someone please explain air fuel ratio
Mar 21, 2006, 08:09 AM
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someone please explain air fuel ratio
all these dyno curve i have been seeing and the a/f ratios i have been seeing, everyone says that 11:1 ratio is good. but i though stoichiometric was 14.7:1 ?
im so confused. here is a curve i found. someone please chime in and explain.
im so confused. here is a curve i found. someone please chime in and explain.
Last edited by deadbeatrec; Dec 13, 2006 at 08:08 AM.
Mar 21, 2006, 08:16 AM
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14.7:1 is the most effecient for burning at sea level on a 15 degree centigrade day at 29.92" of mercury.
That is too high for turbo'd vehicles as you increase the boost you'll increase detonation. Therefore you have to remove some of the air.
That is too high for turbo'd vehicles as you increase the boost you'll increase detonation. Therefore you have to remove some of the air.
Mar 21, 2006, 08:17 AM
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so what should it be like 11:1
Originally Posted by Cirrusly Evolvd
14.7:1 is the most effecient for burning at sea level on a 15 degree centigrade day at 29.92" of mercury.
That is too high for turbo'd vehicles as you increase the boost you'll increase detonation. Therefore you have to remove some of the air.
That is too high for turbo'd vehicles as you increase the boost you'll increase detonation. Therefore you have to remove some of the air.
Mar 21, 2006, 08:28 AM
#4
Originally Posted by deadbeatrec
so what should it be like 11:1
If you have to ask WHY... then why the sky is blue? Why do I have nipples?
It's just the way things are.
Accept it man.
Unless you wanna run your engine at 14.4:1 ratio at WOT to find out first hand. :P
I saw that second hand and that was good enough for me. heh
Mar 21, 2006, 08:38 AM
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when did i ask why? just wondering because i cant find where i said why. maybe u saw something that i didnt. if u dont have anything helpfull about the question at hand then y post. i realize detination will blow up an internal combustion engine, i was asking what it should be. not why.
Originally Posted by Ev0ikon
It just happen to be the ratio that we end up after years of turbo engine trial and errors.
If you have to ask WHY... then why the sky is blue? Why do I have nipples?
It's just the way things are.
Accept it man.
Unless you wanna run your engine at 14.4:1 ratio at WOT to find out first hand. :P
I saw that second hand and that was good enough for me. heh
If you have to ask WHY... then why the sky is blue? Why do I have nipples?
It's just the way things are.
Accept it man.
Unless you wanna run your engine at 14.4:1 ratio at WOT to find out first hand. :P
I saw that second hand and that was good enough for me. heh
Mar 21, 2006, 08:50 AM
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Originally Posted by deadbeatrec
all these dyno curve i have been seeing and the a/f ratios i have been seeing, everyone says that 11:1 ratio is good. but i though stoichiometric was 14.7:1 ?
im so confused. here is a curve i found. someone please chime in and explain.
im so confused. here is a curve i found. someone please chime in and explain.
Yes, 14.7:1 is the ideal ratio, in that at 14.7:1, there will be absolutely zero unburned fuel left after combustion. The problem is that in turbo cars, there is a higher likelihood of detonation due to the increased pressure of the compressed mixture under boost. The increased pressure results in a faster burn, which leads to detonation.
So ... the additional (unburned!) fuel added to get down around the 11:1 AFR ratio acts to slow the flame front propagation, reducing the likelihood of detonation. The additional fuel also has some cooling properties, but that is pretty much negligible compared to something like water which has a significantly higher specific latent heat of vaporization.
N/A motors can run a leaner AFR as the compressed mixture is not under as much pressure and therefore the flame front is not likely to move as fast as when turbocharged and detonation is reduced.
That's my limited understanding FWIW ...
Mar 21, 2006, 08:55 AM
#7
Originally Posted by deadbeatrec
when did i ask why? just wondering because i cant find where i said why. maybe u saw something that i didnt. if u dont have anything helpfull about the question at hand then y post. i realize detination will blow up an internal combustion engine, i was asking what it should be. not why.
I didn't read it properly.
Your question of What is valid. And the answer depends. Believe it or not, it varies from car to car and from one type of fuel to the other.
The best ratio is one where your car makes the most power with the least amount of knock.
