TECH (not so) QUICKIE – Getting Started With Variable Cam Control (not VTEC, yo)

 

Variable cam control is one of those things that requires a lot of fiddling. There are 4 main items that need setting up before anything useful can happen. They are (in order of importance):

  1. Cam static position
  2. Control delay
  3. Cam valve frequency
  4. Base PWM map

Cam static position

As you need to be running sequential injection for cam control, this will already be filled in as it is the same value as camshaft sensor position in the sequential injection settings. However, for sequential injection, this value doesn’t need to be 100% accurate, for cam control it does. Unplug the cam control valve and run the engine. Open the base cam pwm % table and check the value for current cam advance is reading 0. If not, adjust the cam static position value until it is. This value can also change with engine speed so there’s an offset table to compensate for that also.

Control delay

This really depends on how quickly your engine accelerates through the load sites. Start off with 0.05 seconds and adjust up or down – is 20 adjustments per second enough for your engine?

Control valve frequency

This depends on a couple of things and also has an effect on others, so bare with me. PWM is a way of turning a digital on/off signal into something more like an analogue signal. It does this by rapidly cycling on/off in different ratios of on and off. The PWM frequency is the number of times per second the on/off cycle occurs. Some valves/motors/controllers will only work with a certain frequency or within a certain range of frequencies – so a bit of trial and error is usually required. However, a higher frequency can mean you need a higher base PWM and possibly lead to a narrower range of adjustment.

Base PWM map

The values in here want to be the duty cycle (the ratio of on to off mentioned previously) required for the control valve to be half-way between advancing and retarding the cam – i.e. 0° of advance or retard, in the middle! First, find how much duty is required to make the valve start advancing the cam – start with a low value like 30 and work your way up, fill the whole table with the same value for now (F5). Once you have a ballpark figure, zero the table and update the map (to park the valve) and then start approaching the figure you had a moment ago with finer changes. E.g., the cam starts advancing at 70% but does it? Park the valve and start again at 65% then increase 1% at a time until is starts to advance. Make a note of that figure and then up the duty to 100% to ensure the valve is fully advancing the cam. If you ended up with a figure like 67%, put that back into the table and start reducing it until the cam starts to retard (get a ballpark figure and then refine like before but instead of parking the valve, you want to fully advance it). Once you have a figure for how much duty is required to retard the cam after it has been advanced, e.g. 47%, you fill the table with a value half-way between the two. So, if it starts advancing the cam at 67% and once the cam is advanced, it starts retarding it at 47%, you fill the table with 57%.

Aaaaaand relax.


TECH QUICKIE – Getting Started With Variable Cam Control