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iVTEC ('intelligent' VTEC) is a combination of VTEC and VTC. VTC (variable timing control) is another mechanism used by Honda to increase engine output which decreasing emissions and fuel consumption. VTC controls the intake camshaft advance. Unlike VTEC VTC is not a simple on/off control, rather the ECU controls the intake camshaft advance over a range of 50 crank degrees. The effect on tuning of VTC is that there are 5 copies of each major table - for cam advance 0, 15, 30, 40 and 50 degrees. In effect this makes the major tables three dimensional.
The cam angle is the intake cam advance measured in crank degrees. The allowable cam angle range is from 0 to 45 or 0 to 50 degrees, depending on the calibration type.
Note that there is a mechanical limitation in the VTC mechanism. If you increase the values in the VTC tables but do not see further cam advance you may have reached the mechanical limitation.
The intake cam is positioned by an electro-hydraulic mechanism, which uses feedback from the intake cam position to alter the position of a solenoid which in turn rotates the intake cam inside the cam sprocket. Because of the design of the mechanism there is a delay between setting the cam position in the ECU, and the cam physically rotating to this position. This delay is around 0.1 seconds per 10 degrees of rotation.
With Honda cams there is a physical stop limiting cam advance to prevent valve to valve contact and valve to piston contact. With after market cams it is up to the manufacturer to ensure that the cam lobes are positioned so that valve to valve and valve to piston contact is not possible. Because the cam control mechanism uses a closed-loop feedback system, limiting the cam position in the ECU will not guarantee that the cam position will not exceed what is set in the ECU. Because of this all cams must have a physical stop to prevent valve contact.
In short, the better the breathing of the engine; intake, cams and exhaust, the greater the cam advance needed. There is no situation in which best overall performance is achieved by fixing the cam angle to just one setting or using manual cam adjustment wheels for the intake cam. There may be benefits to fitting and adjusting the exhaust camshaft angle, which is not under computer control.
Cam Angle at VTEC
If the cam position tables require the camshaft to rotate a large amount at VTEC (e.g. from 20 degrees on the low speed cam angle table to 45 degrees on the high speed cam angle table) you may lose power for 500-700 rpm after VTEC activates, as the cam rotates into the correct position. The best method is to start advancing the intake cam in the low speed cam angle table before the VTEC point, so the cam has to rotate less once VTEC activates. This usually means sacrificing a few hp just before VTEC point to gain hp after the cams switch. When this is done right the characteristic VTEC change in sound is greatly reduced. For more information see VTEC crossover tuning
Part Throttle Cam Angle
This applies to the portions of the cam angle table below full naturally aspirates load (column 7 and less).
It is normal for the ECU to briefly fully advance the cam on overrun in order to clean the VTC mechanism. It will do this a few times from a cold start approximately 5 seconds after the throttle is released when the engine speed is 2000-3000 rpm. These cleaning actions show in in datalog as small spikes to 45 or 50 degrees.