The best recorders to use are the cheaper ones which do not have an input level control. These will have an automatic volume control (AVC) instead. I have found that the volume level from the knock sensor changes drastically from idle to WOT, making the input level very difficult to set properly.
If this is a stereo recorder, I recommend using just one channel for knock, and put a standard microphone on the other channel. You can then use the spare channel for making verbal notes about where you are and what you are doing (very useful for road testing, though maybe not at the track).