Key takeaways
A log turns an opinion about how a car feels into evidence you can compare. Record a core set of engine and vehicle channels at a sensible rate, look for trends as well as peaks, and repeat the same test routine each time so that differences in the data mean something.
Most problems with a tuned car do not show up as a single dramatic event. They appear as a lean patch at one load, a boost figure that drifts upward, or intake temperatures that climb session after session. You can feel some of that, but you cannot reliably measure it by feel. Data logging gives you the evidence, whether the aim is a safe tune on the road or consistent laps on a track day.
The channels that matter#
Which channels are available depends on the ECU and sensors fitted, but a useful core set is similar for most petrol engines. Engine speed and throttle position tell you the operating point. Manifold pressure (MAP, or boost on a turbocharged car) tells you how much air the engine is being asked to use. Lambda, a normalised measure of air-fuel ratio, shows whether the mixture is right for that load. Ignition advance and any knock indication show how close the engine is to its limit. Temperatures and oil pressure show whether the engine is coping.
| Channel | What it tells you | What to watch for |
|---|---|---|
| Engine speed (rpm) | The operating point; the x-axis of most analysis | Sudden dips or limiter hits |
| Throttle position | Driver demand; separates part-throttle from full load | Inconsistent pedal use between runs |
| MAP / boost | Air load on the engine | Overshoot, creep or drop-off at high rpm |
| Lambda | Mixture quality for the conditions | Lean readings under load, unstable readings |
| Ignition advance | Timing the ECU is applying | Timing pulled back as temperatures rise |
| Knock indication | Whether the engine is detecting abnormal combustion | Repeated activity at the same load and rpm |
| Coolant temperature | Cooling system health | Steady climb during a session |
| Intake air temperature | Charge temperature and intercooler effectiveness | Rise from run to run (heat soak) |
| Oil pressure | Lubrication system health | Falling pressure in long, hard corners or when hot |
| Vehicle speed | Gear and acceleration analysis | Mismatch with rpm, suggesting clutch slip |
| Lateral G | Cornering load and driver consistency | Variation in peak G for the same corner |
Sample rate: fast enough to see the event#
A log is a series of snapshots, and an event shorter than the gap between snapshots can be missed completely. As a rough guide, slow-changing channels such as coolant temperature are usually fine at a few samples per second, while engine speed, throttle, boost, lambda and ignition are commonly logged at somewhere between 10 and 50 samples per second or higher, depending on what the ECU and logger support. Knock-related and fast pressure events can need faster capture than that. More is not always better, as very high rates create large files and can be limited by the bus or the logger, so choose the rate for the fastest thing you need to see.
- Approximate stoichiometric air-fuel ratio for petrol
- 14.7:1
- Lambda at stoichiometric mixture, for any fuel
- 1.00
- Approximate stoichiometric air-fuel ratio for pure ethanol (E85 is about 9.8:1)
- ≈9.0:1
Lambda is useful because it is the same scale for any fuel: 1.00 is the chemically balanced mixture, values below 1.00 are rich and values above are lean. Target lambda under full load is set by the tuner for the engine, fuel and boost level, and is typically richer than stoichiometric on a boosted engine.
How to read a log for problems#
Lean spikes#
Plot lambda against engine speed for a full-throttle pull. A line that sits steadily at the target and then jumps leaner at a particular rpm, or a gradual drift upward as load builds, points to a fuelling limit: an injector, pump or regulator at the edge of capacity, or a map that does not match the engine. Lean mixtures under high load are dangerous because they raise combustion temperatures. If you see them, stop and investigate before driving again.
Boost creep#
On a turbocharged car, boost should follow the target curve. If measured boost climbs past the target as rpm rises, or is higher than the control system expected, the wastegate may not be able to bypass enough exhaust flow, or the control solenoid or its plumbing may be at fault. Boost creep matters because the extra pressure raises cylinder loads and knock risk beyond what the tune allows for.
Heat soak#
Compare intake air temperature and ignition advance across consecutive runs. If the intake temperature starts a little higher each time and the ECU progressively reduces timing or boost, the intercooler and bay are heat soaking. The car feels slower late in a session even though nothing has broken. Cooling time between runs, airflow and ducting are the usual remedies.
Warning
Do not log your way into trouble
Logging is a diagnostic aid, not a substitute for a safe calibration. Hard testing on public roads is dangerous and may be illegal. Use a dyno or private venue for high-load testing, and have changes to fuel or ignition maps made by a competent calibrator.
Building a repeatable routine#
- Record the conditions: date, ambient temperature, fuel, tyre pressures and any changes since the last session.
- Warm the engine, gearbox and tyres to normal operating temperature before any load test.
- Use the same test: for example, a fixed gear, a defined throttle ramp from a set rpm, repeated the same way each time.
- Save each log with a clear name that includes the date and the changes made.
- Compare against the previous log at the same engine speed and load, not against memory.
- Make one change at a time and note it.
What this means in practice#
Good logging is mostly discipline. Choose a core set of channels, sample them fast enough, look for trends, and test the same way each time. The same habits apply to everything from ECU remapping to chassis changes. The log will not tell you what to do, but it will tell you honestly whether what you did worked.
