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Understanding AFR

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Discussion and questions related to the course Understanding AFR

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I have a skyline R34 gtr which is equipped with link g4+ pnp.

Engine is stock with upgrated turbos, DW 1200cc injectors, upgrated DW 340 lph fuel pump.

Car was tuned in road and since i didnt have an afr gauge i was using the lm2 innovate portable unit.

I recently installed innovate dual afr gauge with a sensor in each downpipe and the headache begin.

1 to 3 cylinders was running very rich (10.5-11.0) and 4 to 6 cylinders very lean (15.0-17.0) at idle.

i then connect the portable unit and the average afr was 13.5 to 14.0. Thats with ect at 80c.

The map used is MAP based. I have check the injectors on bench testing machine and it was ok.

I did a compression test on the cylinders and all values are ok.

i used the individual injector map and to get them close to the stoichiometric value i had to go -50 at first 3 cylinders and +15 at the other 3.

there is any way to solve this out?


Do the afrs level out at cruise or higher load?

Have you swapped the dump sensors around to check for any offset in the channels?

I'm guessing the throttles aren't balanced if everything else is ok.

I have replace sensors from down pipes and also from gauges. very slight offset.

I didnt try it on road yet but holding the revs up to 4K the differnce remains like that.

As for the throttles there is an adjustment screw per pair of tb's(1-2 3-4 5-6)

which is not accesible for adjust and after i install new plugs

for testing i remove them in 5 minutes and find that 1st and 3rd cyl was the issue.

are you sure you positioned the sensors in a way that is getting an accurate reading?

Sensors was positioned according to innovate.

More than 24" from turbo outlet and between 9 to 3 o'clock. Brand new kit and calibrated twice.

My first guess is that you may have an exhaust leak that is introducing ambient oxygen to the exhaust system pre sensor. This will result in a lean reading that isn't real. It would be interesting to see if the readings are consistently out under cruise conditions as well as full boost/WOT operation as typically under full boost the exhaust pressure negates the effects of an exhaust leak and the readings would be much closer.

It would also be possible to get a disparity in your readings if the wastegates are not synced and one is opening much later than the other. This would affect the relative exhaust back pressure between the cylinders and hence the VE of the front and rear set of 3 cylinders would be different. This would not show up until you're actually on boost though so idle and cruise mixtures should be similar.

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