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How much cowling to cut to cool things down?

J

jsbc

I'm New Here
Like a lot of RV owners I am having trouble with a hot-running engine, high oil temps, and high CHTs. I can keep the oil to about 200-210F in cruise but during climb, circuits, slow flight etc.. it can get up to 230F pretty quickly and has redlined before now.

After reading a lot I think I am going to try the simplest solution of opening the bottom cowling a little to give more airflow over the cylinders. I know others have tried this with some success.

I would love to hear back from those who have opened the lower cowling a bit, especially regarding how much they have cut out, with photos if possible.

Many Thanks

Oh - I have an RV7a with a CS prop and 180hp carbed Lycoming.
 
Greetings jsbc,
Prepare for the response that tells you to first check the "search" feature, as this is an often covered subject.

I am having some issue as well with temps, specifically CHT that can be greater than 425 on #4 and 410-415 on #2 with #3 trailing by a few degrees and #1 ok. This is with takeoff OATs in the 80-90's F. My oil temps are not bad: 180's usually.

I cut the trailing edge of the lower cowl scoop even with the trailing edge of the left and right sides. When I stand my lower cowl on it's end, the line is straight across.

I have also obsessively sealed the baffles and had several RV savvy A&P's check the baffle seals, including the rubber seals at the top of the baffles, and the metal junctions around the cylinders.

I placed a piece of aluminum tape over the top of the oil cooler as a temporary dam to improve airflow over #4, since that is my hottest cylinder.

I have a washer behind #3 to improve airflow, and removed the "above the equator" baffle from in front of #2.

I have rechecked timing on the Slick mag to be 25* BTDC and the EMAG to be on the "A" curve.

I just ordered a cowl flap from ASA. I will install when it arrives on the pilot side cowl bottom.
 
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Heat

Yes the Search function will bring up lots of info.
Years ago, I had the same problem and tried the same solutions with mixed results. I finally found I had a lean carb and was not giving the engine enough fuel on takeoff. 0-360 / CS / carb.
Short answer is I drilled the jet.

To be clear, my high temps were on TO and Climb, cruise was fine.
So when I was asking the most of my engine, I wasn't feeding it enough fuel.

If yours is hot all the time, you have other issues, like airflow, too small oil cooler, etc. Cowl Flaps seem like a great idea for high demand or peak cooling times, but should not be used to fix a problem.

JSBC, What engine do you have, do you have piston squirters? That usually increases Oil temps.
Steve, Are you carbed? Which one?
 
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There are many things to check before cutting on the cowling. But if you're certain that's the next step, as Steve mentions, there's a nice cowl flap assembly sold by ASA. I was at the point where I needed more flow, but really hated adding drag for all phases of flight. So I installed two of the cowl flaps in my -8. They work as advertised, bringing immediate cooling. But they also add drag and noise (when open). One minor issue with the devices - lock all the fasteners with a thread locking compound before using. I had one come loose after a couple of hours. Easily repaired, sand I locked all the other fasteners. No more loose connections. I also wrapped the actuators with a radiant barrier/insulation.
 
There are thousands of RVs flying with acceptable temperatures without modifying the cowling exit. There must be some reason your engine is running hotter than normal, and it would be very wise to keep looking until you find it. Otherwise you are just curing the symptom of the underlying problem, and that problem may come back to bite you later if you don't fix it.

Are your CHTs acceptable, and the only problem is oil temperature? If so, please give us details on your oil cooler, and how the air gets to the cooler.

If your CHTs are also high in cruise, how much EGT rise can you get from full rich to peak EGT?

If your CHTs are high in climb, what airspeed do you climb at, and what fuel flow are you seeing during take-off?
 
Temps

Opening up the cowl outlet is not the way to go. Kevin and the poster who said to drill the carb jet are exactly correct.
Two great sources on optimizing cowling/cooling:
search Dave Anders RV4 and study everything you find
Get the book Speed with Efficiency(do I have the title correct?) still available from EAA
The oil temp is easily solved.
CHT will probably require more work but also easily solved, sometimes time consuming.
Dave Anders at one point had outlet area around 30% smaller than inlet, this with an angle valve engine with high compression pistons.
If you can find the Sport Aviation article from many years ago about Dave's airplane the pictures will show what to do with the oil cooler.
 
I had the same problems that you are experiencing. Anti splat Aero cowl flaps dropped the CHT's in climb. Moving the remote oil cooler on the firewall to a position behind the baffle above the # 4 cylinder took care of the high oil temps.

John Morgan
 
Thank you all for your comments. I can see cooling is a big topic on the forum and there seem to be lots of different opinions. I will check my CHTs very carefully before I decide on whether to cut the cowl, put in a cowl flap, or change out my oil cooler for a bigger one. (My cooler is baffle mounted). I'll also check the difference between peak EGT and full rich EGT.
 
jsbc said:
Thank you all for your comments. I can see cooling is a big topic on the forum and there seem to be lots of different opinions. I will check my CHTs very carefully before I decide on whether to cut the cowl, put in a cowl flap, or change out my oil cooler for a bigger one. (My cooler is baffle mounted). I'll also check the difference between peak EGT and full rich EGT.
Click to expand...

I cut 2" off the bottom back of my cowl outlet and sloped it to the firewall. This gave me an average of 30-degree oil temperature drop. That was 18-years ago cannot find my notes so unable to tell you what the CHT drop was. At the time I cut the cowl, it was September in SoCal, was the hottest time of the year, and I was in phase I testing.
 
I have higher that desired oil temps on hot days in climb. WOT at altitude runs around 210. I have a Sam James cowl and a large remote oil cooler fed with a 4 inch scat tube. I am tempted to cut the cowling a bit as well but have not done so since I am fine in the winter and 210 is fine with me. I have poston oil squirters and 195 hp
 
I need to look at the invoice from Pacific oil cooler to see which one it was but it was the biggest one they advised me to install. I took it off the top right cylinder facing the prop. Cant remember if its 1 or 4
 
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