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Front Cylinder Air Dam Question

rvanstory

Well Known Member
My RV-10 has always had it's #6 cylinder as the hottest on climbout (I know this is normal). It's usually the only one to get above 400. I had previously cut both air dams off for cooling of front cylinders. My right bank is fine (Cyl. #'s 1, 3 & 5). But I was curious if adding the dam back onto the left bank (#'s 2, 4 & 6) may help #6 cooling. I realize #2 will get a little hotter, but if it cools #6 more, I'm ok with that. Here's a screenshot of engine data on a 100 + degree day in south TX. All cylinders below 400, except #6 which got to 416. Cruise temps are all 380 and below.

Has anyone added the air dam back AFTER taking it off? Is so, what difference did it make for you?
 

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You can experiment with aluminum foil tape. On my O-360 RV-4, I have pretty high tape on the front, and it makes a big difference. I got my cylinders within 10 degrees, (they were almost 50 degrees difference before) and for me, blocking up the front definitely cooled down the back!
 
Not a direct answer to your question but I’ve been adding aluminum tape to raise the air dam by 1/4” at a time on my -10. I’m still breaking in the engine, so my temp changes may not be relevant. But it is easy to add and remove a piece or two or of tape and watch the results.
 
I would experiment. The whole purpose of the **** is to move cooling air from the front cylinder to the middle / rear cylinder. #4 is pretty cool, so could **** up that area a bit as well. My 3/4 are quite a bit cooler than the others and plan to experiment with that.

Larry
 
You can experiment with aluminum foil tape. On my O-360 RV-4, I have pretty high tape on the front, and it makes a big difference. I got my cylinders within 10 degrees, (they were almost 50 degrees difference before) and for me, blocking up the front definitely cooled down the back!

Thank you Tom. Tape is great idea. Just the advice I was looking for. Can you tell me this.... did your front CHT rise in relation to the back CHT cooling down? Did you see a direct correlation of lowering back CHT by 1 degree, raised the front CHT by one degree? Was it an even swap?
 
Thank you Tom. Tape is great idea. Just the advice I was looking for. Can you tell me this.... did your front CHT rise in relation to the back CHT cooling down? Did you see a direct correlation of lowering back CHT by 1 degree, raised the front CHT by one degree? Was it an even swap?

I have about 2 inches of tape up the front. I don’t remember exactly the relationship of warming the front and cooling the back, I could go back and look at my data though, but it seemed to even out the temps for sure. There’s also a difference between climbing CHT spread and cruising CHT spread, so analyze the data in both regimes.
 
I have the opposite issue - the front is hotter than the back. I have removed the dams which has made a difference. #2 CHT is 20 to 40 degrees hotter than others. #1 is fine, close enough. Has anyone played with the shape of the cowl ramp to change the ratio of air front to back?

Or other suggestions?

Thanks
 
Not bad

Those numbers aren't too bad at all.
Your CHT spread is very close and your oil temp is not out of line either.
The fact that your #6 is the hottest by only a few degrees would not be a concern to me.
Rather than concentrate on #6, I would find a way to bring all your cylinders down 20degrees or so.
For comparison, my air dams were left in place and no particular modification was done to the baffling system. There are no leaks or gaps anywhere in my system and even in 100 degree weather I see the hottest cylinder reaching perhaps 370 and running around 330 in cruise.
I think that people look at little gaps and leaks and by itself it does not seem like a big deal. The bigger problem is that it pressurizes the lower cowl and thereby reduces the pressure differential between the upper and lower cowl and as a result the overall cooling efficiency.
 
I have the opposite issue - the front is hotter than the back. I have removed the dams which has made a difference. #2 CHT is 20 to 40 degrees hotter than others. #1 is fine, close enough. Has anyone played with the shape of the cowl ramp to change the ratio of air front to back?

Or other suggestions?

Thanks

Mine exactly the same.
I’ve look, scratched my head but no fix yet.
Looks like a trip to the RV10 isle at Lowe’s to pick up some aluminum tape and start a trial and error.
 
What is your GAMI spread? it might be that #6 is a bit leaner, and a 0.005" increase in injector size on that cylinder could make it very happy. Don Rivera at Airflow Performance is very helpful with changing injector size.

This, of course assumes you have an FI engine. I didn't see mention of that either way.
 
What is your GAMI spread? it might be that #6 is a bit leaner, and a 0.005" increase in injector size on that cylinder could make it very happy. Don Rivera at Airflow Performance is very helpful with changing injector size.

