Enlarging the Cowling cut-out to improve CHT cooling.

[fastj22]

Enlarging my outlet size by 100% solved my CHT temp problems on my Jabiru.

[bakfly]

Rodger that would be great is you can email my some photo's, Enjoy Vietnam.

My Aerovee is using 16 L/hr (4.2 Gal) during cruise at 2950 rpm at 95 kn (110 M/hr) with needle #2. I don't think so it running lean. The electronic ignition can be partly the cause of the high #3 CHT. Next flight I try to run it on magnetos for a while, see if the CHT stay lower during the climb.

[gammaxy]

Here's what I saw on a recent flight with outside air temperature of 75F (24C).



The plane was at about 1100 lbs (me and a friend and 10 gallons of gas). We took off and climbed at around 80-85mph. The CHTs reached a steady state between 3-5 minutes until I let my friend start flying and he allowed us to slow to below 70mph causing #3 to raise a little about 400F. I helped him trim the airplane and we gradually leveled out at 2000ft agl. The temperatures were pretty consistent from 12-17 minutes when we were flying pretty level before we started climbing again.

I have the standard-sized cooling exit (with the aluminum deflector on the leading edge) and the fence baffles. I have the top-mounted oil cooler with no hole cut in the cowl below the engine. I think I did a pretty good job sealing the baffles to the cowl, but have always felt that the side fences could be sealed better at the expense of making them more difficult to remove.

As an aside, I find it interesting that my right cylinders are consistently warmer when I climb and suspect it is due to cooling airflow being different between engine sides when climbing (maybe something to do with the propeller having a higher angle of attack on one side versus the other). Does anyone else see this?

[dtwolcott]

Those of you that are testing the CHT temperatures by shutting off one or the other ignition sources and are seeing lower temps on one or the other might need to give some consideration to the possibility that the fuel may not be burning completely do to not have both plugs firing. This might cause the CHT to be lower do to an inefficient fuel burn in the cylinders. Do you also see a slight reduction in RPM or airspeed?
Don't know but it may be a cause and effect situation.

[radfordc]

Those of you that are testing the CHT temperatures by shutting off one or the other ignition sources and are seeing lower temps on one or the other might need to give some consideration to the possibility that the fuel may not be burning completely do to not have both plugs firing. This might cause the CHT to be lower do to an inefficient fuel burn in the cylinders. Do you also see a slight reduction in RPM or airspeed?
Don't know but it may be a cause and effect situation.

Absolutely no change at all. I flew from Crossville, TN to Kansas City on just the mags and saw exactly the same performance and fuel burn as my buddy Bob. I've never seen a drop in rpm when switching between systems in the air.

[XenosN42]

Here are the numbers I experienced last Sept. in an AeroVee/XENOS flight. The outside air temp was 85 degrees on the ground & 75 at altitude. The engine had been run 153 hours.

Looks like the order of which cylinder was the hottest to coolest was the same for your aircraft and mine.




My engine temps have been very stable the last two years. Once I'm established in cruise the numbers don't move a noticeable amount.

-- Michael

[radfordc]

Those of you that are testing the CHT temperatures by shutting off one or the other ignition sources and are seeing lower temps on one or the other might need to give some consideration to the possibility that the fuel may not be burning completely do to not have both plugs firing.

I've wondered if maybe the VW engine actually operates better with just the "normal" single set of sparkplugs firing. Those Germans at Wolfsburg were pretty sharp designers back in the 30's.

[bakfly]

Here are the numbers I experienced last Sept. in an AeroVee/XENOS flight. The outside air temp was 85 degrees on the ground & 75 at altitude. The engine had been run 153 hours.

Looks like the order of which cylinder was the hottest to coolest was the same for your aircraft and mine.




My engine temps have been very stable the last two years. Once I'm established in cruise the numbers don't move a noticeable amount.

-- Michael

Interesting that you #4 CHT is so much lower the the rest, during a short 20min cruise the CHT were between #3 the highest with 350F and #2 with 315F the lowest.
The problem is not so much during cruise but more during the climb. As soon I start putting the nose up and climbing the CHT start to rise. Yesterday during the cruise I switched off the electronic ignition and I noticed an sudden increase in about 20 or 30 rpm.
I have adjusted the electronic ignition a few degrees less advanced. Will try it out today if weather permit.

Peter

[XenosN42]

Hi Peter,

It appears that you've misread the graph showing my cylinder head temperatures. The lowest is the cylinder #2. Your #2 cylinder is also the coolest. Here is a larger image of the graph.



When I look at the graphs from my flights (and I do save the data my EFIS stores from every flight) I'm more concerned with changes than absolute numbers. The raw numbers could be due to installation errors, a slightly bad CHT cable, errors in the EFIS, etc.

Now let's say that the temperature line for the #2 cylinder starts to match the other cylinders. One might say "it's still well below the maximum allowed CHT so no problem". However, the change indicates that something may be wrong and that should be investigated.

Regarding your high CHT on climb out you may want to consider testing to see if the temperature of #3 is really as high as your instrument is measuring. Harbour Freight has some inexpensive infrared thermometers that will suffice. Just fly around awhile, land, quickly take off the cowling and see if your EFIS numbers match the thermometer.

Also make sure that your mixture is rich enough during climb. Lean engines run hotter.

Good luck. At least you didn't experience what one local builder(?) and pilot(?) did on his first and only flight. ( Don't worry he walked away, but his Sonex didn't fair so well. ) Turns out that soon after take off at about 500 feet his cylinder temps were hitting 500. He quickly completed the pattern and managed to stall about 50 feet above the ground. After the first bounce the plane wing landed on a runway light which cleanly cut thru the bottom and top skins.

So why were his temps so high? A few days before that flight I was showing a few local pilots this guys Sonex, and comparing it to my XENOS. When I looked at his AeroVee I noticed that the super tin was installed on the TOP of the cylinders. So I asked him "Is this installed correctly?" He said "I think so". I pulled out my manual and pointed out the page that describes how to install the super tin. I then said "It will only take you about 2-3 hours to install it correctly. Not much time considering you spent 10 years building the plane. But if you don't do that at least take it off. It's doing more harm than good on the top".

Well guess who decided not to invest that 2-3 hours?

-- Michael

[radfordc]

Sounds like the money he spent on a DAR was wasted.