Oil Cooler/Hot Cylinder

[Sunriver Ken]

I have an O-360 and electronic ignition in my RV-9A (which I did not build). The oil cooler is mounted immediately aft of the left rear cylinder with a door that, when opened rearward, allows air to pass from the baffled engine area through it to cool the oil. With the door shut virtually no air passes through the cooler.

At cruise EGTs run plus or minus 15F but the CHT for the left-rear cylinder is consistently 30F-50F higher than the other cylinders. Good or bad I have trouble getting my oil temperatures, at cruise (9-11K ft.) and leaned, above 160F even with the door closed. It is in this configuration the CHT is 30F-50F higher than the other cylinders. I did not try opening the door to enable more cooling air but doubt if that would lower the CHT.

So, my goal is to raise oil temperature while reducing the CHT of the left-rear cylinder. Any suggestions?

[jjconstant]

First off, my understanding is that absolute EGT numbers are not really useful, I wouldn't worry about them being "uneven". One of their main uses is identifying where the peak EGT is for each individual cylinder and trying to get each one to its own peak at close to the same fuel flow. Regarding oil temp, is the oil cooler shutter on the front of the cooler or back? I know it doesn't make sense but I have seen examples where the shutter was moved from the back to the front and it made a big difference in the effectivenes of the shutter. You can also use adhesive backed metal foil to close off the front of the opening to the oil cooler in cool seasons.
You can also use the same foil to act as an air dam on either the #1 or #2 (front) cylinders to rob some of their cooling air to provide more for the back cylinders. It doesn't always follow that blanking on the front of one side will provide more cooling for the back cylinder on the SAME side. It's a process of try it, test it, modify it, test it. In my case I found that a strip about 1" high and 4" long taped to the front of the cooling fins on #1 helped bring #4 down in cruise. But it changed #3 in climb so you need to determine what flight regime is most critical to you. It's easy to fixate on imperfections in cruise when you have all the time in the world to look at things, but frequently climb is the most critical phase for temperatures.

Some experimentation with metal tape and a test flying mental approach might be some fun flying and then you see what you get and make your choices

All Best

Jeremy Constant

[RV7ator]

Ken,

My 360/RV-7's almost always show #4 coolest CHT. (I'm assuming you typo'd EGT in your post.) The 7-row coolers have ~30% blockage with a piece of aluminum sheet held by two of the cooler bolts. It's installed year around, oil temps ranging from 160 to 190 depending on ambients.

Try removing or at least leaving the door open. I believe this allows more mass flow over and around #4 which is what reduces its CHT. Lower CHTs should be your primary concern for the engine's health. Low oil temps are rather unimportant; the oil's hotter elsewhere in the engine and at higher altitudes water boils off at lower temps.

You may need to mess with baffles in front of #1 and #2 to minimize hot-spot CHTs, particularly in climb. I've found about 1-2" high is a happy height. YMMV.

John Siebold

[JonJay]

What is your CHT spread when not leaned? It is possible that #4 is running richer or leaner relative to your other cylinders. It is also possible that your EI may have some issue with it specific to that cylinder, although again, unlikely. Map your EGT profiles and see if you get any clues.
If this shows nothing obvious, swap your CHT probes and see if the probe is for some reason reading high.
Check your #4 cylinder for obstructions like casting flash that may be plugging up the coolling fins.
You may not be able to get higher oil temps at those altitudes due to OAT. This is a common "problem".

[Sunriver Ken]

Thanks guys, I will give your suggestions a try.