Ignition Advance - Mapping your Distributor-less Ignition System (DIS)

The benefits of mappable, distributor-less ignition systems are well known, car manufacturers today utilise them for greater economy gains and others are now using these systems for greater performance. How do we map / adjust these ignition systems though?

Most car manufacturers make their ignition maps inaccessible for the modern car enthusiast so if any modifications are made to the modern car, the ignition cannot be altered to suit. Unfortunately this often results in a tuned engine that will ignite too early and therefore knock, leading to much poorer performance than expected and/or engine destruction. With these modern, distributor-less engines, a fully mappable engine management system is absolutely essential if any further modifications are going to be made.

With these mappable, distributor-less ignition systems, ignition advance is mapped against both engine load and engine rpm. For the engine load either MAP (Mass Air Pressure) or Throttle Position is measured, using a MAP sensor or TPS (Throttle Position Sensor). The engine rpm is usually measured using a VR sensor and a trigger wheel on the crank or camshaft. 

At this stage it is worth noting that if you are looking to get the final percentage of  performance out of your mappable ignition system, a rolling road (a.k.a. dyno) is  essential. With this equipment you can obtain the highest advance possible before power is lost through too much advance, whereas on the road it is easy to provide too much advance. It may not be enough for it to audibly knock but it will lose you both performance and reliability. The loss in performance will not be evident on the road but obvious on the rolling road.

At Lloyd Specialist Developments we are often required to provide base maps with our engine management kits or ECU’s. The ignition maps are always set on the retarded side at high load as it is virtually impossible to safely provide an optimised ignition map, even if we have already previously optimised an ignition map for the same type of engine! So what exactly affects how much ignition advance we can use? 

1. Engine Load / Vehicle Weight – The amount of load that the engine is placed under is greatly affected by the weight of the vehicle, rolling resistance and aerodynamic drag. You will notice that an engine is most likely to suffer from knock when going uphill at full throttle in a high gear, this is the highest loading condition that the engine is likely to encounter. We have rolling road mapped a 4-litre Rover V8 in a TVR with an ignition advance of 31 degrees under full load at 2000rpm - producing 250 lb-ft, seemingly ideal for a big heavy 4x4 such as a Land-Rover. In reality a 4-litre Rover V8 in a Land-Rover or Range-Rover will only tolerate an ignition advance of 24 degrees under full load at 2000rpm, producing 220 lb-ft. So, if you put a lot of weight in a vehicle optimised with no weight in it, it may knock under full load!
2. Engine Size / State of Tune – A bigger engine will generally tolerate more ignition advance than a smaller engine. However, an engine that breathes more air will actually tolerate less ignition advance than an engine that breathes less well. This also holds true for changing atmospheric conditions. It is often thought that if an engine is exactly the same size and type, it must require the same ignition advance. Although generally true, it is not always the case. As any serious engine builder/tuner knows, sometimes one engine that comes out of the factory just seems to perform better than the other. Car manufacturers will always develop the ignition map to suit the engine that will tolerate the least ignition advance. 
3. Fuel Type & Grade – The higher the octane of the fuel, the more ignition advance the engine will tolerate. Some car manufacturers, such as Saab, cleverly utilise knock sensors to allow the engine management to advance or retard the timing dependent on the conditions seen by the engine. When filling up a Saab 9000 at the fuel station it has been noted that using 98 octane fuel actually increases both performance and/or fuel economy slightly. Many aftermarket engine management systems have the facility to change both fuel and ignition maps at the flick of a switch. This allows the user to change from a map optimised for one fuel to a map optimised for another. This is particularly useful for alternative fuels such as LPG or bio-ethanol, both of which tolerate much more ignition advance than the poor quality petrol available at the pump today! LPG use is becoming more widespread but LPG still has a poor reputation for performance in many circles, this is mainly because many LPG systems simply ‘piggy-back’ off the existing engine management, merely advancing the petrol ignition timing by a small amount everywhere.

As can be seen, ignition advance is affected by many variables and these variables change. Car manufacturers developing an ignition map for a particular model of car need to allow for the worst combined case of all these variables, therefore they will generally not be optimised for performance. A fully mappable ignition system allows us to optimise an engine for both performance and part throttle economy, specific to the driver’s requirements. It also provides us with the flexibility to change the ignition timing to suit practically all engine modifications and alternative fuels.