From time to time someone will mention that the parallel valve cylinders are harder to keep cool than angle valve cylinders. It's one of those things which seem plausible. However, evidence would be nice too.
Jeff Schans at Lycoming was kind enough to supply cooling air charts for the 4-cyl engines of interest to RVers. They are available (for now) at:
Tonight I plotted a 4 inch H2O baffle drop at 5000 ft pressure altitude and 60 OAT for all four engines. Below you see the lower right corner of each chart:
Note the parallel valve charts have two CHT curves, 75% @435F CHT and full power at 500F CHT. The angle valve engines have three CHT curves, 70% @ 400F and 435F (425F for the IO-390), and a full power curve. I point this out because it's easy to get confused....look close at the labels. I've marked the 435F curve on each.
Apples to apples, you can just barely keep the 320 under 435 CHT with 4" of water across the baffles.
4" isn't enough for the parallel valve 360; CHT is heading for the absolute limit.
On the same 4" an IO-360 angle valve is around 420 CHT. To be fair the curve is drawn for 70% power rather than 75%. Even at 75% the angle valve 360 would cool as well as the little 320 on the same mass flow
The 390 chart is plotted for 425F CHT rather than 435F. It appears they just shifted the curves upward a bit. You can even cool a 390 on the same mass flow as a 320.
So, parallel valve owners, the CHT problem is not entirely your imagination. Running "only" a 320 does not make the task easier. O-360 owners (and by extension O-540 owners) get no slack at all. They must
have good baffles, good seals, and good upper cowl pressures or accept the consequences.