VTEC point tuning

VTEC is one mechanism Honda uses to achieve good emissions, fuel economy and engine power from a small displacement engine. The function of VTEC (variable valve timing and lift electronic control) is to provide two distinct camshaft profiles which are switched electro-hydraulically. The smaller camshaft profile is called the low-speed cam; the larger the high speed cam. The camshaft profiles are switched depending on engine rpm and load, usually from 2500 rpm to 6500 rpm. The main effect on tuning of VTEC is that there are usually two copies of every major table - one for the low speed camshaft, one for the high speed camshaft.

•	Do not set the VTEC point too low as the engine will lose oil pressure and possibly damage the engine. It is not recommended to set the VTEC point below 2000 rpm.

•	Do not set the VTEC point too high as the high speed cam rocker arm may float on the lost motion assembly, damaging the valve spring retainers. It is not recommended to set the VTEC point over 6500 rpm.

The determine the best VTEC point perform two dyno runs, one with VTEC set low (e.g. 3000 rpm) and the other run with VTEC set high (e.g. 6500 rpm). Set the VTEC point to the intersection of the high speed cam and the low speed cam. Generally if there is a sudden increase in engine output immediately after the cams switch then lower VTEC. Conversely if there is a sudden dip in engine output then raise the VTEC point. Since the VTEC point will be at the intersection of the low speed and high speed cam torque curves, it is normal for the torque to dip slightly around the VTEC point.

Generally the VTEC point will be 5500-5700 rpm on a stock engine, 4500-4800 rpm with intake & exhaust, 3000-3200 rpm for supercharged applications. Aftermarket cams generally require a high VTEC point.

The VTEC window uses a combination of rpm and manifold pressure to determine when to switch cams. This is so that the cams will not switch at light or medium throttle - which is undesirable for fuel economy or driveability reasons. See VTEC parameters for more information on the settings.

1.	Determine the optimum VTEC point by performing a dyno run on the low speed cam and then the high speed cam.

2.	Set the VTEC rpm window lower rpm to the VTEC point value.

3.	Set the VTEC rpm window higher rpm to 6000 - 6500 rpm.

4.	Set the VTEC window pressure to 80 kPa up to the lower rpm value. For forced induction set the pressure to 110-120 kPa.

5.	Set the VTEC window pressure to 40 kPa at the higher rpm value.

6.	Smooth the pressure transition between the high and low point. The pressure values are interpolated between rpm points, so it is not necessary to have multiple rpm points between the high and low rpm window values.

1.	As above, but set the lower pressure values to 120-150 kPa at the lower rpm value, decreasing to 70-80 kPa at the higher rpm value.

Normally there will be a small dip (less than 5 lbs torque) at VTEC. If there is a greater dip, try changing the VTEC point up, in 100 rpm increments, to see if the dip is reduced.