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First, a quick primer on how the factory disc is configured:
The factory disc has two slots for the cam sync ("cam teeth") and four to indicate crankshaft position ("crank teeth"). As the disc rotates, the slots pass between an LED and a photo diode, which detects whether or not there is an opening. Specifically, the sensor detects the edge which signifies the beginning of a slot (rising edge) and that which defines the end of a slot (falling edge). The sensor is a logic type and outputs a square wave signal (either on or off).
The longer of the two cam teeth in the picture above represents TDC for #1, or the point in cylinder #1 at which all of the valves are closed and the piston is at the top of the cylinder. The ECU looks for this cam tooth to determine when to ignite the fuel mixture in the cylinder. It also uses this point to determine when to fire the fuel injectors. In a stock configuration with the factory ECU, the cam teeth are at fixed, known locations with respect to the cam because they are keyed to the cam. The stock ECU knows this information and uses it to make decisions as it sees the cam teeth go by. With an AEM EMS, the cam tooth can be located anywhere on the disc, as the EMS compensates for differing locations via the Ignition Sync parameter. This value is set when the operator, with the help of a timing light, adjusts the ignition timing via the Set Ignition function under ECU Setup. The second, shorter cam tooth on the factory disc pictured above indicates TDC for cylinder #4. The factory ECU uses the two different length slots to differentiate between TDC for #1 and #4, and to calculate fuel injector timing.
The four crank teeth, relative to the cam teeth, describe to the ECU where the engine is in its rotation at a given moment. Note here that the ECU uses time-based calculations from the last tooth seen to "guess" where the crank is in its rotation. Although there are four crank teeth in the factory disc, because it is driven by the camshaft it is only rotating at half the true engine speed. This reduces the number of effective crank teeth from 4 to 2. More on this later.
The AEM EMS functions differently from the factory ECU in that, while cranking, it needs to see a certain number of crank teeth in order for the injection and ignition events to be syncronized (this syncronization is evident when the Stat Sync'd parameter shows ON in the EMS; the engine starts almost immediately thereafter). Because the factory disc offers so few effective crank teeth, the EMS waits until it sees enough teeth to sync. The result is the notorious slow engine starting characteristic of many EMS-equipped Mitsubishis. Another drawback of having only two effective crank teeth is degraded engine timing resolution at high RPM.
Because AEM preaches the plug 'n play approach to its EMS, it offers a few software work-arounds for the Mitsubishi. Although the need for a custom trigger disc is mitigated by the software approach, it is nonetheless a compromise. The EMS is designed to function the easiest with 12 crank teeth and 1 cam tooth (using the Sync Cam Count strategy). For cars with greater or fewer than these, certain tactics are employed to compensate for the lack, or abundance, of signal. Since the Mitsubishi has far fewer than 12 crank teeth, the EMS counts the available teeth multiple times to satisfy the needs of its internal buffer, and discards half of the readings. This helps it to reach the Stat Sync'd condition more quickly. Additionally, the EMS has code that allows it to ignore the second cam tooth on the factory disc, since it only needs one.
Fortunately, the way that the signal from the cam angle sensor is interpreted by the EMS is completely configurable via the Advanced Cam/Crank Pickup settings. All that's needed is a trigger disc that will give the EMS what it wants.
Our DSM trigger disc eliminates the second cam tooth and provides 24 crank teeth, which equates to 12 effective crank teeth due to the disc rotating at half engine speed. With the proper tweaks to the Advanced Pickup settings, injector phasing and coil dwell, the engine will function just as it did with the factory disc. Improvements will be observed in engine starting, ignition timing drift, and ignition timing resolution.
The EVO (and DSM 7-bolt) crank trigger suffers the same shortfalls as the DSM cam trigger. Although the EVO is different in that it has separate sensors for the cam and crank signals, it is the same in that there are two cam teeth (long and short) and two effective crank teeth. Since the trigger containing the crank teeth is actually driven by the crank instead of the cam, only two physical teeth are needed, rather than four, to end up with the two effective teeth. The end result is the same number of cam and crank teeth as is seen on the DSM. This is why the stock syncing strategy is the same between the 1G DSM, 2G DSM, and EVO.
Another major difference with the EVO is that both sets of cam and crank teeth are detected by hall effect sensors, rather than an optical sensor with which the early DSMs were equipped. The hall effect sensors are still logic type and output a square wave, but are generally more accurate and reliable than their optical counterparts.
The same engine cranking and tuning obstacles apply to an EMS-equipped EVO. This time, a custom trigger disc with 12 physical crank teeth takes the place of the factory disc (pictured above). The cam angle sensor is then modified by cutting off the longer of the two cam teeth, leaving only the short one. The result for the EMS will be the ideal 12 crank teeth and 1 cam tooth. Tweak some settings as mentioned above and that's all there is to it.
The AEM EMS forums are a great resource, but it can sometimes be difficult to find information on obscure topics there. To alleviate this somewhat, we have put together the following document which consolidates much of the useful information scattered around the forums, particularly about cam/crank signal interpretation. The document is continually being updated, so check back periodically for newer versions.