Turbo Miata Losing Power and Popping at WOT? How to Diagnose Ignition Timing Pull on a MegaSquirt Setup
What’s Actually Happening When the ECU Pulls Timing
When a turbocharged Miata suddenly feels sluggish, starts popping at WOT, and sounds rough instead of pulling hard, the ECU is almost certainly retarding ignition timing aggressively. On a MegaSquirt-based setup like the ME442 PnP, the system can pull multiple degrees in milliseconds. When it does, power collapses fast.
Timing advance — degrees before top dead centre — determines when combustion pressure peaks relative to piston travel. Pull it back too far and combustion happens late. The piston is already moving down before peak pressure arrives, efficiency tanks, and hot gases are still burning as the exhaust valve opens. That is your pops and that tractor grumble.
The Main Suspect: Knock Control
MegaSquirt’s knock control watches a piezoelectric sensor on the engine block. When it detects vibrations that match the frequency signature of detonation, it pulls timing. This is protective and intentional. The problem is the sensor can also pick up valve train noise, injector tick, or general mechanical clatter from a modified engine and treat it as knock. False triggers are common.
When knock retard kicks in, the results are dramatic. Timing can drop sharply in a fraction of a second. Under boost, that means no power and exactly the rough, popping sensation the original poster described.
Reading the Log: Columns That Matter
Open the CSV in MegaLogViewer or a spreadsheet and look at these columns during your wide-open-throttle pull:
- SPK: Knock retard — degrees pulled by the knock control system. Any non-zero value during a WOT run means knock control is active and taking timing away.
- Knock in — the raw sensor reading. Real detonation typically registers 3–5 times above the engine’s normal noise floor. Marginal spikes that barely clear baseline are more likely false triggers than actual knock events.
- Spark advance or Ign Adv — compare this against what your ignition table should be commanding at the same MAP and RPM cell. The difference shows exactly how much timing is being removed.
If SPK: Knock retard is climbing during your pull, that is your answer. What you do not yet know is whether the ECU is responding to real detonation or to sensor noise.
Real Knock vs. False Triggers
Check AFR or lambda during the same log run. On a turbocharged engine, running lean under boost is a direct path to real knock. Lean mixture burns hotter and faster, and the end-gas conditions that cause detonation get worse as the fuel-air ratio climbs. If AFR is off during the pull, fix fueling first before touching anything else.
If AFR looks correct and the car is running a healthy rich mixture under boost, false triggering becomes more likely. The fix is recalibrating the knock threshold — you establish a noise baseline using conservatively retarded timing, then advance slowly until actual detonation appears. That process belongs in a proper tune session with a knowledgeable tuner, not on the road.
Other Corrections That Can Pull Timing on the ME442
Knock retard is the most dramatic, but three other corrections can stack up quietly:
- IAT correction — MegaSquirt applies a spark retard correction table based on intake air temperature. If your intercooler is undersized or you are doing multiple back-to-back pulls on a hot day, this table alone can take several degrees off the ignition advance.
- CLT correction — coolant temperature correction. Small in most tunes, but worth checking if the engine was running hot when you logged.
- Overboost protection — some tune maps apply a timing cut or retard as a secondary safety measure when boost exceeds a threshold, separate from the wastegate control. Check whether yours has this configured in TunerStudio.
The Rough Tractor Sound at Other Throttle Positions
If the engine sounds rough not just at WOT but at cruise and part-throttle too, that is a different layer of the problem. Check ignition table values at light-load cells — overly conservative numbers anywhere across the map make the engine feel harsh and unresponsive. Short, mis-timed combustion events at low load produce exactly the lumpy, uneven character the OP described.
Also check spark plug gap and condition. Turbocharged applications generally run a tighter plug gap than stock because the compressed, denser charge is harder for the spark to bridge. A worn plug or a gap that is too wide misfires under boost, and misfires show up in the log as AFR spikes — unburnt fuel passing the oxygen sensor reads lean even when the mixture is actually rich.
Where to Go From Here
Post the log back to the thread with the SPK: Knock retard and Knock in columns visible. If knock retard is spiking during the WOT run, the path splits into two clearly defined problems: real detonation caused by fuelling or boost pressure, or false triggering caused by sensor threshold calibration. Both are solvable. Neither requires guessing.
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