Tuning

Datalogging: The Numbers Worth Watching on a Modified Car

September 5, 2026 · 6 min read · GZ Performance, Tampa FL

A datalog is the only way to see what your engine is actually doing rather than what you assume it is doing. On a modified car it is the difference between catching a problem and paying for one.

The channels that matter

Knock activity and ignition correction

How much timing the ECU is pulling because it detected knock. This is the single most important channel on a boosted car.

A small amount of correction occasionally is normal. Persistent correction, or correction in the same place every pull, means something is wrong — fuel quality, heat, a mechanical issue, or a tune that is too aggressive.

Air-fuel ratio

From a wideband sensor, not the factory narrowband. You want to see it tracking the commanded target across the whole pull, not just at peak.

Lean at the top of a pull is the classic sign of a fuel system at its limit. Fuel system limits.

Injector duty cycle

How hard the injectors are working. Above roughly 85% there is no margin left. This is the number that tells you when the fuel system needs upgrading, before it fails.

Fuel pressure

Should hold steady under load. Pressure dropping at high RPM means the pump, lines or filter cannot keep up.

Boost — target vs actual

Both channels together. Spiking, creeping or falling boost are all visible here and each has a different cause. Boost control diagnosis.

Intake air temperature

Critical in Florida. Watch how much it rises across a pull and how quickly it recovers between pulls. A large rise that does not recover is heat soak, and it is telling you about the intercooler. Intercooler selection.

Coolant and oil temperature

Both, not just coolant. Oil temperature is the one that limits many Florida cars and the one fewest people monitor.

Fuel trims

Long and short term. Trims that have drifted meaningfully from zero are telling you the engine is receiving different air than the ECU calculated — usually a leak, a failing sensor, or a fuel delivery problem.

Throttle position and pedal position

On a drive-by-wire car these are different channels, and a gap between them means the ECU is limiting torque for a reason it has not told you about.

Log a full pull, in one gear, at full throttle

The most useful log is a single sustained wide-open-throttle pull in a gear that takes some time — usually third or fourth. Short bursts and part-throttle logs are much harder to read. Log at a high enough sample rate that fast events are visible.

What healthy looks like

Air-fuel ratio tracking the target closely through the whole pull
Little or no ignition correction
Boost reaching target smoothly without overshoot, then holding
Injector duty cycle with real headroom left at peak
Fuel pressure flat under load
Intake air temperature rising modestly and recovering between pulls
Fuel trims near zero

Reading trouble

Timing pulled in the same place every pull — a specific cylinder or a specific load point has a problem.
Lean at the top only — fuel system limit.
Boost overshoot then settle — boost control gains or a wastegate opening late.
Boost climbing above target at high RPM — creep, a wastegate flow problem.
IAT climbing steeply and not recovering — heat soak.
Positive long-term fuel trims — unmetered air entering, often a boost leak.
Rising oil temperature across a session — cooling capacity.

Logging habits worth having

Log after any change — a new part, a new tune, a different fuel.
Log periodically even when nothing is wrong, so you have a healthy baseline to compare against.
Log on a hot day as well as a cool one. The car behaves differently and you want to know how.
Keep the logs. A comparison across months shows drift that a single log never will.
Send them to us if something looks off. It is far quicker to diagnose from data than from a description.

Send us a log

If your car feels different and you are not sure why, a datalog usually answers it faster than a diagnostic appointment. Get one and bring it in.

Questions we get asked

What is the most important thing to watch in a datalog?

Knock activity and how much ignition timing the ECU is pulling. Occasional small correction is normal; persistent correction, or correction in the same place every pull, means fuel quality, heat, a mechanical issue or an over-aggressive tune.

How do I take a useful datalog?

A single sustained wide-open-throttle pull in a gear that takes some time, usually third or fourth, at a high enough sample rate that fast events are visible. Short bursts and part-throttle logs are much harder to read.

What does it mean if my car goes lean at the top of a pull?

Almost always a fuel system at its limit — the injectors or the pump cannot deliver enough at peak flow. Check injector duty cycle and fuel pressure in the same log to see which one ran out.

Should I log even when nothing is wrong?

Yes. A healthy baseline is what lets you spot drift later, and logging on both a hot and a cool day tells you how the car behaves across the conditions it actually sees in Florida.

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