Just an update. I also purchased the 3" SS downpipe and installed it today. I broke one of the bolts off that attaches it to the muffler and couldn't drill it out so I will have a muffler shop remove it tomorrow. But when I took it for a spin, I didn't have any knock retard at any speed. When I tried it out with just the PLOG, it was getting high knock retard. The PLOG so far doesn't seem to heat the A/C comressor up, since I ground down enough of the compressor that it's not touching.