How AFR affects Torque?

I was doing some knock testing today and thought this might interest readers, How change in AFR affects Torque output.

One of the advantages of my setup is that my car is a "moving dyno" since it is equipped with Race-Technology DL1 and it can measure the torque.

Torque is measured from the acceleration of the vehicle.

Below are two graphs, one from yesterday and one from today. It is a 2nd gear WOT acceleration.

Black is torque, Red is AFR and orange RPM.

 

Upper graph is normal torque graph, with normal 12.6 target AFR. Nice and smooth torque curve.

Lower graph is a log with leaner AFR, about 13 and shows the problem. Look at the AFR around 441secs, about 14.7.

Take a look at the black trace now and see the significant drop in torque. I also felt that the acceleration wasn't normal.

WARNING: 14.7 AFR is way too lean for Wide open throttle and may destroy your engine.

I have a data logger connected, I always record and listen for the knock so I would have lifted if there would have been any knock.

 

Other good performance indicator is how fast RPM's are rising. Autronic can log RPM/Sec values, which indicates how RPM's change.