High CHT's On Climb
 

I am experiencing high CHT's on climb. Density Altitude the last few flights has been around 1500' - 2000', and mixture is full rich against the stop on the carb. This is on an airplane (RV-8) I bought with ~300 hours. Cruise CHT's are fine. If I don't climb at less than 110 kts, the CHT's will start climbing. My goal is to always keep CHT's at 400 or less, 415 an absolute limit for me. When I first started flying the airplane I'd see 435 which is way too high, and it's not even hot yet (~ 80 - 85 deg). My engine is an O-360 A1A turning a Whirlwind 200RV. The carb is a brand new Avstar 10-3878. I have looked in the air intake of the cowl with a flashlight and can see no gaps that would allow air leakage, and the baffles look to be in good condition. High temp RTV is used for a sealant where there is metal to metal contact on the baffle system. EGT's are 1300 to 1325. Timing is set at 25 deg (Slicks), and the plugs are cleaned and gapped. Take off fuel flow runs 15 - 16 gph. I have considered two options to help: 1. resize the carb jet to a little larger. And/or 2. retard the timing a degree or two.

Any thoughts and suggestions would be greatly appreciated. Thanks

Normal ops Jim. I think you meant, you need to climb faster than 110 knots to lower the CHT's. If that is the case, thats exactly what I do. My RV8 has a 3878 carb, and I accept the Lycoming recommendation of 500 deg. redline and 400 degrees continuous as my limit. I lower the nose and climb at a faster speed to regulate my temps to stay under 425 for climb( still over 1000fpm). I do not view 5-10 minutes in climb as continuous! A 4164 carb. might help slightly, but if you expect to be able to achieve Van's rate of climb #'s and remain within your CHT limitations as opposed to Lycoming's, well good luck!
Best regards,

More exit area will help almost for sure.

In a full power climb to 8500', I see 375F CHT. Oil temp 170 with a RV-10 cooling set up on the firewall.

Not many guys want to mess with the Van's cowl. But I think they cut it a bit too close on intake vrs exit area to keep drag to minimum. The ratio is about 108%. If it is 120%, cooling is better.

Yep...I had not had any coffee yet :)

The exit area on the cowl does appear to be relatively small. That may be what it needs. I'll try to take some measurements and look at the ratios.

Jon, it's been a while since I have flown a Lyc, but on a big bore Cont, if the CHT's are allowed to go to 500 you'll see dramatically lower cylinder life. Aluminum does not care if it's on a Lyc or Cont, a cylinder will not last as long if it's allowed to get hot on a regular basis. I went to the Avanced Pilot Seminar put on by the GAMI folks (Braly, Atkinson, Deakin) at Ada, OK, and they say keep em under 400. They preach keeping them under 380, but that is not totally practical so their limit is 400, even in a climb. On a big bore injected Cont, you jack up the take off fuel flow to somewhere around 28 GPH to keep em cool. They excess fuel cools everything. Heat kills cylinders which is why I'm concerned about what I'm seeing. Thanks

Jim

One thing I don't see mentioned here is the cowling inlet eyebrows inside the cowling. If your cruising temps are good and the problem is only in climb, you might take a look here. If the internal eyebrows are not sealed properly you can get turbulence during high AOAs, thereby increasing CHTs.

And you're climbing with full throttle open all the way up for the enrichment circuit? Full throttle for fuel cooling, and slow the prop down some (2500 or so) in the climb to reduce power output. This is working great for us.

Mel - checked that too. The rubber baffle material is sealing the inboard and the outboard portions of the eyebrows, both left and right/

If I look hard, I can find it, but that involves work. I have read what Mel refers to before. The inlet ramps on the top cowl half, at least one end needs to be sealed off. I am told the air will bypass the cylinders if it is not sealed.

Two things I plan on doing with my cowl
1. Cut the exit area back at least one additional inch.
2. Seal the inlet ramps on the top cowl.

From all of the experience I have read here, these seem to be the biggest bang for the buck to lower cylinder head temps.