I was looking into the reason for the differance between the large lip on the ragwings and the early A modles. I was looking at some early pictures in a book on the history of Cessna. There is a picture of two 170A's one with a large lip and one with a small lip. The caption under the picture said the reason for the change in size was to reduce buffeting felt under the rear seat.
FYI to this old post. My 48 had a 3" lip and always ran about 180 degrees or so in warm temperatures. I trimmed it to 3/4" lip and it still runs 180 degrees in warm temperature. The other noticeable gain in going with a smaller lip is an increase in cruise speed. The 3" lip is nothing but drag, it's like flying around with the cowl flaps open on an old 180. I saw a speed increase of 2 - 3 mph in smooth air with the small lip.
I'd be interested to see Cessna dwgs. 0552001 or 0552002 to see whats actually stated about the opening and said lip.
There is a thread on here where a person with an IO-360 had no lip. In fact, the lower cowl was extended back along the contours of the cowling to make a smaller opening all together. The experimentals (RVs) do this as well and have rather small openings without cooling issues.
On the other hand, a good friend had problems with very high CHTs on his newly-completed Rocket. His first fix was a big lip on the lower cowl outlet, and that worked very well. He's since moved on to lower-drag solutions, but the lip sure worked. I joked that he had the only seaplane Rocket in the world.
Dave,
You are more than welcome to try the one we made and see how it works on your bird. It made no difference on mine. Well, it did make more of a rumble on my floor boards but your mileage may differ.
I had to rebuild my lower cowl when I put the o-360 in it and the bottom of my cowl is flat and my opening is 2.5ins wide . My lip started out about 3ins wide and now is about 2ins wide at a angle of about 40 degs . I originally had it at 90degs and it sounded like a 747 at full bore in the cabin . So a little experimentation was carried out and the lip was slowly brought back to where it is now . In a full climb at max power my CHT,s run between 360 to 380 degs . Max. However in a prolonged climb at around 80 to90 kts the CHTS , drop to 345 degs . WHY ??????? I do not know . Oil temps sit at 183 degs constant . However your baffle condition is very important and no leaks between the top chamber of cooling air and the bottom chamber of used hot air should be allowed.
I don’t think John Minton is a member anymore. The most recent directory he shows up in is from 2013. Your best vet might be to write him at the address he gave.
Miles
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne