It's just easier to troubleshoot if you have problems getting it running, or to stay running, if you are only controlling fuel OR spark. My OHC engine was the first engine I started from scratch, without previously running the engine, that I was controlling both fuel and spark. It was my third MegaSquirt install.
Looking at some of my YouTube videos, here is one from when I went distributorless on my Buick 455, you can see the grey wire coming off the pickup coil, which is now acting as a cam sensor for my MegaSquirt. http://youtu.be/iGSrYLsvxPM
