VTEC engagement depends upon:
rpm
engine temp (temp sensor)
throttle position (throttle position sensor)
oil pressure
vehicle speed (speed sensor).
there are several trigger points or thresholds that have to be met for each of these (not just rpm) before the VTEC solenoid will activate and lock the pin across the 3 rockers. It usually varies by 200 rpm around the main VTEC switchover (4200-4600 rpm for a main GSR switchover of 4400 rpm depending on engine loading conditions) not by 500 rpm. Is your distributor going (rotor or bearing)? The tach is usually off by as much as 600 rpm from the true rpm at the distributor on old cars. When we dyno some older cars we see this difference from the dyno pickup for rpm compared to the tach.
in the
dyno tuning basics article, we show you one way to find the best VTEC engagement point for your particular engine and the unique way that it breaths (IM, head, cams, header with no backpressure to work against or not).
another way is to run on the VTEC lobe all the time (ver very early VTEC engagement set by reprogramming or a piggyback VTEC controller) and race N/A engines or high performance street engines with porting and big CR sometimes use this approach but it requires a complete ECU reprogram of the fuel tuning for the car to be driveable on the street and idle properly. You can't do this on a stock ECU with a piggyback VTEC/fuelcontroller because it just doesn't have enough tuning points to adjust the part throttle and full throttle fuel tables and their programs. Plus you need to redo the ignition tables.