Well if your timing gets to far advanced you will start getting symptoms like A/F shows rich and you keep taking fuel out but your EGT temps will rise showing you have a bad timing split or to close together. This leaves exhaust gas left after the combustion cycle messes with you WBleading you to believe you are rich!

 

Have the car on a dyno will help you nail down exact numbers.
Every EGT gauge is different, so try to aim low and start leaning it out from there.

I prefer the turbo exhaust manifold, but divided systems (and a single EGT probe) necessitate going to the downpipe. You'll need to adjust for the delay if you're going into the downpipe, as the EGT reacts faster in the turbo exhaust manifold.



-Ted