So, in order for you to find that out for your own car, you'll need some way to monitor the AFR, the EGT, the Knock voltage, and your igition timing, and other pertinent temp/pressure info.
Depends on your level of comfort (knowing how long your engine will last) it could vary...
Tune it!
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Mar 21, 2006, 08:56 AM
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this is what i was thinking, due to the compression of the turbo, the AFR's need to be richer to be on the safe side of things. i was thinking about getting a aem uego gauge to monitor my AFR's . i had recently found them for like 2 bills and was concidering it a safe upgrade. where does the bung need to be welded, post cat or pre-cat?
Originally Posted by joshd
Here is my lame shadetree mechanic attempt at an answer.
Yes, 14.7:1 is the ideal ratio, in that at 14.7:1, there will be absolutely zero unburned fuel left after combustion. The problem is that in turbo cars, there is a higher likelihood of detonation due to the increased pressure of the compressed mixture under boost. The increased pressure results in a faster burn, which leads to detonation.
So ... the additional (unburned!) fuel added to get down around the 11:1 AFR ratio acts to slow the flame front propagation, reducing the likelihood of detonation. The additional fuel also has some cooling properties, but that is pretty much negligible compared to something like water which has a significantly higher specific latent heat of vaporization.
N/A motors can run a leaner AFR as the compressed mixture is not under as much pressure and therefore the flame front is not likely to move as fast as when turbocharged and detonation is reduced.
That's my limited understanding FWIW ...
Yes, 14.7:1 is the ideal ratio, in that at 14.7:1, there will be absolutely zero unburned fuel left after combustion. The problem is that in turbo cars, there is a higher likelihood of detonation due to the increased pressure of the compressed mixture under boost. The increased pressure results in a faster burn, which leads to detonation.
So ... the additional (unburned!) fuel added to get down around the 11:1 AFR ratio acts to slow the flame front propagation, reducing the likelihood of detonation. The additional fuel also has some cooling properties, but that is pretty much negligible compared to something like water which has a significantly higher specific latent heat of vaporization.
N/A motors can run a leaner AFR as the compressed mixture is not under as much pressure and therefore the flame front is not likely to move as fast as when turbocharged and detonation is reduced.
That's my limited understanding FWIW ...
Mar 21, 2006, 08:59 AM
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thanks again, and i do have it tuned but at the time of tune i was pulling some timing, but im not sure if i was running lean or not. thats the weird thing, i think i was actually running rich, but i was pulling timing. so thats y i want to get a AEM uego so i can see for myself.
Originally Posted by Ev0ikon
Sorry, my bad. :}
I didn't read it properly.
Your question of What is valid. And the answer depends. Believe it or not, it varies from car to car and from one type of fuel to the other.
The best ratio is one where your car makes the most power with the least amount of knock.
So, in order for you to find that out for your own car, you'll need some way to monitor the AFR, the EGT, the Knock voltage, and your igition timing, and other pertinent temp/pressure info.
Depends on your level of comfort (knowing how long your engine will last) it could vary...
Tune it!
I didn't read it properly.
Your question of What is valid. And the answer depends. Believe it or not, it varies from car to car and from one type of fuel to the other.
The best ratio is one where your car makes the most power with the least amount of knock.
So, in order for you to find that out for your own car, you'll need some way to monitor the AFR, the EGT, the Knock voltage, and your igition timing, and other pertinent temp/pressure info.
Depends on your level of comfort (knowing how long your engine will last) it could vary...
Tune it!
Mar 21, 2006, 09:12 AM
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Originally Posted by deadbeatrec
where does the bung need to be welded, post cat or pre-cat?
Originally Posted by deadbeatrec
i do have it tuned but at the time of tune i was pulling some timing, but im not sure if i was running lean or not. thats the weird thing, i think i was actually running rich, but i was pulling timing. so thats y i want to get a AEM uego so i can see for myself.
Keep in mind that boost, AFR, and timing work together and are interrelated and interdependent ... and you can't always just add fuel to overcome knock. It's possible that the timing is simply too high to begin with. In general, the idea in tuning is to achieve the minimum best timing advance for highest torque (MBT - also sometimes referred to as minimum timing for best torque). In a turbo car, sometimes you are knock limited before you reach MBT, and you can possibly use a little extra fuel in order to be able to achieve MBT without knock. Again, this is my simplistic understanding, but at least it gives you some of the picture I think.
