This outlines 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. 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 at the VTEC point is about 12 degrees. The optimum high cam angle at the VTEC point 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.
VTC tables
These are tuned composite cam angle maps for the low and high cam VTC on a stock engine.
Low cam VTC table
High cam VTC table
Low cam VTC optimization
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 |
Increasing table resolution around 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.
Re-indexed low cam VTC table
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.
Optimized low cam VTC table
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 high cam (in this case 37 degrees)
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 high cam VTC map to a Hondata optimized one.
Stock high cam VTC table
Hondata optimized high cam VTC table
VTEC crossover datalog
This datalog shows a VTC transition from 26 degrees low cam to 50 degrees high cam in about 0.3 seconds. CAM CMD is the cam angle target value from VTC tables. CAM is the measured cam angle.