This, of course assumes you have an FI engine. I didn't see mention of that either way.

That is almost a 20% increase in size over the .028 that Don recommends for the 540. I think you meant .0005"

OP listed climb temps and we can probably assume he is 200-250 ROP in that regime. One cyl running a hair leaner at those levels will NOT account for the size of CHT spread seen in the chart. If 5 cyl are 250-300 ROP and #6 is 100 ROP (best power and highest CHT), maybe. But that is a lot more than .0005 in injector size.
 
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My experience

I've just gone through this process with my new 7 I built, almost done with Phase I. Here's how I did it. Superior IO-360 with a Whirlwind RV-200 prop. My CHT's were all high all the time, and they would go over 400 even cruising if I brought the power up high. Mind it's summer and hot as **** here in Oklahoma. On a particularly hot day I had to reduce power shortly after takeoff just to keep the front cylinders below 430. During the build a tech counselor advised me to install a washer between #3 and the rear baffle, which left a, well, washer-thickness gap back there.

So, high CHTs most of the time, with #1 and 2 higher than 3 and 4, preventing me from using high power. But I wanted to balance the fuel flows before I did anything with cooling because I wanted to make sure any cooling changes I made were due to airflow issues, not mixture differences. I wasn't exceeding any limits so I wasn't worried about hurting my engine - I just was power-limited. Props to Kyle at Airflow Performance; He spent a lot of time with me on the phone and over email, and long story short, I went from a GAMI spread of 1.8 to 0.1 with Kyle's help. I owe him lunch if I ever meet him. His advice was educational for me. I can cruise LOP smoothly if I want to and my ROP deltas are generally within about 20-30 degrees at most.

So, fuel flow balanced, I started looking at my baffle seals and baffles. I used a scope with the cowl on to see how the seals sat against the inside of the cowl. Found three slight buckles that looked like they might be leaking. Fixed those by adding baffle seal "doublers" to those sections to force the original seal flat against the cowl. No difference in CHTs. The upper seals just behind the prop were a mess. I trimmed and added a doubler to the right side. Still no difference. I retarded the dual p-mag timing 5 degrees (running the B curve). Surprisingly, no difference. Because 1 and 2 were hotter, I trimmed the front air dams down about 3/4" each. Only a very slight difference - the heat "moved" rearward a little. I checked the wrap-around parts of the baffles on the lower forward and rear faces of the fins. The left side had a gap so I tightened up the tensioner to force the wrap-around much closer to the fins. Still no difference. I used high-temp silicon on some leaky areas. Still no noticeable difference.

Finally I looked more closely at the upper front seals behind the prop. Wish I had taken pictures. Maybe I'll post them later if someone wants to see. The very front edges of those seals on both sides, behind the upper inboard sides of the cooling inlets were not pressing tightly against the upper inside contour of the cowl (that I built up with foam and glass. Ramps were installed and sealed per plans at build time). This probably allowed a significant leak. During flight, it's possible that the airflow was causing the front edges of those upper front seals to pry even farther away from the upper contour of the cowl, allowing a much bigger leak. So I came up with a highly technical solution: I installed another rivet on each side of the seal above the forward-most rivets, that caused the seals to hug tightly against the upper inside contour, closing the little triangular opening at the forward edges of the seals. At the same time, I replaced the AN-960 spacer with an L version between #3 and the right rear baffle, halving the gap (I now know about the "hammer relief" method of allowing cooling air by the zero-fin-depth section at the back of Lycoming cylinders, but I haven't done it yet). Also, because 1 and 2 were still warmer than the back, I trimmed the air dams down to approximately half their original height. I did all three of these things before the next flight so I don't know what made the biggest difference.

With all those changes, my CHTs are about 30 degrees lower. I can run max power, best power mixture at 6500 feet and my CHTs are around 370-385 at most on a hot day (today, in fact). TAS 173 knots. Before, I couldn't keep them below 400+ in those conditions. My engine manual says normal operating CHT is 360-385. I'm squarely in that range now at high cruise. At lower power and peak EGT I'm below 360, cruising at 153-ish knots true. I can hold takeoff power now, etc, etc. It's totally fixed my cooling problem and my fuel flows and CHTs are tightly balanced. I'm almost worried that in the winter I'll have trouble keeping the cylinders warm enough at low power. We'll see.

Hope this helps someone.
 
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