Does trimming lower cowl to increase exit area really lower CHT?

[rv7adriver12]

I have a RV-7A with an O-360 and fixed pitch prop. I routinely see CHT reach 410 or more on all 4 cylinders if I don't immediately throttle back after take-off and climb at a very shallow angle. It doesn't matter what the ambient temp is. It can be 40 degrees and I still exceed 400 degrees if I use full throttle in a shallow climb. CHT is always fine once I level off or throttle back. Baffle seals are all new and in good shape. I'm debating whether to trim the lower aft part of the cowling to increase exit area. But am hesitant because of a new paint job and feedback from one poster that this made his situation worse. However, if trimming off some cowling material is really worth the effort, I'm ready to cut. Is it effective, and if so, how much should be trimmed - 2" or so?

[Ron RV8]

On my -8, I cut mine forward an inch at a time. I'm now at 5" and the temperatures came down incrementally.

I did a bunch of other "stuff" as well, lots of info in other threads.

The biggest remaining factor is getting the baffles "just right" in the very front of the cowl. I'm planning to install some "backing" for the rubber seals to help hold them in position.

Good luck...

[painless]

A general rule of thumb is that you want the exit area to be twice that of the inlet area. I trimmed about 1 1/2" from the rear of the exit area on my 6A to approach that ratio and have had no issues with overheating.

[brad walton]

Take a look at the servo actuated vents on the Anti Splat website. More exit area may lower CHT, but will also increase cooling drag. The servo actuated vents may help cool in climb and then can be closed in cruise so you dont add to drag

[f1rocket]

Well, it works IF, and it's a big if, you are getting enough high pressure inflow on top of the engine. I've solved cooling issues on about a half dozen RVs so far and while I'm not an expert, I can say that what I've found to be the two most consistent problems are:

1. Restricted inflow above the cylinders where the upper ramps are bonded to the top of the cowl. As a general rule, if you cannot get a clenched fist up inside your cowl, then the flow is restricted. Cut out the ramps and move them forward so they make a more abrupt transition to the inside of the upper cowl.

2. Leaky baffling. Make them as tight as you can. Consider a plenum if you just can't get them tight. Seal all the little slots, cracks, and openings around the engine to drive as much air as possible through the cylinder fins.

My previous RVs all cooled extremely well without the need for other "remedies". Work at it and I'm sure you can get the temps down where you want them.

[N941WR]

Those kind of temps in climb are not a big deal. What are your temps like in level cruise at 75% power?

[rv7adriver12]

CHT in cruise has been running about 330 or so.

[TomVal]

FYI:

General cowl cooling flow tips by Jabiru:

http://usjabiru.com/wp-content/uploa..._Flow_Tips.pdf

[DanH]

To answer your question directly, yes, increasing exit area will generally increase mass flow. I have actually taken that experiment to the extreme, as my modified cowl can be flown with no exit chute in place, i.e. with about half the bottom as an exit.

The cure is usually what Randy said, times two. Barring a poor engine setting, it's pretty much all there is.

That said, with 330F in cruise, 410 is about what I would expect for a fixed exit. Variable exit area would be the next step, so you could lower climb temps, and raise cruise temps to cut drag.

Originally Posted by painless

A general rule of thumb is that you want the exit area to be twice that of the inlet area.

Proven untrue, over and over again. Dan Raymer has a few words on the subject in his latest book.

[vlittle]

Use the search function. The solution to high CHTs on climb out in a carburetor engine is to have a bigger diameter jet in the carb. Don't cut your cowl!