Hondata VTEC Crossover and VTC tuning techniques

This tech article details the optimal method of tuning the low to high cam switch point for K series engines. The assumption is that the low and high cam angle maps composite maps are already tuned. (Watch the tuning videos here ). This technique is not used in any Honda maps and was developed over a period of time by Hondata. It has finally been perfected for use in the 06-09 K20 powered Civic.

Theory

The VTEC crossover point is determined by overlaying the low and high cam torque curves. The optimum cam angle for maximum power on the low cam at the VTEC point is often 20 to 35 degrees less than the cam angle for optimum power on the high cam.

In the example below the VTEC point is 4350 RPM. The optimum low cam angle is about 12 degrees. The optimum cam angle for high cam power is 37 degrees.

The problem is that the cam angle does not rotate instantly into the position at the VTEC point. The cam angle rotates at about 10 degrees every 10th of a second. Thus in this example the cam takes about 3/10ths of a second to rotate from 12 to 37 degrees. While the cam is rotating into position the engine is not making optimal power, often resulting in a power dip after VTEC.

The technique Hondata has developed is three-fold:

•  Advance the cam into to match the high cam VTC angle before the VTEC switch point.
•  Alter the RPM indices to make this happen as quickly as possible.
•  Keep the cam angle constant across the high cam RPM rows.

This results in a small power dip on the low cam as it is rotated into position but more power on the high cam after the VTEC point.

How to optimize the VTC tables

This is a stock, tuned, composite cam angle map for the low and high cam VTC.

How to optimize the VTC tables

With the VTEC point at 4350 RPM we do not need the RPM resolution in the 5000-7000 area.

Right click the RPM row to edit the RPM indices.

• Make one RPM index 150 RPM less than the VTEC point
• Make one RPM index 50 RPM less than the VTEC point
• Make one RPM index 50 RPM more than the VTEC point

In this example for a VTEC point of 4350 RPM the RPM indices are set to:

• 4200 rpm
• 4300 rpm
• 4400 rpm

The highest RPM break point is reduced from 8100 RPM to 7000 RPM.

Set the cam angle at the higher of the two RPM rows to the high cam angle at the same RPM value - in this case 37 degrees.

This has the effect of keeping the low cam at its optimum cam angle until 4200 rpm - 150 RPM before the VTEC switch point. The intake cam is then rotated rapidly over a small time/RPM  interval to the correct angle for the highcam (in this case 37 degrees)

VTEC crossover datalog

This datalog shows a VTC transition from 26 degrees low cam to 50 degrees high cam in about 3/10ths of a second. CAM CMD is the value from tables. CAM is the measured value.

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High Cam VTC optimization

With a VTEC point set to 4350 RPM there is no need to have VTC RPM tuning rows under 4000 RPM. You should never be on the high cam at these points. These RPM rows can be deleted and more resolution can be added in the area where the camshaft is rotating rapidly which is typically in the 6500-10,000 rpm range.

In addition we keep the cam angle advanced across the RPM rows. This stops the cam angle from retarding to zero through the gearshifts and keeps the car on power longer. Compare a  stock highcam VTC map to a Hondata optimized one.

Hondata optimized VTC map