What a datalog is, and why a tune without one is a guess

A dyno measures what an engine made. A datalog records how it made it. Of the two, the log is the one a tuner cannot work without, and this is why.

What is in it

A modern engine controller measures hundreds of things many times a second and will report them over the diagnostic port to any tool that asks. A datalog is a recording of a chosen set of those channels through a chosen stretch of driving, saved as a table: one row per sample, one column per channel. For a tune, the stretch is a single full-throttle pull, and the channels are the ones that show whether the calibration is safe: engine speed, throttle, boost requested and actual, ignition timing and the timing pulled for knock on each cylinder, the mixture requested and actual, and the intake air temperature.

Why it beats every other signal

The seat of the pants measures acceleration and nothing else. A dyno measures torque at the wheels and nothing else. A car can make a fine number while pulling six degrees of timing on cylinder three and running lean at the top of the pull, and the only thing in the world that will say so is the log. Knock, in particular, is inaudible in a modern car at the level where it matters; the ECU hears it, acts, and the driver feels a car that is a little flat. The log shows the ECU's actions.

How a tuner reads one

In a fixed order. Timing pull first, per cylinder, because it is the engine's own verdict on the calibration. Then boost against request, because a gap is a leak or a limit. Then mixture against target, because lean under boost is the dangerous direction. Then intake temperature, which is the context for all three. A log is judged as a whole: a degree of pull on a hot day with boost on target is fine; the same degree on a cool day with boost falling short is a fault.

What "checked against a datalog" means

It means the file was written from a stock read and then confirmed against the engine actually running it. A tune delivered without that step is a prediction. Most predictions on a healthy stock car on the right fuel come true, which is why plenty of tunes are sold without logs and most of those cars are fine. The ones that are not fine are the reason the datalog tiers of this service exist, and the reason even the base tier includes a check of the first log.

How to take one

The procedure is short and specific: warm car, declared fuel, a few minutes of steady driving so the intake is not heat-soaked, one pull in third gear from about 2,000 rpm to just short of the limiter, on a private road or a dyno, logging at the tool's fastest rate. One clean pull beats ten partial ones.

The simulator on this site does not produce a log, because it is not a car; it produces the same signals instantly from a model instead. Below, drag the timing past the ceiling and watch the knock gauge. That gauge is the column a real log is read for first.