In fact, the presenting problem is that I have not been solidly below
400=B0F. The hottest peak CHT just after takeoff on a standard day would
be more like 430=B0F, and the coolest about 400=B0F. On really hot days I
have seen even higher values, albeit for only a short time. I think I
have solved this problem at this years's annual by installing a fairing
around the big gap between the PowerFlow exhaust pipe (much smaller
than the standard Cessna exhaust). With the fairing installed, peak
CHTs just after takeoff are now between 400=B0F and 375=B0F as a result
of better airflow inside the cowling.

Regarding design limits, I do appreciate that Deakin was referring to
bigger engines. But I still wonder what a limit for the O-360 might be
as a result of adding the tuned exhaust.

One step at a time, let's check the prop governor and maybe the
wide-open mixture.

Thanks.