When the VVT solenoids are left unpowered, the cams will be in their most retarded state. This cam position produces the greatest amount of peak power, at the expensive of low end torque. Therefore, without VVT control the engine will have a significantly noticeable reduction in power below ~4000RPM. When power is applied to the VVT solenoids the cams advance, increasing low RPM power. The use of VVT greatly increases the width of the powerband, giving the advantages of both retarded and advanced cam timing.
The VVT control routine that has been added to M4.4 is open loop control – there is no feedback from the cam sensor to the ECU regarding cam position. This means that cam control is not as refined as it is with the ME7 engine management system. However, it still provides a significant boost in the width of the power band.
With a VVT head on non ME7 cars it is important to get the cams timed perfectly. ME7 will correct for the deviation in cam timing and will set a DTC if the cam timing is off by more than 11º. Since M4.4 doesn’t have the ability to dynamically adjust the cam angle, it is important that the cam timing is set as close to perfect as possible. Even when using the cam lock tool it is possible to get the cam timing a bit off. Therefore, I recommend adding a few more steps to the normal process when using a VVT head on a M4.4 car:
- With the timing belt on and tensioned, rotate the crank to the reference position and lock the cams in place with the Volvo style tool.
- Take the timing belt off.
- Rotate the crank 1/2 tooth counter clockwise. The indicator on the block should be pointing to a valley of the crankshaft gear.
- Rotate the VVT hubs to their limit position in the clockwise direction.
- Loosen the three 10mm (or 8mm) cam gear bolts a bit so that gears can move independently from the cams.
- Install the belt, starting at the crank, working your way clockwise. Since the cam gears are free to spin, line up the reference tick marks with the indicators on the plastic cover.
- Once the belt is installed, tighten the 3 perimeter cam bolts on both cams.
- Set the tensioner to the correct preload position.
- With the cams still locked in place, rotate the crank clockwise until the free slack in the belt is taken up and you can no longer rotate the crank.
- The crank should now be exactly at the reference position.
- Inspect the tensioner and retighten it if necessary, which will probably be necessary. Remove the cam lock tool and rotate the crank 2 full turns. Check the position of the crank and cam gear reference marks.
Make sure to never rotate the engine counter clockwise, as this will remove slack from the VVT hubs and throw off your timing.
Timing the motor this way guarantees that the cams are in the exact correct location. Having done a bunch of timing belt jobs on ME7 cars and making note of the cam deviation, I have found that when I use this procedure, I can get the cams timed to within 1º of spec. Whereas, without these extra steps the cam deviation is usually 5º-8º.
For those who don’t want to run VVT control
For those who don’t want to run VVT control, the cams should be timed so that they are centered in their total range of adjustment. This gives a compromise between maximum high RPM power and low end torque. To do this, time the cams as mentioned earlier and remove the cam locking tool from the back of the cams, leaving the exhaust side of the tool still in place. Put a magnetic protractor on the exhaust cam side of the locking tool, using the flat space on the valve cover, between the two cams, as your 0° datum. Loosen the 3 perimeter bolts on the VVT cam gear and rotate the camshaft clockwise 8º, using the T55 center bolt on the camshaft, not the exterior T55 cap. When rotating the cam, make sure that the gear stays fixed in place. Perform this same operation on the intake side, but rotate the cam 14º, also in the clockwise direction. After you’re done, make sure to torque the 3 perimeter cam gear bolts to 7 ft-lbs.