Reducing CHT's

Ever since I put the O-360, Sam James Cowl, and cooling plenum on my plane I have been struggling with high CHT's.

The only way I could keep my CHT's below 400* at 75% power is to DUMP fuel into the engine. Even then cylinders two and four were always much hotter than one and three.

While the plane was down for some unrelated maintenance I decided to see if I could locate any possible air leaks. Boy did I ever find some leaks.

The largest leak was where the cylinder baffles for #2 and #4 come together. They had bowed out crating an air scoop which was large enough that you could slide a quarter between the baffles. (See Image below.) The same baffle splice exists between #1 and #3 but the baffles were flat against each other.

The old RTV was removed, the cracks were sealed with fresh RTV, RTV was put on each cylinder head were it mated up to the baffles, and a new bead was added to seal the cooling plenum.

The net result was my CHT's came down 25*F. Problem solved!

Here is a picture of the major leak area:

It never ceases to amaze me how much "little" leaks like these can make such a big difference in CHTs. Thanks for sharing the pics!

Gash said:

It never ceases to amaze me how much "little" leaks like these can make such a big difference in CHTs. Thanks for sharing the pics!

Click to expand...

Yeah, but you can say "Hey, your baffling needs work", and they look at you like you've sprouted a second head

Truth is, standards are very low. Don't believe it? Here are two snapshots taken in the engine compartment of a well known RV. These baffles were installed by an entire team of experienced builders, including a few A&P's. Sure, they work, but they're a long way from optimum.

I agreee that the cylinder baffles that Dan posted pictures of are very important. However if you have a good install (per Van's plans) I'm not so sure the little leaks really make that much difference. For example on my own aircraft I generally take off the heater hoses/shroud when it starts warming up for better access and install it when it gets cold (so its off most of the year). I have found ZERO difference in temps when I plug the huge 2" hole in the rear of the baffle for the heater. Defies logic.

Gives me an idea.

Walt said:

I agreee that the cylinder baffles that Dan posted pictures of are very important. However if you have a good install (per Van's plans) I'm not so sure the little leaks really make that much difference. For example on my own aircraft I generally take off the heater hoses/shroud when it starts warming up for better access and install it when it gets cold (so its off most of the year). I have found ZERO difference in temps when I plug the huge 2" hole in the rear of the baffle for the heater. Defies logic.

Click to expand...

The friction in the spiral tube is huge, but that gives one an idea for future testing. Take some known orifices for "bleed air" losses from upper to lower plenum, then plot temps and delta-p for upper and lower chambers. That should provide some definitive data for future estimates. The idea is that it maybe easier to measure what makes it worse than what makes it better.

Just an idea for when I am actually flying that is.

Dan, as always you bring up some good points.

My cylinders are the ECi tapered units. To keep the cooling air over the heads I made the air dam recommended on page four ofthis technical document. Prior to installing the baffles, I sealed the air dam to the baffle with RTV and after installing them, I put a bead of RTV along the outside cylinder fin and the baffle to keep the air moving down between the fins.

It all adds up and my goal is to over cool the engine so I can then "add heat" where I want it.

The other trick I did was to discard the air dams provided with the baffle kit, replace three of the rivets in front of cylinders #1 & #2 with plate nuts and make replaceable air dams. That way I can remove them, trim them until all my CHT's are in line, and reinstall them. Or, in winter months they can be replaced with taller air dams to help keep the cylinders warm.

N941WR said:

Ever since I put the O-360, Sam James Cowl, and cooling plenum on my plane I have been struggling with high CHT's.................

Click to expand...

Ah hah! So it's not just the older Jabiru's with cooling problems
Sorry Bill, I just had to.

Seriously, hopefully you'll get her cooling like you want. Sounds like a bunch of good suggestions will probably be posted on this topic, which will help us all learn new ideas.

Take care,

txaviator said:

Ah hah! So it's not just the older Jabiru's with cooling problems
Sorry Bill, I just had to.
...

Click to expand...

That was good!

The difference is that I was able to control my CHT's with fuel and power and even though I had what I consider "High" CHT's they were well within the allowable continuous operational range described by Lycoming.

As for the Jabiru's, I sure hope they have solved their cooling problems because the engine really does look promising. That said, people thinking about installing one need to know the history. (The same goes for those installing P-mags, LSI, or whatever.)

Walt said:

I agreee that the cylinder baffles that Dan posted pictures of are very important. However if you have a good install (per Van's plans) I'm not so sure the little leaks really make that much difference. For example on my own aircraft I generally take off the heater hoses/shroud when it starts warming up for better access and install it when it gets cold (so its off most of the year). I have found ZERO difference in temps when I plug the huge 2" hole in the rear of the baffle for the heater. Defies logic.

Click to expand...

I propose an experiment!

If I understand correctly, the above two configurations are:

(a) 2" scat blocked, or
(b) 2" scat feeding a heater shroud on a header pipe, then ducted to a firewall heater box, then vented into the lower plenum or the cabin.

I'd suggest that (b) has the same pressure drop, end to end, as the drop across the cylinder fins, and maybe quite a lot more.

The experiment? Disconnect the 2" scat at the rear baffle wall. Go fly with that 2" hole open into the top of the lower cowl volume, sort of like a giant cool air source for the magneto area. Pick a particular power setting, fuel flow, and altitude, let things stabilize, and record the CHT and oil temp.

Now go back to the airport, seal over the 2" hole, and repeat the flight.