Plenum for O-360 on a 337 ??

[Bryan Bowlsbey]

I am having some difficulty getting a new engine to run cool. I am just at 50 hrs now, and the second oil change. The oil consumption has dropped down to about one quart every 8 hours, but no. 3 cyclinder runs pretty hot during any extended climb during 85 degree days. To mitigate this I have been backing off to 25 squared at about 500' on the takeoff run, and then climbing at 100 kts, (which sometimes doesnt quite get me 500 fpm). With this procedure, no. 3 CHT was climbing to 455 or possibly 460 before we would get to 3500 or 4500' (break in cruise altitudes).

We have since sealed off every hole I can see with black RTV silicone, including the gap between the metal baffle and each front cylinder. Seal to the top of the cowling looks ok with the cowling open, but it seems difficult to be sure we are getting a good seal on the right hand, (copilot side) of the engine compartment because the metal baffling sits a little lower on that side due to the cylinders being a little lower as well. After 10 minutes of 65% cruise at altitude no. 3 drops down to 385-390 while still running full rich. No. 4 is usually within 15-20 degrees lower than no. 3, and the front two cylinders can be as much as 95 degrees lower in climb, and 60 degrees lower in cruise.

I can clearly see gaps through the piano hinge on each side, but that must be exactly the same on every Comanche single, so I cant imagine that is the source of my problem.

Now, with all that said; my queston is as follows.

Has anyone every heard of a plenum retrofitted to a Comanche 180? My hangar mates all have Mooneys, and their stock aluminum plenum box around the cyclinders and heads looks like it would be easier to ensure a good seal. Even better would be one of the slick fiberglass or carbon plenums I see on O-360s on experimentals. with the rounded internal shape of the later, it seems as if you could optimize air flow past the heads.

Is it a pipe dream to think some FSDO might approve either a fabricated AL plenum ala the M-20 crowd, or approve the installation of one of the "experimental" parts on a 337 for the purpose of engine cooling? Does anyone know of any precedence for this, or have any other methods?

Apreciate your thoughts

Bryan Bowlsbey
ICS 17556
N6056P located at Sherman Army Airfield (FLV), Leavenworth KS.

[MULEFLY]

You need to have an extensive conversation with our President... Zach Grant... he is wonderful guy... most willing to share his knowledge that is EXTENSIVE... and he flies one of the fastest 180s in the fleet.

All the best!
Jim

[N3322G]

Bryan,

Overhauled my twin's engines a few years ago, followed the recommended break-in and the temps dropped in 10 hours.

Did twiddle with the badly installed baffles - the shop that hung the engines is no longer in business. That got me 20 more degrees.

I'm assuming you've already checked that everything went back together as designed?

Second thought would be to check with whoever sold you the engine for their thoughts as well.

Other 180- owners may have more specific advice.

[DAVEG24]

You state that #3, and 4 run hotter than 1 and 2. That is normal. If you want to lower those temps on the rear cylinders, you have to raise the temps on the front cylinders. The way you do this is to make a baffle that extends in front of the two front cylinders. You can experiment with aluminum duct tape. Tape off about 1 1/2 inches high across the two front cylinders (from the front baffle up). Fly the plane and note the temps. You should see a dramatic drop in the rear cylinder temps. If the front two are now to hot, trim off (or add) some of the tape and do it again. When you find the right combo, make some permanent baffle extenders that match what you did with tape, and permanently mount them (rivet) to the front baffle. I did this many years ago. My temps run within 10 degrees of each other, and is typically around 350. On climb out, full power, I may see 400 on a hot day. These temps are well within reason. Try it. You may like the results. If not, just remove the tape and your back to where you started.

Dave Gitelman

[Bryan Bowlsbey]

You are exactly right. I used some AL HVAC tape and the temps came back into much closer ranges. Still running hot all around though. Seeing 440 briefly during extended full power, full rich climbs to 7500 or 8500 ft. Once at altitude the head temps back off to 375-380. I am learning to use the new JPI to lean, and 50 degrees ROP seems to give me about 405 cht on number three consistently

Going to try to get the seal against the top cowl very close by using silicone to seal up any leaks I can find with a high powered light from behind the baffling while in a dark hangar.

I still need to follow zach's suggestion to loosen the inter cylinder baffle support and bend the back baffle plate on number three away from the head to provide more clearance there. My current baffling has cyclindrical projections coming off the back to direct air onto the magnetos. I am planning in going to electro aire EI once we get this new engine stabilized and getting rid of one Mag so there may be some other things to seal up for more flow past the heads.

[DAVEG24]

Glad to hear my suggestion worked. While your numbers are a little high, they are well within the parameters that Lycoming publishes. The original baffle setup for the O-360 was pretty loosey/goosey, with lot's of places for air to leak by. I used felt strips to fill the big gaps, with silicone for cement and to fill some of the smaller holes. How much time is on your engine? You may not be entirely broken in, which would also lead to higher CHT's. A lean carburetor will also cause high CHT's. There's more places to look besides the baffle setup. Mag timing? These are just a few of the things that come to mind. Good luck. You'll get there eventually. For what it's worth, really cool CHT's are not desireable either, so if you can get down to where they are running around 350, I would quit there.

Dave Gitelman