and one all the way to the top with sand, it should be obvious to most folks that the gallon of sand will weigh more, even though both containers have the same unit of volume: one gallon.
So, the air we breathe, much like the air our engines draw in, gets less dense as we go up in altitude, but the volume of air inside the engine does not change. We say that the “Volumetric Efficiency” of our engine stays the same!
So, if the engine’s Volumetric efficiency is the same at any altitude, then why does the engine make less power when we go up?
Because the quantity of air we are concerned about when supplying fuel to the engine is called the “Mass” of air. The “Mass” is determined by multiplying the volume of air you have by what it weighs.
So, M = V x D, where:
M = Mass in Lbs/ minute
V = Volume of air in CFM
D = Density in Lbs/ Cubic Ft
So, if our engine is flowing 650 CFM for example, then at sea level where one cubic foot of air weighs about .076 pounds we would have a Mass of about 49.4 Lbs of air per minute.
Ex: 650 * .076 = 49.4
However, if we take the engine up to the top of a mountain where one cubic foot of air might only weigh .064 pounds, we would only net 42.18 Lbs of air per minute!
Ex: 650 * .064 = 42.18
That’s about a 15% reduction in power at the SAME volumetric efficiency!
So, the question becomes then, since I’m making 15% less power, should I change the A/F ratio to be 15% leaner?
The answer: NO! Leave it the same.
We are talking about a “ratio” of air to fuel, so since the air is 15% less, we will want the fuel to be 15% less as well, which would still net the same Air to Fuel Ratio!
This means we can build some automatic compensation tables into our ECU settings to detect a change in altitude or barometric pressure and add or subtract fueling as necessary. Wouldn’t that make life so much easier?
If we reference our previous document, “NACA Report #189 “Relation of Fuel-Air Ratios to Engine Performance” to try and solidify the above statements we’ll see what our old buddy Stanwood Sparrow has to say.
On page 111 of the report, we find following:
These tests were done at various air pressures from sea level all the way to 30,000 feet in elevation, and the results show that it is desirable to maintain the same ratio of air to fuel at all altitudes.
Of course, what we are talking about here is only the A/F ratio that produces maximum power. This is not to say that better fuel economy might not be possible by a reduction in A/F ratio due to the fact that since we are making less overall power, we may not need as much fuel for cylinder cooling, and could find some savings there. Then again… we are talking about RACING, not grocery shopping, so lighten up a little and get out there and have fun!
There are always a lot of variables in engine tuning that we could continue to converse about until the cows come home, but the only thing left to answer in our three part series is whether or not we would need a different A/F ratio when the intake air temperatures get hotter or colder.
Tune in next time for an in depth description of how air temperatures affect air density, as well as how they affect the way our engine runs! See you there.
Ben Strader Said:
Hello again everyone! It’s time for the final installment of our three part series called “Air/Fuel Ratio Management For Racers”!
In our last issue, we discussed how the engine reacts if we tune it on a dyno or at a track in one location and then take the engine to a totally different altitude or location. We found a government study put out in the 1920’s shows that the same air to fuel ratio would be required of the engine at any reasonable altitude. The engine made less power overall due to the lack of air density, but the ratio of air to fuel did not need to be changed because of this. Knowing this, the only thing a racer needs to do is make sure to maintain the same air fuel ratio at the track that they found to work when they were on the dyno!
In this final article of the series, we wanted to ask the question: “What happens to my engine at various inlet air temperatures, and how does this affect my choice of air fuel ratios?”
Let’s take one last look at our favorite document, “NACA report 189” to see if we can find the answer!
PART III
When I find the right Air Fuel Ratio for maximum power, will that number change when I race at tracks with different temperatures?.
If you’ll remember back to the last article, we used a mathematical formula to calculate the mass of air that went something like this:
Mass = V*D
Where:
V = the CFM of air the engine was breathing,
and
D = the density, (or weight) of one cubic foot of air.
There are primarily two things that affect the density of air. One is the air pressure, and the other is the air temperature.
We can use the following formula to determine how much one cubic foot of air weighs:
Density = 2.7 P/T
Where:
P = PSI (absolute)
And
T = Temperature in degrees Rankine (Degrees F + 460)
If we use the standard temperatures and pressures at sea level, we will find that one cubic foot of air weighs around .076 Lbs.
Ex: 2.7 [14.7/(60 + 460)] = .076
Now, if we simply plug in different values for various altitudes or temperatures, we can find out how much change in air density we have and then add or subtract fuel from the engine accordingly to maintain the same air to fuel ratio.
Take a look:
Lets say we are up in the mountains, and the barometric pressure is down to around 12 psi absolute, (which is around 24.4 inches of mercury, or about 82 Kpa), and the outside temperatures are about 40 degrees F.
Using the above formula, we see that:
D = 2.7 P/T
D = 2.7 [12/(40 + 460)]
D = .0648 Lbs per cubic foot
So, .0648 / .076 = .85 or about 85% of the original air density at sea level!
That means in order to keep the same air to fuel ratios, we would need to subtract about 15% of the fuel we were previously giving the engine!
We can very easily program a table into the engine computer to automatically measure the intake air temperatures, and then add or subtract fuel to maintain a constant air fuel ratio at all temperatures.
The question is though, do we need a different air fuel ratio when the air gets very hot, or very cold?
Well, to find the answer, we must once again visit “NACA Report 189”.
On pages 111 and 112 we see this following statements, (which are paraphrased here):
“An analysis of a large number of tests covering an inlet temperature range of –20 C to +40 C has shown maximum power to be obtained with approximately the same air fuel ratios at each temperature.”
This would indicate that one would always want the same air fuel ratio, regardless of the inlet temperatures. However, the report goes on to state the following:
“The volatility of the fuel is in reality the determining factor in this question. A constant fuel air ratio is desirable only so long as a change in air temperatures does not appreciably change the relative quality of the mixtures supplied to the various cylinders or the amount of fuel that has been vaporized at the time the compression stroke is completed.”
Essentially, what they are saying is that if the intake temperatures are so hot or cold that they cause the fuel to be ignited prematurely, causing detonation, or cause the fuel to remain in a more liquefied, un-vaporized state, which would make it not ignite so easily then the need for a richer or leaner air fuel ratio might exist.
Overall, what we learned from this is that if the fuel being used is fairly stable, and the temperatures encountered while racing are not extreme, then a constant air fuel ratio is desirable across a wide range of air temperatures. If however, the temperatures your engine will see are extreme, then there is a possibility that a change in air fuel ratios might be warranted.
However, most ECU manufacturers have understood this for some time, and nearly all give you one or more tables to create a method for adding or subtracting fuel as the inlet temperatures increase or decrease.
Hopefully, this series of articles has given you a small amount of insight into understanding the engine’s requirements when it comes to selecting, and maintaining a given air fuel ratio.
Only thorough testing and some trial and error will tell you what is exactly right for your engine, but perhaps with a better understanding of the factors involved we can shorten the time spent tuning, and increase the time spent racing and enjoying your vehicles! See you at the track!
FOUND HERE.
http://www.chevytalk.org/fusionbb/showtopic.php?tid/214216/
