VTC tuning

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.

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With a naturally aspirated engine the cam advance should be set to maximum just after VTEC engagement until about 6500-7000 rpm. From 7000 rpm (where the cam advance should be near 50 degrees) to redline the cam is retarded back around 25 degrees. This procedure is correct for all commercially available after market cams at the date of release of this software, but camshafts which are substantially different from a Honda camshaft may require different settings.

•	With a supercharged engine the cam advance needs to be set to maximum (50 degrees) throughout the rev range, with only a 10 degrees or so retard above 7000 rpm.

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With a turbocharged engine the cam advance generally needs to be less than stock. This is because a turbocharger generates much more exhaust back pressure than a naturally aspirated or supercharged configuration. The higher the back pressure the more cam retard is needed. With small turbos and stock catalytic converters you may need to retard the cam fully to 0 degrees at 8000 rpm. A log style turbo manifold results in a high VTEC point and low VTC angles - a manifold with longer, equal length, runners requires a lower VTEC point and higher VTC angles - closer to that which a naturally aspirate engine would require.

•	Perform dyno runs at each cam angle. This will give you with 6 dyno curves with different cam angles. Set the cam angles in the cam angle map to those which give you maximum power then re-dyno. The power curve you get should be a maximum of all the 6 individual dyno runs you have just done.

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Now we need to "bracket" the composite cam angle map we have just created. Add 5 degrees to the cam angle map and re-dyno. Subtract 5 degrees and re-dyno. This will verify you have an optimum cam angle map. You will probably find a few RPM points, particularly where the cam is changing angle, that need a little modification to make more power. If you wish you can then bracket the resultant power curve by dynoing with plus or minus 2 degrees cam angle change. The power change at this degree of cam angle change is likely to be about 1 – 1.5 hp on a naturally aspirated engine.

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

This applies to the portions of the cam angle table below full naturally aspirates load (column 7 and less).

•	Above normal cruise rpm set the cam angle to the same value as under full load. This will smooth out gearshifts as the cam shaft will not start to rotate back to zero during the gearshift.

•	Remember that the cam cannot rotate instantaneously. It takes about 0.1 seconds to rotate 10 degrees. Cam angle changes should not be great over a small rpm interval.

•	The intake cam is locked at 0 degrees for 10 seconds after a hot start. Let the engine run for at least 10 seconds after starting the engine before performing a dyno run.

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.