Tweaking CHT's

[David-aviator]

On a recent cross country flight I recorded some engine numbers just to stay awake and maybe learn something about what was going on. That data indicated #1 and #4 CHT's were running about 20°F warmer than 2 and 3, wonder how come?

An oil change was due after that flight so things were checked over.

In front of #1 cylinder is an air deflector as per Van's baffle plan. It is about 2" tall and a similar device is installed at #2. I decided to trim the #1 deflector .75" from the top and see what gives. It is now about 1.25" tall and more air is available for direct cooling of #1.

Yesterday I finally had a chance to fly to see if it made a difference. It did. #1 CHT is down about 12F. The CHT's were 352,346,346,363 across the board.

OK, so what's going on with #4 now riding high compared to its neighbors?

It is the oil cooler stealing air from it. I have a remote oil cooler ducted with 4" scat as per the RV-10. When air flow to the cooler is shut off oil temp rises, like from 170 to over 190 and #4 CHT begins to drop.

For a change it all makes good sense, at least in stable cruise.

[schristo@mac.com]

i have a similar setup using the oil cooler and plenum on the firewall from the -10 with a 4" sceet tube and my number 4 is the coolest CHT... at least 30 degrees cooler than the others. also, this cylinder has the highest EGT... it must mean something...

[Frank Smidler]

20 to 30 degrees difference on cylinder head temperature is great. If you get it closer it will only be due to luck and not design. With fuel injection you will have a better chance of getting them closer but there still are differences in spark plug gap, spark plug contamination, valve springs, valve facing, probe variation, wire connection to probes, injectors, etc. Prior to the electronic engine monitors that read out digitally to one degree, you would never have even noticed 20 degrees difference between cylinders on the individual gauges, if you had more than one CHT. As mentioned on other posts, pilots have flown, successfully, for years with only a single CHT (if that) in Piper's and Cessna's. I am sure you have seen many of the aged baffling on these planes that are not even close to as nice and tight as on most RV's. Blue skys.

[49clipper]

I had that problem early on and made a plate over the 3" intake to the oil cooler and proceeded to make holes in it to restrict some air to the cooler. It solved the problem. My oil now stays about 184f during this very hot STL wx and the cyl dropped down to about 350 during cruise on my O-320. I did also put the **** in front of #1 and its 1.25" if I remember right. I ended up with about a 1.625" hole in the plate over the oil cooler inlet on the baffle with the hole curved inward for a smoother transition, which does make a difference.
Jim
RV-6

[DENMACRES]

CHECK THAT THE GAPS ON THE BOTTOM OF THE BAFFLING, BARREL AND HEAD IS CORRECT. YOU MAY NEED TO ADJUST THEM.