FireMedic_2009

FireMedic_2009

Well Known Member
I have an RV3 and for a long time my CHT’s on cyl #3&4 have been problematic in the climb reaching 415-420 range. Once in cruise all the cylinders level out within 5-7 deg of each other around 380. I’ve leveled out to get the temps down and then climb again. Even at an airspeed in the 140 kt range climbing at 700-1000 fpm will increase cyl 3&4 into the 410 plus range. I even have pulled back the power 2450-2500 rpm to 2250-2300 range. The baffling looks to be pretty tight when I checked close to 2 yrs ago using a light behind the rear baffle an a borescope through the front inlets on cyl 1&2. I can understand if the airspeed on takeoff is around 100kts, but after leveling off cruising at 155-160 kts and climbing at 140kts to see cyl 3&4 increase to 410 doesn’t seem right. Currently I have the air to my oil cooler shut down just to get oil temp above 170 deg so no air being robbed from there. My concern is cylinders 3&4 are going to need to be overhauled before 1000 hrs. I currently have 325 hrs of tach time

Does anyone have suggestions?
Thanks
Daren
 
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Per the OEM and most resident experts you’ll find 410F isn’t an issue, but it if you wanted to reduce it a little you could check the cylinder flashing between the fins. I bought a set of needle files and cleaned up the flashing between the fins for a 10-15F improvement. My -3 would never exceed 400f with high compression and p mags.
 
I experienced higher than desired CHTs a while back and tried a number of remedies. Finally I bit the bullet and replaced my rubber baffling and sealed up leaking areas around the metal baffling. My CHTs experienced a significant decrease. Oh yea, one other thing, I fabricated air dams for both my intakes to help move more air towards the back towards cylinders 3&4. My CHTs went from a high of around 420-430 to never getting above about 370 in a climb or doing a lot of slow flight.

1707090171119.jpeg
 
I added a cooling channel behind #3 as others have described that helped that cylinder as well as cleaned out the flashing as much as possible.

Also, assuming you are carbureted, there is very often an uneven mixture distribution at WOT, which, at least in my case causes the rear cylinders to run leaner and hotter. By very slightly closing the throttle, I can alter that airflow enough to get better distribution and more even temperatures.
 
FireMedic_2009 said:
my CHT’s on cyl #3&4 have been problematic in the climb reaching 415-420 range. Once in cruise all the cylinders level out within 5-7 deg of each other around 380.
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Neither Lycoming nor Mike Busch think these temps are a problem. For what that’s worth.
 
YellowJacket RV9 said:
I added a cooling channel behind #3 as others have described that helped that cylinder as well as cleaned out the flashing as much as possible.

Also, assuming you are carbureted, there is very often an uneven mixture distribution at WOT, which, at least in my case causes the rear cylinders to run leaner and hotter. By very slightly closing the throttle, I can alter that airflow enough to get better distribution and more even temperatures.
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Yes I’m carbureted. When the temps start rising above 400 I started pulling back the throttle to slow down the temp increase but it still increases. I’ve looked for flashing between the cylinders but I don’t think there is any that I can see.
 
Tankerpilot75 said:
I experienced higher than desired CHTs a while back and tried a number of remedies. Finally I bit the bullet and replaced my rubber baffling and sealed up leaking areas around the metal baffling. My CHTs experienced a significant decrease. Oh yea, one other thing, I fabricated air dams for both my intakes to help move more air towards the back towards cylinders 3&4. My CHTs went from a high of around 420-430 to never getting above about 370 in a climb or doing a lot of slow flight.

View attachment 55464
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What do the air dams look like on your intake? I’m not sure how that would cause more air towards the back cylinders unless you mean the intake area had leaks at the very front of the intakes. My intake area is sealed up pretty good
 
This is always a fun mystery to solve since there are so many possibilities!! ;) Adding to the list, a carb that is setup too lean and/or a mixture control that doesn't move to 'full rich' can also be issues.

I didn't see mention of the CHT's on the other two cylinders, what are they reporting when the 3 & 4 are high?
 
