bruceh

bruceh

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Five flights into this Phase I exercise. Engine (Lycoming IO-320) is running great. I've been downloading the data logs from the Dynon Skyview after every flight and pouring over the graphs over on savvyanalysis.com.

It looks like cylinders #1 and #2 are consistently the hottest over the aft cylinders #3 and #4 by about 25 degrees. Cylinder #2 being usually the hottest by a few degrees and #3 being the coolest.

I have the stock air dams from Van's baffle kit in place on the front cylinders and I'm wondering what the concensus is here on how much to trim them down so I can balance the CHT's a little bit closer. Any suggestions on a starting amount to trim off? 1/4" or less?

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Should I keep the relative shape of the stock air dams or just start trimming from the top edge downward?

Overall cooling has been just fine, and the past couple of flights have been in rather hot weather.
 
In my experience the little dams at the front two cylinders need removing or cutting back a fair bit.

25dF difference is not much and quite OK but more importantly are what are your real in flight CHT's during say a 115knot climb, at takeoff, and in the cruise either ROP or LOP or both.

DanH on this site so read his posts on the topic of cooling.
 
A 1/2" trim off will get you pointed in the right direction. You will find it needs more.
 
Like you I followed Van's instructions and installed the air dams prior to first flight. Like you I started trimming to even the cylinder CHT's. Like you (most probably) I eventually removed the dams entirely. Like you, CHT's are now balanced perfectly. Just like David (RV10nOz) stated.
 
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Like Kahana said above, a half inch cut is a good first try. I did it with a cutoff wheel and a piece of sheet steel between the dam and the cylinder for protection.
 
Remove them

I have found better results with them completely removed. You can always add some cover/blockage with aluminum heat tape if needed.

Vic
 
On the recommendation of a multi RV builder, I removed mine and reinstalled with three no. 8 screws so they are easily detached an reattached. Then it was easy to cut them down a little at a time until CHT balanced to within about 10 degree spread.
 
Bruce, I had the exact same problem. After about the second flight I removed both dams, with the idea that I would fly and see what the results were, and start adding aluminum tape a little at a time until I found the right height for the dams to be.

Turns out my CHTs were quite well balanced without the dams, and so I haven't even experimented with the tape. Of course things may change as I still don't have gear leg and and wheel fairings on yet.
 
I started with no air dams and found the front cyls too cold. Added some air dams based on guesswork, and then went thru 3 iterations of trimming /retrimming until I got the CHTs perfect. Now my cruise CHTs are all within 10F. :D
 
Fuel flow check before removing metal

Have you checked your GAMI spread? IMO if you have not, it would be best to first balance (or at least check) the mixture before doing much with the baffles.
My #2 was running significantly richer than #1 when I got my FI back from rebuild making a comparison more difficult.
 
"dams"

From my own experience, what folks are calling dams should be called deflectors. In my case, a full size ?dam? ( Vans design) actually helped to cool my #2 cylinder. It was not blocking airflow, it was steering. To view them as dams is a complete oversimplification of what is actually
going on with the airflow inside the cowlingl/baffle. Each firewall forward is a little bit different. And unless we somehow meter the inside of this area, we are only guessing. So?trial and error are the order of the day. I installed nut plates so I could remove the ?dams,deflectors, whatever? and I am still tweaking them and I am close to getting temps just right.
 
jdeas said:
Have you checked your GAMI spread? IMO if you have not, it would be best to first balance (or at least check) the mixture before doing much with the baffles.
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Great point. The engine is freshly rebuilt, so I expect that the CHT's are going to be a bit high while things get broken in. Highest I've seen in climb on a very hot day was around 420F (very briefly), and down around 375F in cruise. I've pretty much been running full rich on the mixture, but I did try the LEAN function on the Skyview a couple of times.

I will hold off on trimming anything until I have a bit more time on the engine, and have made sure the spread on the injectors is close.

One thing I did notice in the data is that at cruise speed cylinder #2 is the hottest, and in slow flight (higher AOA) cylinder #1 is hottest. They swap back and forth when I speed up or slow down.
 
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