afr confusion
Mar 8, 2008 | 07:28 PM
#1
afr confusion
when I had my truck dyno tuned the afr on the dynojet showed 13.1 afr . only changes since tune was removed my catalytics. well now that i have a plx wideband at wot it shows a 12.0-12.3 afr. checked my tune and my pe shows a commanded 1.229 which should be like 11.94 which dosent make sense. also when i did the dyno tune the o2 sensor was placed at exhaust dump before axle. why would i have one say 13.1 and now 12.0-12.3. yeah that was kind of confusing.
Mar 8, 2008 | 09:02 PM
#2
If the dyno wideband was inserted into the tail pipe and you had cats it would read leaner then actual because the cats are burning up some of the unburt fuel. Now as too why your commanded afr and actual dont match then the answer lies in your VE or MAF table. You set the commanded where you want it and tune your VE and MAF until the commanded and actually match.
Mar 8, 2008 | 09:52 PM
#3
I am not sure why your AFR did what it did, but I have been told to try to shoot for 12.5 N/A so you can run more timing without knock....
More than likely your commanding a richer mixture to make up for an uncalbrated MAF and/or VE table... If your VE and MAF tables are tuned correctly whatever mixture you command is what you will get... If you comanded AFR does not equal your actual AFR then something is off... VE+MAF is used for <4000RPM, MAF is used >4000RPM, so which ever RPM range does not equal your commanded AFR, that is where you need to look.
More than likely your commanding a richer mixture to make up for an uncalbrated MAF and/or VE table... If your VE and MAF tables are tuned correctly whatever mixture you command is what you will get... If you comanded AFR does not equal your actual AFR then something is off... VE+MAF is used for <4000RPM, MAF is used >4000RPM, so which ever RPM range does not equal your commanded AFR, that is where you need to look.
Mar 9, 2008 | 11:17 AM
#4
so if i change my pe command value from the 1.229 to let say 1.20 it wont change anything ? im assuming that since its at 1.229 =11.95 but really showing 12.3 then maybe changing pe command value to 1.20=12.23 should show a 12.58....
Mar 9, 2008 | 07:51 PM
#6
Mar 9, 2008 | 08:50 PM
#7
True, this is kinda a simpliefied version of how I believe the tables work. The uses the values in the VE and MAF table to provide the amount of AIR needed to provide a AFR of 14.63 or 1.00... If the AFR is off that is where STFT and LTFT come into play.... The computer will use the AIR value but it will adjust the amount of fuel it applies with the fuel trims to achieve the commanded AFR, so if your trims are more than +/- 5% your losing power. So this is why these two table need to be tuned for the mods you have, every mod will change the way your engine breathes and exhales.... The VE and MAF table are the base of every AFR calculation the computer makes, so when they are wrong the AFR is wrong and your STFT and LTFT have to compensate... Does that make sense?
Now on to how PE works....
PE calculation with VE table...
Given a certain MAP and RPM value. For example in my tune MAP = 95 and RPM = 3200 that intersection in my VE table is 95.02, so the computer will use 95.02 in the AFR calculation to achieve 14.63.... Next throw in the PE value of 12.5, this is the value that I am commanding according to my PE table at 3200 rpm.... The computer does the following to calculate how much air is needed to acheive that AFR... 12.5 = 1.17 (14.63/12.5=1.17) the computer uses the eq value and multiplies it by the VE value so in our example 95.02*1.17 =111.17 amount of air needed to achieve a AFR 12.5... Does all that make sense.... I may be way off but this is the way I understand the way the tables work together...
Now on to how PE works....
PE calculation with VE table...
Given a certain MAP and RPM value. For example in my tune MAP = 95 and RPM = 3200 that intersection in my VE table is 95.02, so the computer will use 95.02 in the AFR calculation to achieve 14.63.... Next throw in the PE value of 12.5, this is the value that I am commanding according to my PE table at 3200 rpm.... The computer does the following to calculate how much air is needed to acheive that AFR... 12.5 = 1.17 (14.63/12.5=1.17) the computer uses the eq value and multiplies it by the VE value so in our example 95.02*1.17 =111.17 amount of air needed to achieve a AFR 12.5... Does all that make sense.... I may be way off but this is the way I understand the way the tables work together...
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Mar 9, 2008 | 09:08 PM
#8
Correct, if you are not in PE then you should be around 14.7afr. However in your initial post you were referancing WO correct? If so then PE is the commanded table and you tune the VE and MAF to match. If you are running a MAF most of your fueling calculations are comming from there at WO. To properly tune your VE table if you are running a MAF you have to disable the MAF and tune the VE. Once VE is right then enable the MAF and any error will be MAF related so you then rescale the MAF table. There are step by steps on how to do this on this site and HPT forums.
Mar 9, 2008 | 09:25 PM
#9
True, this is kinda a simpliefied version of how I believe the tables work. The uses the values in the VE and MAF table to provide the amount of AIR needed to provide a AFR of 14.63 or 1.00... If the AFR is off that is where STFT and LTFT come into play.... The computer will use the AIR value but it will adjust the amount of fuel it applies with the fuel trims to achieve the commanded AFR, so if your trims are more than +/- 5% your losing power. So this is why these two table need to be tuned for the mods you have, every mod will change the way your engine breathes and exhales.... The VE and MAF table are the base of every AFR calculation the computer makes, so when they are wrong the AFR is wrong and your STFT and LTFT have to compensate... Does that make sense?
Now on to how PE works....
PE calculation with VE table...
Given a certain MAP and RPM value. For example in my tune MAP = 95 and RPM = 3200 that intersection in my VE table is 95.02, so the computer will use 95.02 in the AFR calculation to achieve 14.63.... Next throw in the PE value of 12.5, this is the value that I am commanding according to my PE table at 3200 rpm.... The computer does the following to calculate how much air is needed to acheive that AFR... 12.5 = 1.17 (14.63/12.5=1.17) the computer uses the eq value and multiplies it by the VE value so in our example 95.02*1.17 =111.17 amount of air needed to achieve a AFR 12.5... Does all that make sense.... I may be way off but this is the way I understand the way the tables work together...
Now on to how PE works....
PE calculation with VE table...
Given a certain MAP and RPM value. For example in my tune MAP = 95 and RPM = 3200 that intersection in my VE table is 95.02, so the computer will use 95.02 in the AFR calculation to achieve 14.63.... Next throw in the PE value of 12.5, this is the value that I am commanding according to my PE table at 3200 rpm.... The computer does the following to calculate how much air is needed to acheive that AFR... 12.5 = 1.17 (14.63/12.5=1.17) the computer uses the eq value and multiplies it by the VE value so in our example 95.02*1.17 =111.17 amount of air needed to achieve a AFR 12.5... Does all that make sense.... I may be way off but this is the way I understand the way the tables work together...
blackgmc so you get 111 ve value?
Last edited by phasemaster1; Mar 9, 2008 at 09:39 PM.
Mar 9, 2008 | 09:30 PM
#10
Correct, if you are not in PE then you should be around 14.7afr. However in your initial post you were referancing WO correct? If so then PE is the commanded table and you tune the VE and MAF to match. If you are running a MAF most of your fueling calculations are comming from there at WO. To properly tune your VE table if you are running a MAF you have to disable the MAF and tune the VE. Once VE is right then enable the MAF and any error will be MAF related so you then rescale the MAF table. There are step by steps on how to do this on this site and HPT forums.