Rev limit parameters

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Activation and recovery

Most rev limits have two parameters - an activation rpm, which is the engine speed when the rev limiter operates, and a recovery rpm, which is where normal engine operation resumes. Normally the recovery rpm should be set 100-300 rpm lower than the activation rpm.  A larger different will result in a slower, rougher rev limit.s


Rev limiter type

The ECU uses a fuel cut type of rev limit. The ignition cannot be cut without a fuel cut as this allows the cylinder walls to be washed down by the unburnt fuel. Note that if using a wet type nitrous system with a fuel type rev limit it is important to set the nitrous to be switched off before the rev limiter, otherwise a lean condition will exist when the ECU activates the rev limiter.


Overall limiter


The overall rev limiter is the maximum engine speed the ECU will allow under any conditions.  Note than a lower rev limiter may operate when the engine is cold or when the low speed camshaft is in use.


The overall rev limiter will not protect the engine against a 'mis-shift' overrev as the engine is mechanically coupled to the wheels, so rev limiting the engine will not affect the engine speed.


Overrev rpm

The overrev rpm is the engine speed where the ECU will generate a fault condition.  This should be set about 200 rpm higher than the overall rev limiter.


Launch limiter


The launch limiter is active when the vehicle is not moving for drive-train protection.  The launch limiter acts the fastest when the cut and recover rpm are the same.


Some vehicles have an adjustable launch control.


Speed limiter


The speed limiter activates at the indicated speed.  To disable the speed limiter enter a value larger than the top speed of the vehicle.


Boost Cut


The boost cut will rev limit the engine if the manifold pressure rises above the specified pressure.  This helps to protect the engine from over-boost.  The boost cut is only active over 2000 rpm, to ensure the engine can always be started.


Boost Cut with a stock MAP sensor

Care must be taken when setting a boost cut near the limit of a MAP sensor.  The stock MAP sensor can read to around 1.78 bar (approx 11.5 lbs boost pressure), but to ensure the MAP sensor will generate a voltage high enough to trigger a boost cut it is not recommended to set the boost cut over 1.75 bar (11 lbs) with a stock MAP sensor.


Full Throttle Shift

It is possible to 'flat shift' when changing gear without reducing the throttle pedal.  If the ECU sees that the clutch pedal is depressed and the throttle is wide open (or almost wide open), the ECU will cut the fuel injectors to allow the engine speed to fall for the next gear.  This is designed to work at full throttle and high rpm.


The rpm limit is not critical as the gearshift can be performed quicker than the engine speed can fall.  It is suggested to use a value 2000-3000 rpm lower than the shift rpm.

The minimum throttle prevents the shift limit acting during part throttle / low speed shifts.