FireMedic_2009 said:
What do the air dams look like on your intake? I’m not sure how that would cause more air towards the back cylinders unless you mean the intake area had leaks at the very front of the intakes. My intake area is sealed up pretty good
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Here’s another picture with my “air dams“ that help direct air towards the back cylinders. The CHT difference between front and back cylinders is normally around 10 degrees using theses “dams.”. YMMV of course.
1707144201123.jpeg
 
FireMedic_2009 said:
What do the air dams look like on your intake? I’m not sure how that would cause more air towards the back cylinders unless you mean the intake area had leaks at the very front of the intakes. My intake area is sealed up pretty good
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Air dams divert cooling air from the front cyl's to the rear. If your problem is imbalance between front and rear start experimenting with air damns' There is a reason that they were included in your baffling kit. Some need them and some don't, as each installation is a bit unique. Did you install the airflow assister FG pieces on the inside of the top cowl at the inlet area? Those are also important. Don't mean to offend anyone, but have seen a few planes where the builder treated them as spare parts and didn't install them.

Larry
 
YellerDaisy said:
This is always a fun mystery to solve since there are so many possibilities!! ;) Adding to the list, a carb that is setup too lean and/or a mixture control that doesn't move to 'full rich' can also be issues.

I didn't see mention of the CHT's on the other two cylinders, what are they reporting when the 3 & 4 are high?
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They are usually 10-20 deg less. Although looking at Savvy data I’ve seen where cyl 1&2 have exceeded 3&4 by a few at times during the climb into the 405 range but 3&4 end up passing them up in a minute or so and 1&2 start to decline. However once in cruise they are all around the 380 range within 4-7 deg of each other
 
LR172 brings up a valid concern. Any kind of turbulence at the D-shaped cowl inlets will create a pressure drop. Imagine a rock in a stream and how the water flows around it. Those vortices are essentially lost pressure. Van's drawings show cross-sections of how this should look at the fiberglass inlet at the top of the cowling, and these are important. Also, what about your ignition timing? Have you checked both sides recently? Even if they were set correctly, some installations will drift over time, and it can make a significant difference. If it's a standard magneto set-up and it's already at spec, you may be able to move the timing to achieve a little less advance. For instance, if it calls for 25 degrees BTDC, you may be able to go to the 22-23 degrees BTDC range. Mike Busch talks about this on one of his webinars on high CHTs, but that doesn't necessarily apply to every engine out there. If you have E-mags, they're set at TDC and it does all the advancing for you depending on conditions. The E-mags also have a jumper you can install to have less aggressive timing, which can also lower CHTs.

Lastly, check all of the baffling and inter-cylinder baffles (especially where they fit around the cylinder bases) with the top cowl off, then check the flexible baffles again with the top cowl on. Easier said than done, but ANY leaks will cause problems. It doesn't take much. If you have a bud that's got lots of experience in this area, maybe he can put another set of eyes on it.
 
Between the cylinders is not a totally correct statement. I think what these gentlemen are saying is look down through the fins between the pushrods on each cylinder or intake valve and exhaust valve. During the casting process, that area can get plugged up with metal flashing and it blocks off the passage way. I've seen some cylinders completely blocked. It doesn't take much to break through the flashing and open up the fins. Make sure you intercylinder baffling is also sucked up to the bottom of the of the fins and even RTV the gap between the engine cash. Air leaks are no different than water leaking from a bucket. ANY air leaking means it's not cooling your engine and just blowing out of the cowling. The front bottom cylinder baffling and rear cylinder should also be tight up against the cylinder fins. IF those aren't pulled together, it will spill out the side gaps and not go over the fins and out the bottom holes.
 
When I'm full throttle in climb #1 and #2 get the hottest. When I pull back the throttle a bit my #3 and #4 get the hottest. Thus, I'm able to manage temperatures in climb most days by alternating between full-throttle and slightly less than full throttle. As I understand it, this often just comes with the territory on carbed models that often have poor fuel distribution. You might experiment a bit more and see if you can replicate it like I can.
Hope this helps.
 
I have an SDS CPi2 dual electronic ignition that’s set to 25 deg. I guess I could retard it back to 22 or 23 at full power in the map table.

So I do have the air dams in front of cyl 1&2. The lower baffles between cyl 1&3, 2&4 are tight. I have looked between the fins for flashing over a yr n half ago, everything looked clear from memory. I see about taking pics next time I have the cowl off.

Thanks everyone for your input!
 
FireMedic_2009 said:
I have an SDS CPi2 dual electronic ignition that’s set to 25 deg. I guess I could retard it back to 22 or 23 at full power in the map table.

