I bought my 2002 X5 4.4 about a month ago, and have noticed that it's either down on power below 4k, or has a surge of power right at 4k. I've never driven an m62 powered vehicle, much less another X5, so I don't know what's normal. It feels strong down low, and up top as well, but I can notice that at full throttle from idle, when it hits 4k, the sound changes, and it begins pulling harder. I'd say it feels like 20-30 whp more between 3900 and 4000. It's not a gradual thing, it's an instant change of some sort. It's not really bad or anything, just something I notice if I pay attention. Looking at stock m62 dyno graphs, I know I shouldn't be feeling that, as the power curve is smooth in that RPM range.

I'm familiar with single vanos engines, as the m50b25tu that I put in my e30 is single vanos. That is an on/off solenoid though, 6 degree advance to 6 degrees retarded, at 1k RPM's and again at 5200 RPM's, depending on throttle load. From what I understand, the vanos in the m62 is continuously variable, and has 20 degrees of motion. Could something potentially be causing it to not operate properly, until 4k?