lower cowl cooling vents

I am working on the bottom cowl assembly with the large air filter on my wide body Backcountry Cub. The two vent flanges which came with the kit are too long to fit between the air filter fairing and the edge of the cowl. The Piper PA-18 drawing #12357 shows these parts. I have found photos on this site showing different combinations of vents flanges. Some have these two on either side. Some do not. Some have the aft cooling lip at the main air outlet and some do not. Some do not have any vent lips. What combinations are some of you using and what cyl head temps are you getting with a 180hp engine when you operate on hot 80+ degree days. Also what is the distance between the side cowl and the boot cowl at the maximum point.

Cooling is important. I want to get it right the first time. Thanks for your inputs.

This is a wide body Cub Crafters lower cowl. The two vents are closed off



The hole is for a Sutton exhaust.
Darrel

QUOTE:
You seem to be on the rich side, all right. As a maybe not-too-useful comparison, my 150 PA-18 runs 1480 EGT peak, and 1430 cruise, at 2400 rpm and 5,000 MSL with 6.5 to 7.0 gph with a Borer 44 prop. My CHT's started out at 390 + OAT=CHT, and now run 310 .... looks like you are at 352...
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Just a quick table of my(successful) mods:
1. close off the hole down the front of the engine: 27 F
2. front ramps: 20 F(on rear cylinders)
3. lower cowl lip: 10 F
4. deflection vane over #4: 20F
5. finding binding bushing on #4 exhaust rocker arm: 20 F (on #4)
I started with +2.7 inches of pressure on the suction side of the cowl, and wound up with zero after the mods.. UNQUOTE
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Do a search on "CHT".......my lower vents gave me 10 deg F on CHT's....

The red cowling in the picture above goes on a friend's 180 hp SC with an MT prop. The cowl door-to-boot cowl gap is 3 inches. This plane often runs a little too cool. My SC with stock cowling and a 160hp 0-320 has a cowl door-to-boot cowl gap of 2.5 inches. I would like to have this gap a little greater, not because the temps are too high but because my baffle seals are very tight and at cruise of 92+mph the top cowl puffs up some from being pressurized, telling me that the air can not flow down and out quite fast enough. I think a gap of 2.75 to 3 inches would be about right but this also might cut into airspeed a little. My setup is a 160 hp with a Leading Edge exhaust system and an 82/44 Borer prop.
Darrel

Thank you Darrel- That is a big help. Were the two vents in the picture closed in an attempt to raise the temperature?

I also did not install the vents. If you copy the CC 180 Top Cub exactly you will be good to go. Another big difference between Piper Production and CC is the CC does not use any kind of Baffle around the airbox, its a straight shot clean through. I have no flange lip either. Sutton exhaust, firewall mounted large oil cooler, 116 mph cruise. Tested at 5000 to 11,500 ft during the summer. Only broke 200 on a hard climb once. normal oil summer temps around 162. I'm now looking at how to warm it up and reduce cooling drag for a few mph's. Speed to RPM ratio has a lot to do with proper cooling.

No the cooling vents were not covered, the cowling came like that from Cub Crafters. And the area around the carb is straight through as Fortysix 12 says.


Darrel

I never thought about it until now but I suspect the reason the vents were there to counter the oil cooler air. On the original the oil cooler was below the baffling therefore reducing the negative pressure on the low side of the cowl. They were also ramped which is very efficient when trying to exit air. CC takes oil cooler air from up top as do I and every other experimental builder I've seen whether it be firewall mounted or rear baffle mounted. I cool with a 2 inch supply. Truley experimental until proving this summer that that was all it needed.

Thank you Darrel & 4612 you were a big help.
4612 your idea of the two mid point vents being for the oil cooler makes sense. I will leave them off as I am planning to mount the oil cooler on the rear baffle. I will install the aft lip as I am going on floats which will give me about 10 mph less ram air and I believe that the lip will help. I participated in some cooling tests long ago in a PA-12 on floats to certify a 168hp 0-360 (low compression 80 oct.). We needed to add a larger lower aft lip.