9 Dec 2017

Drivability tuning continues.

While maintaining a constant low speed at zero boost, lambda (air/fuel ratio) swings back and forth quite a bit between about 0.80 and 1.2. A side effect is feeling an oscillation in the car, like someone’s pushing and letting off the gas about once a second.

The fact that it was oscillating reminded me that closed-loop lambda control has its own PID loop. Comparing the values with an old tune showed that the tuner had set the (P)roportional constant about 4X what I had and the (I)ntegral constant at about double. I suspect he just set it to something in order to get on with the main tuning. Well… that’s fine, but it leaves the car with yet another drivability quirk. It’s harmless, but it’s stuff like startup, idle, and partial-throttle performance that gets noticed far more often because 99% of the time that’s where the car operates. If I’d taken the car for a test drive, we’d have easily been tuning the rest of the afternoon, but since he doesn’t fit on the passenger side(!), I don’t think sticking around would have helped.

I’ve talked to other tuners and they said this is where all the time goes – drivability issues. Customers get all excited about loud pulls that produce dyno curves for bragging rights, but it’s the little, nuanced variables that complete the tune and make the car act, what, like a real car?