Baro and MAT ( IAT ) correction.
Posted: Mon Mar 02, 2009 4:13 am
Another squirter "tjabo" (http://www.msextra.com/memberlist.php?m ... ile&u=9525) recently asked me specifically about baro correction via PM but I want to reply on the forum in case I talk rubbish (which I know I am quite capable of) hoping that others will correct me.
Both MAT and BARO have a substantial effect on the AFRs resulting from a tuned VE table. EDIT, likewise both MAT and BARO can influence the VE table while being tuned. This is my biggest challenge while tuning the VE table.
Regarding MAT correction (which adjusts PW as a function of inlet air temp)
Do yourself a favour and do the following experiment:
Set values for all correction %'s in the MAT correction table = 0. Then , with stim, keep all inputs constant except IAT and see what happens with inj PW.
My point is that the built-in adjustment for IAT is too aggressive and adds/removes too much fuel. Others have reported likewise. What I now do is that I first adjust the MAT correction table using stim so that PW is the same for all values of MAT. This effectively cancels out the built-in MAT adjustmens that MS make. Then I modify the MAT correction table as needed on the basis of different MAT temps being experienced. Some tuners leave it up to AMC or EGO to correct for the AFR deviations from the AFR table. Personally I prefer to tune the table correctly so that even in open loop, the AFRs will be close to target.
EDIT: I have since realised that one should actually even tune the MAT correction table before the final tune of the WUE table. (Explanation futher down).
Regarding BARO correction (which adjusts PW as a function of altitude)
Highly experienced tuners on this forum hold the view that real time baro correction is unneccesary (when driving with altitude ( baro ) changes). Obviously if you only use the car at only one altitude then this is true. I have not yet been able to understand how that can be, as my AFR gauge clearly tells me that the engine is going richer or leaner for as little as 150m (500') change. This is why I fit a real time baro sensor as std to the MS's I build (not that I do many). Even using "initial map reading" is IMHO not good enough as one can easily experience much baro change on one start - depending on where you live and drive.
Similar to the MAT correction, I set the two point baro correction as follows: "At total vacuum = 100", "rate = 0". This has the effect of cancelling out the effects of the two point baro correction and ALL baro correction is then done with the Barometric Correction table. This gives an interesting insight as to how much fuel needs to be added / removed as a function of barometric changes.
Between MAT and BARO its not easy to get a VE table tuned that was not influenced by either MAT nor BARO during the tune as it means tuning the entire VE table at the same MAT and the same altitude. One needs a very long flat quiet road to be able to do this on the road. Then once the VE table is tuned one should then tune the entire MAT table at constant baro (altitude) and the tune the entire Baro table (ie all altitudes) at a constant MAT (although if the MAT table is now tuned already, MAT changes should not affect the baro table much).
I have found it better to first get the MAT table sorted at constant baro and then when I drive to different altitudes I ONLY adjust the baro table to keep the AFRs where they should be.
EDIT: In an ideal world, the MS tuner (OK, me) would therefore like to first have the MAT correction and Baro tables 100% correct and then tune the VE table. But, like I teach my kids, there's what you want and there's what you can have
EDIT 2: Please also read this if you are interested in Barometric Timing Correction for normally aspirated high compression engines: http://www.msextra.com/forums/viewtopic ... 01&t=46373
Other opinions?
Both MAT and BARO have a substantial effect on the AFRs resulting from a tuned VE table. EDIT, likewise both MAT and BARO can influence the VE table while being tuned. This is my biggest challenge while tuning the VE table.
Regarding MAT correction (which adjusts PW as a function of inlet air temp)
Do yourself a favour and do the following experiment:
Set values for all correction %'s in the MAT correction table = 0. Then , with stim, keep all inputs constant except IAT and see what happens with inj PW.
My point is that the built-in adjustment for IAT is too aggressive and adds/removes too much fuel. Others have reported likewise. What I now do is that I first adjust the MAT correction table using stim so that PW is the same for all values of MAT. This effectively cancels out the built-in MAT adjustmens that MS make. Then I modify the MAT correction table as needed on the basis of different MAT temps being experienced. Some tuners leave it up to AMC or EGO to correct for the AFR deviations from the AFR table. Personally I prefer to tune the table correctly so that even in open loop, the AFRs will be close to target.
EDIT: I have since realised that one should actually even tune the MAT correction table before the final tune of the WUE table. (Explanation futher down).
Regarding BARO correction (which adjusts PW as a function of altitude)
Highly experienced tuners on this forum hold the view that real time baro correction is unneccesary (when driving with altitude ( baro ) changes). Obviously if you only use the car at only one altitude then this is true. I have not yet been able to understand how that can be, as my AFR gauge clearly tells me that the engine is going richer or leaner for as little as 150m (500') change. This is why I fit a real time baro sensor as std to the MS's I build (not that I do many). Even using "initial map reading" is IMHO not good enough as one can easily experience much baro change on one start - depending on where you live and drive.
Similar to the MAT correction, I set the two point baro correction as follows: "At total vacuum = 100", "rate = 0". This has the effect of cancelling out the effects of the two point baro correction and ALL baro correction is then done with the Barometric Correction table. This gives an interesting insight as to how much fuel needs to be added / removed as a function of barometric changes.
Between MAT and BARO its not easy to get a VE table tuned that was not influenced by either MAT nor BARO during the tune as it means tuning the entire VE table at the same MAT and the same altitude. One needs a very long flat quiet road to be able to do this on the road. Then once the VE table is tuned one should then tune the entire MAT table at constant baro (altitude) and the tune the entire Baro table (ie all altitudes) at a constant MAT (although if the MAT table is now tuned already, MAT changes should not affect the baro table much).
I have found it better to first get the MAT table sorted at constant baro and then when I drive to different altitudes I ONLY adjust the baro table to keep the AFRs where they should be.
EDIT: In an ideal world, the MS tuner (OK, me) would therefore like to first have the MAT correction and Baro tables 100% correct and then tune the VE table. But, like I teach my kids, there's what you want and there's what you can have
EDIT 2: Please also read this if you are interested in Barometric Timing Correction for normally aspirated high compression engines: http://www.msextra.com/forums/viewtopic ... 01&t=46373
Other opinions?