Engine Speed (RPM) Calculation
Overview
Once the synchronization strategy is configured and the ECU is able to synchronize with the engine, the engine speed (RPM) can be calculated.
Engine Speed Calculation
The 
Engine Speed channel is calculated using the time intervals of crankshaft position sensor readings.
A value must be specified in the 
Speed mul option otherwise Engine Speed will read zero.
The Wheel Teeth option defines how many internal teeth time intervals are summed to produce the Rev Time MSB and Rev Time channels. Depending upon whether Wheel Teeth is equal to the number of internal teeth per revolution determines whether or not Rev Time is really the total time for one revolution of the crankshaft (it may not be).
If Wheel Teeth was 2x the number of internal teeth per revolution then the engine speed would be summed over 2 revolutions of the crankshaft.
If the Speed cycle/Rev option is ON then the speed is derived from 150ms/“Rev Time” instead of 300ms/“Rev Time”, which can act as a tradeoff between maximum engine speed and precision.
The Speed mul option is used to scale the result to a correct RPM value, depending upon the combination of the settings.
Valid settings, e.g. for a calibration with 12 internal teeth:
| Wheel Teeth |
Speed mul |
Speed Cycle/rev |
Max Engine Speed |
|---|---|---|---|
| 12 (1 rev) | 50% | OFF | 12799 rpm |
| 12 (1 rev) | 100% | ON | 25599 rpm |
| 24 (2 revs) | 100% | OFF | 25599 rpm |
The Pickup comp option compensates for fixed timing drift. The ECU’s calculated RPM value can be compared to that measured using a calibrated tachometer to determine the correct value for this option.
When Starting / Cranking
When the engine is starting, the ECU will be in an unsynchronized state and therefore can’t identify the internal teeth, so a different strategy is required to calculate the engine speed.
This strategy is a lot less accurate - actual crank teeth may be irregularly spaced due to missing / extra teeth.
If cranking, a number given by the Start Actual Teeth (7 maximum) of real crank teeth times are accumulated, and left-shifted (i.e. doubled) Start Actual Teeth <2 times, to give Rev Time until the T1 Count Sync’d channel exceeds the Start Crank Teeth option.
The T1 Count Sync'd channel is the number of actual crank teeth seen since engine synchronization was achieved and counts up until it reaches 255, at which point it stops counting. This channel can be useful to monitor short-term loss of synchronization; it is reset to zero if synchronization is lost.