Ignition timing basics: spark tables, knock and safe advance

What ignition timing does

The mixture takes time to burn, so the spark fires before the piston reaches top dead centre (TDC). The aim is for cylinder pressure to peak shortly after TDC, where it pushes the piston down hardest. Advance is measured in crank degrees before TDC.

• Too little advance: pressure peaks late, power drops and exhaust temperature rises.
• Too much advance: pressure peaks early and can trigger knock, where the last of the mixture ignites on its own. Heavy or repeated knock damages pistons, ring lands and head gaskets.
• MBT (minimum advance for best torque) is the point where adding more timing stops adding torque. On pump fuel, many engines knock before they reach it.

Why advance changes across the table

• RPM: at higher RPM there is less time per degree, so the engine wants more advance, up to a point.
• Load: a denser charge burns faster, so light load and cruise take the most advance and full load takes less.
• Boost: more pressure and heat raise knock risk, so timing comes out as boost rises.
• Torque peak: the cylinders fill best there and the burn is fastest, so many engines want a dip in timing at the VE peak.
• Idle: usually a moderate, steady value, so idle speed control has room to work.

How common ECUs handle timing

Speeduino

A 16×16 spark table in degrees, RPM against load. A cranking advance angle is used while cranking, and a fixed timing setting locks timing so you can check it with a timing light. Knock input comes from an external knock controller, as a digital or analog signal.

rusEFI and epicEFI

A 16×16 ignition table in degrees, a cranking timing angle, and a fixed timing mode for checking with a timing light. Boards with knock sensing can retard timing on knock, with a setting for how much to remove.

MegaSquirt MS3

16×16 advance tables in degrees before TDC, with fixed timing and cranking advance settings. The INI's own help says to verify timing before starting.

TunerStudio

TunerStudio shows the spark table, cranking and fixed timing settings as the INI declares them, and you set the values yourself.

A safe order for tuning timing

1. Check base timing first. Lock the timing with the fixed timing setting, compare it with a timing light, and correct the trigger offset until they match.
2. Start from a conservative table for your fuel and compression.
3. Get the fuelling right before adding timing, especially at full load. A lean mixture knocks sooner.
4. Log RPM, MAP, advance, knock, AFR, air and coolant temperature on every pull.
5. Add timing in small steps, one area at a time. Stop at the first sign of knock or when torque stops rising, and back off.
6. On the road, rely on knock detection, not your ears. Light knock is rarely audible over road noise.
7. Keep cruise timing for economy and full-load timing for safety. They are tuned separately.

Using efi2000 for ignition timing

Generated spark table

Generate tables builds a starting spark table from your engine and the fuel you run, with more advance at light load and less as boost rises. It's a sensible first table to confirm with knock detection. Cranking advance is its own setting on most firmware, so check it before the first start.

Checks

Set your fuel grade and how conservative you want to be in Project → Recommendations, and Spotter reviews the spark table against your engine and fuel. It flags things like:

• Too much timing at full load for your fuel.
• Timing that rises when load rises.
• More timing at boost the turbo can't reach than at boost it can.
• Idle timing outside the usual window.

While you drive, autotune tells you to lift off on knock. Log coverage on the spark table shows which cells your logs actually visited.

Questions

How much timing should my engine run?

It depends on the combustion chamber, compression, fuel, boost and the table's load and RPM. Start conservative for your fuel, check base timing with a light, and add timing in small steps with knock detection.

Why does timing come out at the torque peak?

The cylinders fill best there and the mixture burns fastest, so the engine needs less advance to reach peak pressure at the right point and is closest to knock.

Can I tune timing without a dyno?

You can make it safe without one, using knock detection and conservative values. Finding MBT needs a dyno or very careful, repeatable logging.

Does efi2000 check my spark table against my fuel?

Yes. Spotter checks full-load timing against your engine and the fuel grade you set in Project → Recommendations, for gasoline, E85 and methanol.