Shock Cooling
 

At temperatures hovering around 110 F it seems like oil temps and CHT's are always pushing red line on a C-207. I was told by my cheif pilot to leave the cowl flaps open during this time of year even during a decent. Do you think this is hard on the engines?

I would say you'll probably be fine. I Unless you're doing super long high speed descents, you're fine. The big thing is to just monitor the CHT's and they'll tell the story. If you ever fly a C310, you'll see that that one you can leave them open all summer. It's an internal cowl-flap, so it doesn't "Breath" as well as the other types.

We have a Lyco O-470 in one of our planes and keeping the flaps open we do steep descents from 12,000' to pattern using speedbrakes (ie. fast) and it doesn't seem to hurt the engine. There's no EGT gauge so I do not know what it does to it tempwise, but it seems to be ok for decades of this. One thing though, is we keep some power in to help heat the engine on the way down.

EGT is only related to the power, you'll need a CHT to tell what the cylinder heads are actually feeling.

1 inch of MP per minute reduction will usually take care of keeping the engine running right. If you can get a hold of the engine manual it will tell you how many degrees of temp reduction per minute are acceptable. Realisticly unless you have a graphic engine monitor (Insight or JPI) then it is pretty hard to tell how much temp change you are creating with your specific engine management. With the old guages you only get CHT from one Cylinder and egt from one exhaust port.