What air pressure delta should I shoot for in the cowl?

[Desert Rat]

Hey all, I want to do some experimenting under the hood to try and optimize my cooling efficiency and cowl exit air velocity. Before I get too far down in the weeds, I need a way to benchmark what I've got going on in there already.

I know that several folks have gone down this road and I'm trying to save myself some steps here. Can somebody give me a ballpark idea of what sort of pressures I should be seeing in the cowl on a "good" installation? i.e. gross upper cowl pressure in cruise flight and then minimum delta P between upper and lower where you start to see cooling issues?

Bonus points if you can point me in the direction of a reasonable manometer for this shade tree science.

 

[agent4573]

DanH has some good posts on building piccolo tubes to measure cowl differences, I'll see if I can dig up the links. In the meantime, Lycoming requires 6" H2O differential in cruise.