All JDM Honda models have an artificial 180kph top-speed limit. This limit, existing even on the NSX Type-S, is implemented by Honda via the engine ECU. The following explanation of how the 180kph limit is implemented is by kind courtesy of Rennie.
The Vehicle Speed Sensor (VSS) is a device attached to the transmission that measures the car's speed via tracking the number of revolutions of the wheels. The output of a VSS is in either sine wave, square wave or in the form of pulse counts. When the engine ECU detects the car is travelling at 180kph via the VSS signal, it reacts in one of two ways. It will either stop 2 or more injectors or it can cut the ignition (probably intermittently). This reduces engine power output drastically, slowing down the car until its speed drops below 180kph.
According to Rennie, in JDM cars, cutting off the VSS signal to the engine ECU can lead to one of three conditions. One is the ECU will just ignore the absence of the VSS signal and continue operating as normal. In another, the engine simply won't start or will die when gear is engaged. The final condition is the ECU program, because it now cannot determine how fast the car is travelling, goes into a backup safe mode in which it runs the engine rich. This unnatural rich mode is undesirable and will actually lead to a slight loss in engine power as well as oil dilution (contamination of engine oil with petrol).
In Hondas, loss of the VSS signal will cause the ECU program to go into the backup safe mode. In addition, it will invoke a trouble code of 17 (for EF8/EF9 CRX/Civic SiR) indicating 'faulty vehicle speed sensor'. In the HPL archives, Doug MacMillion said that cutting off the VSS signal will also stop the ECU from engaging the high cam lobes for the B16A.
The HKS Speed Limit Defencer (SLD) device works by intercepting the VSS signal and substituting its own signal to the ECU. This device has four wires, two of them for power and ground and of the other two, one receives the VSS signal and the other feeds a substitute signal into the ECU.
When the SLD device detects that the VSS signal is indicating that the car is travelling at 180kph, it substitutes a signal to the ECU indicating a lower speed. Because the VSS signal is not a major parameter used by the engine ECU to control engine operation, this method effectively bypasses the artificial 180kph top speed limit in the best manner.
©1998 Temple of VTEC
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