Reading your datalog: RPM, boost, AFR
The channels the platform draws from your log — and what healthy actually looks like on each one. Four traces tell the whole story.
A datalog is a recording of what the ECU did, frame by frame, when you put your foot down. You don't need to be a tuner to read the four channels that tell the story.
When you flash a tune and drive it hard, the computer records a stream of channels — manifold pressure, engine speed, oxygen in the exhaust, and the wastegate's effort — sampled many times a second. We draw four of them, each on its own honest axis: RPM, Boost, AFR, and WGDC. Here's what each one is telling you.
RPM — the x-axis of everything
Engine speed is where the pull happens. A healthy log shows a clean, linear rise through a pull rather than a plateau or a stutter. The decisions that matter — spool, peak torque, the taper toward redline — live between roughly 3,000 and 6,500 rpm. What you do below 2,500 is interesting but rarely the story.
Boost — requested vs. actual
This is manifold pressure above atmospheric, in psi. What healthy looks like: a quick, smooth climb to the boost target, a steady hold through the meat of the pull, then a taper as the turbo runs out of compressor flow near redline. What tells a story: a spike that overshoots and gets pulled back, a droop where the turbo runs out of duty, or a target that was never reached at all.
AFR — air-fuel ratio
AFR is how much air is going in relative to fuel. Chemically perfect (lambda 1.0) reads about 14.7:1 on pump gas. Under load a healthy tune deliberately runs richer — richer mixtures burn cooler and resist knock — so you will see the trace fall into the low 12s on pump and roughly low-to-mid 11s on ethanol. The one you call about is an unexpected lean spike while the manifold is pressurized.
WGDC — how hard the turbo is working
Duty cycle is the percentage of effort the ECU is spending on the wastegate to reach the boost target. Low duty at target means headroom. Duty climbing toward its ceiling while boost falls means the turbo has nothing left — a target that's too high, or a leak that's quietly stealing the pressure. It gets its own article.
One honest caveat: a single pull is not the whole truth. Heat soak, ambient temperature, altitude, fuel in the tank and how the car was driven all move the traces. A healthy log is a signal to read, not a report card — and a second pull, in the same conditions, is worth more than a screenshot.