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Ignition timing for performance

Revision 2

by llandowtuninguk@gmail.com
Link the knock and octane guides from the ignition timing page.

This is a historical revision. Read the current page or return to its history.

Page organisation in this revision
Page type
Technical article
Subject
Performance
Parent
Section level
Applies to
No ECU-specific applicability
Tool groups
None
Manufacturers
None

Two timings

There is a spark timing that would make the most torque from a given mixture, often called minimum advance for best torque, and there is the earliest timing the engine can run without knock. On many production engines the second limit arrives first, especially once boost or inlet temperature has risen. Advancing the spark toward the best-torque value helps only while the engine is not knocking and the exhaust temperature stays acceptable. Advancing it past knock costs power and can damage a piston.

What that knock sounds like, how to hear it, and the fuel rating that resists it, are covered in engine knock and fuel octane.

The ME7 family keeps those pieces separate in the calibration: a base timing, a faster timing that would be preferred, temperature corrections, and knock control that retards the spark when the sensor detects it. That layout is described in ME7 ignition and knock. Other ECUs split the same job differently. A single "ignition map" on a printout is not evidence that knock control is absent.

What uses up the knock margin

  • Inlet temperature. A hotter charge raises the temperature at the end of compression. BMW's account of water injection is built on this: cooling the plenum air is what lets them advance the spark and raise boost. See charge cooling.
  • Compression and boost. Geometric compression ratio and compressor pressure both raise the starting point for that temperature. A timing table from a low-compression naturally aspirated engine is not a starting point for a boosted one.
  • Mixture. A mixture that is lean of the full-load target, or a fuel of lower knock resistance than the table assumes, removes margin. The fuel hardware is in fuel system upgrades.
  • Hot spots. A glowing plug range that is too hot, a sharp edge in the chamber, or carbon, can ignite the mixture without the spark. That is not cured by adding more advance.

Exhaust temperature

Retarding the spark to avoid knock moves more of the combustion into the exhaust stroke. Exhaust temperature rises, which loads the exhaust valve, the turbine and the catalyst. A calibration that shows no knock and a climbing exhaust temperature is not finished. On a turbocharged engine that heat is also energy the turbine can use, so the boost, the wastegate and the spark are one system. ME7 torque, load and boost is the family-specific version of that link.

Diesel

A diesel does not have a spark. The equivalent timing is the start of injection or the start of delivery, and it affects noise, efficiency, cylinder pressure and emissions together. Bosch describes start-of-injection targets that depend on speed and fuel quantity, with temperature corrections. The EDC15 page summarises that and warns that pump, unit-injector and common-rail systems do not time the fuel in the same way: EDC15 fuel control. Do not transfer a petrol spark correction into a diesel injection map.

Sources