Datalog Analyzer

How to Read Your Own Datalog

A datalog is the closest thing to ground truth your car gives you. Dynos flatter, butt-dynos lie, but the log shows what the ECU actually did. Here is what this page looks for and why it matters:

The Channels, in Plain English

• RPM + pedal/throttle - how pulls are found. This page treats 85%+ pedal held for 1.5 seconds or more as a WOT pull. Pedal is what your foot asks for; throttle is what the ECU actually opens, and the two splitting apart under load means the ECU is intervening for a reason worth finding.
• Boost vs boost target - the tune requests a target and the wastegate chases it. The interesting number is never peak boost, it is the gap between the two lines.
• Ignition timing - advance in degrees before top dead center. More advance makes more power until the fuel can't take it. Timing should climb fairly smoothly through a pull; a sawtooth pattern means corrections are happening.
• Timing corrections per cylinder - the knock sensors talking. A value of -2.25 on cylinder 4 means the ECU heard something it didn't like in that hole and pulled 2.25° of advance there. Sporadic small corrections are adaptation; repeated deep ones are knock.
• AFR / lambda - mixture. Lambda 1.00 is chemically perfect, fine at cruise and dangerous at full boost, where you want 0.78-0.85 (about 11.5-12.5 AFR on gas) for cooling and knock margin. Most loggers display gasoline-scale AFR even on ethanol, but some widebands show true-fuel AFR (E85 cruise reads ~9.8 instead of ~14.7). This tool works out which scale your log uses, from the closed-loop cruise sections and the logged ethanol content, and judges everything in lambda so the fuel never skews the verdict.
• IAT - intake air temperature after the intercooler. Cold air is dense air is power. Watch the climb during a pull, not just the starting number.
• Fuel trims (STFT/LTFT) - how much the ECU is correcting fueling versus what the map predicted. Near zero means the tune and reality agree.
• HPFP / LPFP - fuel pressure on the high side (direct injection rail) and low side (in-tank pump). Both should hold steady under load; sag here shows up in logs long before a misfire code does.
• Wastegate duty - how hard the boost control is working. Duty pegged at max while boost falls short is a turbo at its limit or a leak.

Knock and Timing Corrections

The knock sensors are the canary. Small corrections (up to about -1.5°) on one cylinder here and there are normal adaptation, especially on pump gas in summer. Repeated pulls of -3° or more, or several cylinders correcting at once, means the tune is past what the fuel can support. The fix is usually better fuel, less boost, or less timing, in that order of preference.

Boost vs Target

The tune requests a boost target and the wastegate tries to deliver it. Overshoot of more than ~2 psi on tip-in points at wastegate preload or PID tuning. Boost that sags away from target up top usually means the turbo is out of breath, a boost leak, or a clogged filter. Never hitting target at all is classic underboost: check couplers, diverter valves, and the wastegate arm before blaming the tune.

AFR Under Load

Most turbo street cars target lambda 0.78-0.85 (about 11.5-12.5 AFR on gas) at full load. A lean spike past lambda 0.95 under boost is the single scariest thing a log can show, because lean + boost = heat + knock. Common causes: failing LPFP, undersized injectors at high duty, or a tune scaled for different fuel than what is in the tank. The injector calculator will tell you if you are out of headroom.

Heat: IAT and Heat Soak

Intake air temps climbing hard during a pull means the intercooler cannot shed heat fast enough. Modern DI motors quietly pull timing as IAT climbs, so two identical pulls can make very different power. If your IATs start high and climb 25°F+ per pull, an intercooler upgrade buys you real, repeatable power.

Trims and Fuel Pressure

Combined fuel trims beyond ±10% mean the ECU is correcting for something: a vacuum leak, a weak pump, or bad MAF scaling. High-pressure rail dips below ~1500 psi (DI cars) under load mean the HPFP is maxed. These show up in logs long before they show up as a misfire code.

Log Like You Mean It

For a useful WOT log: third gear, from ~2500 RPM to redline, steady road, AC off. One clean pull beats five ragged ones. Log the same gear every time so runs are comparable.