Air Temp Relocation Kit

[Marc Haibeck]

My strategy for the inlet air temperature sensor is to improve its operation when the air temperature is rapidly changing. Especially in a drag racing situation.

I have not observed the fuel trim changing significantly with respect to engine inlet temperature. At light engine loads the oxygen sensors have great authority over the AFR. They set the AFR.

At a stop light or at the starting line at the track the sensor in the OE location is measuring the temperature of the plenum. Maybe 150 degrees F. When the car starts moving, by 20 mph or so the plenum is flooded with ambient temperature air. Maybe at 75 degrees F on a nice summer day. The air is going so fast that it picks up little heat from the plenum. The air going into the cylinder heads is nearly at ambient temperature. Meanwhile the air temperature reported by the sensor is only a few degrees below the 150 degree plenum temperature by the time the car gets to 20 mph. Mounting the sensor in an area that corresponds to the temperature of the air going into the engine provides a more accurate engine inlet temperature when the air flow is rapidly increasing. With the sensor in the OE location it can be observed that the air inlet temperature being reported to the ECM is at about 120 degrees when the car is halfway through the quarter mile going 70 mph. At the same time the engine is consuming air nearly at the ambient temperature.

The OE calibration retards the spark 3 degrees when the inlet air sensor is reading 150 degrees F. This is a horrible way to start a drag race. With a relocated sensor I retard the spark 1 degree at 90 degrees F. I figure that if the ambient temperature is 90 degrees or more you are probably not looking to set a new personal record that day and it's better to protect the engine from detonation.

[Paul Workman]

Thanks Dom n Marc 4 the info. It helped me to "fine tune" my understanding - the cause/effect placement of the sensor has. Tho I seldom drag race since the group we raced at US-41 got tired of the FBI winning the team trophy, year after year and quit inviting us to come play (kinda like the SCCA, huh?)

[XfireZ51]

Originally Posted by Marc Haibeck

My strategy for the inlet air temperature sensor is to improve its operation when the air temperature is rapidly changing. Especially in a drag racing situation.

I have not observed the fuel trim changing significantly with respect to engine inlet temperature. At light engine loads the oxygen sensors have great authority over the AFR. They set the AFR.

At a stop light or at the starting line at the track the sensor in the OE location is measuring the temperature of the plenum. Maybe 150 degrees F. When the car starts moving, by 20 mph or so the plenum is flooded with ambient temperature air. Maybe at 75 degrees F on a nice summer day. The air is going so fast that it picks up little heat from the plenum. The air going into the cylinder heads is nearly at ambient temperature.

Marc,

The relocation of the MAT sensor was an old trick, like disconnecting the battery cable just before coming to the line. Both were bandaids when not much was known or could be done about modifying the calibration.
Disconnecting the battery cable was done essentially to reset the BLMs to 128. Why was that necessary? At a BLM of 128 or below in Closed Loop, the ECM does not modify the commanded AFR in PE mode. GM used this strategy as a means of protecting the motor from possibly running lean when in PE mode since the O2 sensors(which are narrow band) no longer affect the fuel delivery. When the ECM enters PE mode, it takes the last BLM closed loop reading prior to entering PE and modifies the fuel calculation for Commanded AFR based on that last BLM.
So if ur BLM just prior to entering PE is 133, the ECM considers that lean, and so applies an additional modifier to the Commanded AFR calculation, richening up the mixture to “make up” for the lean condition and thereby protecting against detonation. Disconnecting the battery results in the ECM not changing the commanded AFR calculation since the BLMs reset to 128.
Yes the O2s are there to modify fueling but depending on them to bring the BLMs into proper range isn’t the right strategy. The objective of tuning is to get the base parameters to as correct a point as possible thereby minimizing the intervention by the ECM. IOW, u want to get the BLMs in as close a range of 128 as possible so that there is little need for ECM correction.
While ur sitting at the line or coming up to it at a drag strip, airflow to the plenum is lower, giving it more time to heat up as it passes over the radiator and as it picks up heat from the asphalt.
Without a properly adjusted IAT compensation to maintain BLMs in the proper range, the ECM will likely see a BLM that would cause the ECM to introduce an adjustment to the Commanded AFR when entering PE mode.
Launching off the line, coming out of closed loop and having entered PE, the IAT and O2s no longer modify fueling. Its pretty much left to using the VE table and the correction factors applied for Commanded AFR in PE model. So ur WOT AFR, may in fact not be what would be considered optimal, but just safe.

[Marc Haibeck]

Dominic,

I use a wide band oxygen sensor tune the WOT AFR for 12.4 from 2000 rpm to 5000 rpm. Then 12.6 from 5000 to 7500. I like to stay on the rich and safe side.

[tpepmeie]

I have concluded that GM understood the MAT sensor behavior in the factory location and compensated for it in the calibration. The table is "F26 INVMAT MAT FACTOR VS. MAT". This modifies the air charge density based on the MAT value. What's interesting is that the OE calibration doesn't match the true physical air density calculation. It has approximately a 10% offset.

In the attached graph, the blue curve is the mathematical air density curve. The Orange curve is the OE calibration. Horizontal axis is air temp. Aside from some obvious high air temperature protection built-in (the dip after 56C), you can see the OE curve is approx. 10% under the real curve. The grey curve is the physical curve minus 10%. Fits remarkably well with the OE curve.

Edit: the OE cal richens the mixture, which make sense if the true air temp is cooler than being reported by the MAT sensor.