So it doesn't necessarily follow that running rich will automatically cure all knock ... you have to look at the timing and boost as well ... and factor in EGT and all the other engine operating parameters like Ev0ikon said.
Last edited by joshd; Mar 21, 2006 at 09:15 AM.
Mar 21, 2006, 09:21 AM
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there are a number of reasons why you run a rich mixture (i.e. lambda < 1, or A/F < 14.7:1 ) the biggest reason as people have pointed out is to avoid detonation. when you run a stoic. ratio you get a very complete burn, no fuel is wasted, and you make more power. however temps also increase in the intake charge which can cause detonation if you run high boost. by adding the additional fuel, you help to cool the intake charge and keep EGT's down, and cylinder pressures/temperatures are kept down as well. with keeping the cylinder temperatures down you avoid autoignition.
ideally, 11:1 is extremely rich and a if i were tuning my own car i would aim closer to 12-12.5:1. running that rich you can run the risk of washing your cylinder walls with the fuel. interestingly, if you used E85 as a fuel, you could run right on stoic. ratio and be fine because the fuel itself cools the air far greater then gasoline does (you wouldn't even need an intercooler for moderate boost levels). thats why the alcohol injection kits are a hot commodity for running high boost.
basically what it boils down to, you don't need to run 11:1 A/F. if you don't know what you are doing with tuning, then certainly stay super rich to sort of "mask" your errors. the factory tunes really rich for safety and reliability, but 11:1 ratio is not a max power tune (IIRC lambda of .85-.9 is ideal, which will put you at 12.5-13:1. if you are going to tune for that range, you need to be spot on with your A/F ratios... you need to know the latency of the sensor and readout, you need to know what timing you can run, and you need to know how consistant your boost is. boost spikes with A/F ratios close to stoic will be disasterous if you didn't tune for them.
this is only the tip of the iceburg really. i suggest reading some books on it if your interested in tuning your own car.
FYI, my housemates shifter kart actually makes max power running lean. it is a two stroke engine, and it is really dangerous running lean (things get really hot and pop), but if you want to win thats what you do.
ideally, 11:1 is extremely rich and a if i were tuning my own car i would aim closer to 12-12.5:1. running that rich you can run the risk of washing your cylinder walls with the fuel. interestingly, if you used E85 as a fuel, you could run right on stoic. ratio and be fine because the fuel itself cools the air far greater then gasoline does (you wouldn't even need an intercooler for moderate boost levels). thats why the alcohol injection kits are a hot commodity for running high boost.
basically what it boils down to, you don't need to run 11:1 A/F. if you don't know what you are doing with tuning, then certainly stay super rich to sort of "mask" your errors. the factory tunes really rich for safety and reliability, but 11:1 ratio is not a max power tune (IIRC lambda of .85-.9 is ideal, which will put you at 12.5-13:1. if you are going to tune for that range, you need to be spot on with your A/F ratios... you need to know the latency of the sensor and readout, you need to know what timing you can run, and you need to know how consistant your boost is. boost spikes with A/F ratios close to stoic will be disasterous if you didn't tune for them.
this is only the tip of the iceburg really. i suggest reading some books on it if your interested in tuning your own car.
FYI, my housemates shifter kart actually makes max power running lean. it is a two stroke engine, and it is really dangerous running lean (things get really hot and pop), but if you want to win thats what you do.
Mar 21, 2006, 09:27 AM
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ok when i was dynoed and reflashed by AL he showed me my graph and told me that i was pulling timing. here is the graph, he said that once the graph drops, thats when the car was throwing the knock sensor and pulling timing. so tell me what u think.
Originally Posted by joshd
Wideband O2 sensor bung i believe should be pre-cat around 18" from the turbo outlet, but you might want to search up on that and also reference the manual for the particular product.
How are you currently tuned? Reflash, AEM, etc? And what are you using to monitor timing such that you know timing is being pulled?
