Dip in AFR at peak boost
Nov 8, 2013, 07:58 PM
#16
Its just the "lag" between the MAF sensor and the actual engine. I'll try to explain.
The MAF is at the beginning of the intake stream and measures there, so when spooling, it see all this air coming in and tells the ECU to throw in a bunch of fuel, but the air isnt actually in the engine for a half a sec or so, which creates that rich dip.
A couple things help me find this out. First, on Suby's with a top mount, you have no rich dip. But when you put a FMIC on a suby, you start getting the same rich dip the evos have. Also, when EVOs go to SD, theres usually no rich dip, as the air is measured at the manifold, instead of the beginning of the intake track.
You can usually tune it out but it takes some time. Also, think of the lag of the MAF and the lag of the wideband (since its not inside of the engine and is actually 16"-36" away) when tuning. ie if it says the rich dip is at 300load/4000RPM, you probably actually need to tune the 3750RPM/250-270load area. Hopefully you get what Im saying.
The MAF is at the beginning of the intake stream and measures there, so when spooling, it see all this air coming in and tells the ECU to throw in a bunch of fuel, but the air isnt actually in the engine for a half a sec or so, which creates that rich dip.
A couple things help me find this out. First, on Suby's with a top mount, you have no rich dip. But when you put a FMIC on a suby, you start getting the same rich dip the evos have. Also, when EVOs go to SD, theres usually no rich dip, as the air is measured at the manifold, instead of the beginning of the intake track.
You can usually tune it out but it takes some time. Also, think of the lag of the MAF and the lag of the wideband (since its not inside of the engine and is actually 16"-36" away) when tuning. ie if it says the rich dip is at 300load/4000RPM, you probably actually need to tune the 3750RPM/250-270load area. Hopefully you get what Im saying.
Last edited by Boosted Tuning; Nov 9, 2013 at 12:05 PM.
Nov 9, 2013, 12:20 AM
#17
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This makes perfect sense. Is there any concrete data on the lag between standard location o2 readings and the other variables(rpm/load) in the logs? I have been wondering if this time difference should be considered significant or negligible.
I need to hurry up and go SD
I need to hurry up and go SD
Nov 9, 2013, 03:25 AM
#18
Evolving Member
Its just the "lag" between the MAF sensor and the actual engine. I'll try to explain.
The MAF is at the beginning of the intake stream and measures there, so when spooling, it see all this air coming in and tells the ECU to throw in a bunch of fuel, but the air isnt actually in the engine for a half a sec or so, which creates that rich dip.
I couple things help me find this out. First, on Suby's with a top mount, you have no rich dip. But when you put a FMIC on a suby, you start getting the same rich dip the evos have. Also, when EVOs go to SD, theres usually no rich dip, as the air is measured at the manifold, instead of the beginning of the intake track.
You can usually tune it out but it takes some time. Also, think of the lag of the MAF and the lag of the wideband (since its not inside of the engine and is actually 16"-36" away) when tuning. ie if it says the rich dip is at 300load/4000RPM, you probably actually need to tune the 3750RPM/250-270load area. Hopefully you get what Im saying.
The MAF is at the beginning of the intake stream and measures there, so when spooling, it see all this air coming in and tells the ECU to throw in a bunch of fuel, but the air isnt actually in the engine for a half a sec or so, which creates that rich dip.
I couple things help me find this out. First, on Suby's with a top mount, you have no rich dip. But when you put a FMIC on a suby, you start getting the same rich dip the evos have. Also, when EVOs go to SD, theres usually no rich dip, as the air is measured at the manifold, instead of the beginning of the intake track.
You can usually tune it out but it takes some time. Also, think of the lag of the MAF and the lag of the wideband (since its not inside of the engine and is actually 16"-36" away) when tuning. ie if it says the rich dip is at 300load/4000RPM, you probably actually need to tune the 3750RPM/250-270load area. Hopefully you get what Im saying.
Last edited by Biggy VIII; Nov 9, 2013 at 03:31 AM.
Nov 9, 2013, 08:43 AM
#19
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Well I hate to throw in another layer of complexity but my last set of injectors were PTE 780's and it was doing the exact same thing with them if not a few tenths worse. This is what led me to build a COP setup as I thought I was getting spark blow out at peak but I've since found that not to be true.
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