So I do have the air dams in front of cyl 1&2. The lower baffles between cyl 1&3, 2&4 are tight. I have looked between the fins for flashing over a yr n half ago, everything looked clear from memory. I see about taking pics next time I have the cowl off.

Thanks everyone for your input!
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You ABSOLUTELY SHOULD reduce the timing to say 22-23 degrees. This should be done on ALL EI's and here is why. The magneto is statically set up at 25 degrees but the plugs actually fire at 22-23 degrees. They naturally have this retardation as a function of what they are, and this is part of the "proven design". When you install an EI, it fires precisely at 25, no lag. So the actual plug firing is 2-3 degrees advanced of what Lycomig actually built them for. If you have an angle valve, they are static at 20 degrees, same again, use 17-18.

Not many people in the world have the ability to measure actual spark events, cylinder pressures and crankshaft torsional swings. When you have seen this data live on the GAMI dyno, it changes they way you think.

More agrees is not helpful at all, in fact the opposite. They only help at high altitudes with LOP mixtures and then ONLY about 2-3 degrees. On an SDS that is easy to set up. I have set up quite a few, and they work well. But you never even at low MAP and LOP need more than 26-27 on an SDS, probably 26 really.

Next issue is your fuel flow, at sea level and full RPM you should see 15-15.5GPH for a 320, or 17-17.5GPH for a 360. If you are fixed pitch scale those numbers based on RPM/2700. Like every Avstar FCU and Carby I have ever seen they all flow way too little. Get that sorted also.
 
Can someone with the air channel behind the #3 cylinder post a picture of the setup? I’m having issues with cooling #3 in my RV-4 and am exploring my options to fix the problem.
 
alpinelakespilot2000 said:
When I'm full throttle in climb #1 and #2 get the hottest. When I pull back the throttle a bit my #3 and #4 get the hottest. Thus, I'm able to manage temperatures in climb most days by alternating between full-throttle and slightly less than full throttle. As I understand it, this often just comes with the territory on carbed models that often have poor fuel distribution. You might experiment a bit more and see if you can replicate it like I can.
Hope this helps.
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Alpinelakespilot2000's post shows exactly how the carb throttle position effects mixture between the front and rear cylinders. Suspect that In the OP's case, the throttle plate top at full throttle is tipped slightly forward for one reason or another. (not quite full open) This will cause the fuel from the nozzle below to be deflected toward the front cylinders. The rear cylinders 3 and 4 will then be leaner and hotter than 1 and 2.

So, I would first make sure that the throttle linkage permits the throttle arm to be tight against the stop at full throttle. Only a degree or two of the throttle plate position will have an effect on the mixture division between front and rear cylinders at full throttle.
 
RV10inOz said:
You ABSOLUTELY SHOULD reduce the timing to say 22-23 degrees. This should be done on ALL EI's and here is why. The magneto is statically set up at 25 degrees but the plugs actually fire at 22-23 degrees. They naturally have this retardation as a function of what they are, and this is part of the "proven design". When you install an EI, it fires precisely at 25, no lag. So the actual plug firing is 2-3 degrees advanced of what Lycomig actually built them for. If you have an angle valve, they are static at 20 degrees, same again, use 17-18.

Not many people in the world have the ability to measure actual spark events, cylinder pressures and crankshaft torsional swings. When you have seen this data live on the GAMI dyno, it changes they way you think.

More agrees is not helpful at all, in fact the opposite. They only help at high altitudes with LOP mixtures and then ONLY about 2-3 degrees. On an SDS that is easy to set up. I have set up quite a few, and they work well. But you never even at low MAP and LOP need more than 26-27 on an SDS, probably 26 really.

Next issue is your fuel flow, at sea level and full RPM you should see 15-15.5GPH for a 320, or 17-17.5GPH for a 360. If you are fixed pitch scale those numbers based on RPM/2700. Like every Avstar FCU and Carby I have ever seen they all flow way too little. Get that sorted also.
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Today I adjusted the timing for 22 deg from 1100-2700 rpm, MP add/retard to 0 throughout, and set the LOP button to +6 with a max MP of 25.4”. I just saw this so I guess I’ll set the LOP to +4. I did fly the plane today and the max CHT in the climb was 405 deg. I also ran wot at about 2200’ and rpm was 27200 with about 15.5 gph getting 179 kts
 
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