Keep in mind that boost, AFR, and timing work together and are interrelated and interdependent ... and you can't always just add fuel to overcome knock. It's possible that the timing is simply too high to begin with. In general, the idea in tuning is to achieve the minimum best timing advance for highest torque (MBT - also sometimes referred to as minimum timing for best torque). In a turbo car, sometimes you are knock limited before you reach MBT, and you can possibly use a little extra fuel in order to be able to achieve MBT without knock. Again, this is my simplistic understanding, but at least it gives you some of the picture I think.
So it doesn't necessarily follow that running rich will automatically cure all knock ... you have to look at the timing and boost as well ...
How are you currently tuned? Reflash, AEM, etc? And what are you using to monitor timing such that you know timing is being pulled?
Keep in mind that boost, AFR, and timing work together and are interrelated and interdependent ... and you can't always just add fuel to overcome knock. It's possible that the timing is simply too high to begin with. In general, the idea in tuning is to achieve the minimum best timing advance for highest torque (MBT - also sometimes referred to as minimum timing for best torque). In a turbo car, sometimes you are knock limited before you reach MBT, and you can possibly use a little extra fuel in order to be able to achieve MBT without knock. Again, this is my simplistic understanding, but at least it gives you some of the picture I think.
So it doesn't necessarily follow that running rich will automatically cure all knock ... you have to look at the timing and boost as well ...
Last edited by deadbeatrec; Dec 13, 2006 at 08:08 AM.
Mar 21, 2006, 12:01 PM
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Originally Posted by deadbeatrec
ok when i was dynoed and reflashed by AL he showed me my graph and told me that i was pulling timing. here is the graph, he said that once the graph drops, thats when the car was throwing the knock sensor and pulling timing. so tell me what u think.
Al has tuned a crapload of Evos and I've tuned exactly 0, so I'm not in any position to second guess anything he's told you. I would think he could smooth that out, but potentially at the loss of some power (by lowering timing overall). Did he leave it with that state of tune? Did he have recommendations for how to fix the problem?
Mar 21, 2006, 12:07 PM
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he didnt know what was going on other than the high flow cat messing things up. since then i have taken the hfc off and replaced it with a test pipe. feels strong and i dont feel any loss of power up top what so ever. but i hate to get on it when i dont know if im still having the knock sensor issues, so i drive like a g-ma all the time. he basicly told me to get rid of the cat and see how it feels. that and get some cams and come back for a retune.
Originally Posted by joshd
Well something 'not good' is obviously happening there just before 6K RPMs ...
Al has tuned a crapload of Evos and I've tuned exactly 0, so I'm not in any position to second guess anything he's told you. I would think he could smooth that out, but potentially at the loss of some power (by lowering timing overall). Did he leave it with that state of tune? Did he have recommendations for how to fix the problem?
Al has tuned a crapload of Evos and I've tuned exactly 0, so I'm not in any position to second guess anything he's told you. I would think he could smooth that out, but potentially at the loss of some power (by lowering timing overall). Did he leave it with that state of tune? Did he have recommendations for how to fix the problem?
Mar 21, 2006, 12:10 PM
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Originally Posted by KevinD
however temps also increase in the intake charge which can cause detonation if you run high boost. by adding the additional fuel, you help to cool the intake charge and keep EGT's down, and cylinder pressures/temperatures are kept down as well. with keeping the cylinder temperatures down you avoid autoignition.
My understanding is that the two phenomenon: pre-ignition and detonation are actually two separate (bad) things that can occur in an internal combustion engine.
Pre(auto)-ignition, as you state, is when the mixture spontaneously combusts prior to the spark event due to a high heat/low octane (or combination thereof) situation.
Detonation on the other hand, I think, is an uncontrolled (too fast) burn of the mixture after being ignited by the spark.
Both of these are very bad obviously and can happen more frequently in turbo cars, but I think you might have these two conditions somewhat mixed together in your description above.
Also, my understanding is that fuel doesn't really lower intake temps much at all, that is a bit of a misnomer. Extra fuel I believe slows the burn which helps to prevent detonation. This is different than water/alky, which have much higher specific latent heat of vaporization (water having the highest i think) and can both lower intake temps a significant amount.
Anyway, just rambling here and trying to galvanize my own knowledge. Feel free to point out my mistakes