I have a question about an EGT probe install. Does the stock uppipe have a bung for an aftermarket EGT? If not, what is the best way to install the probe? Tapping the uppipe, welding a nut onto the uppipe, getting a probe with a stainless steel strap?
I would recommend installing your EGT probe in the header/exhaust manifold downstream of cylinder #3. Subarus are notorious for leaning out cylinder 3 (the one closest to the driver). I haven't heard much about the STi in regards to leaning out cylinder 3 (I have yet to install my EGT gauge) but if it's similar to the WRX (which many things seem to be), this is definitely the place to monitor EGT. If the engine is nearing the detonation/preignition threshold, cylinder 3 will likely show you exceedingly high EGTs before the others lean out too much. The best way to do it is to drill a hole and weld the bung on. The up-pipe EGT bung is where the factory EGT sensor is located.
Sti's don't have a cat in the up-pipe so the factory didn't put an EGT in the up-pipe. The manifold is the best place to put it. There's a lot of debate about which cylinder is the best to monitor. A lot of people say #3 is best, but it's tough to get to and a lot more people are experiencing problems in the #4 cylinder. I pretty sure I'm going to mount mine off of cylinder four.