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  #1  
Old 07-01-2021, 09:28 PM
dbaflyer dbaflyer is offline
 
Join Date: Oct 2009
Location: Iowa USA
Posts: 194
Default High CHT #4 and Fuel flow

In the spring/summer of 2020 I spent a lot of time sealing up baffles and had some positive results. This is from a flight 7/12/2020 outside air 25C. FF at take off showing 14-15 gal/hr. CHT peaked around 415 but curve was pretty flat

A year later and my #4 cylinder is really getting hotter (reaching 430 quickly) as you can see by the graph. Outside air a bit warmer at 28C. FF at take off was only showing about 12-12.5 or about 1-2 gal/hr less than a year ago.

I have checked the fuel inlet screen and it was clean. I'm not sure what might be causing this reduced FF and rapid increase in CHT on #4. I'm sure you'll want more information and will provide it as I can

This is an O-320 carb with one slick mag (left) and one EFII (right). The manifold pressure is being supplied by a port on cylinder 4. The prop is a 3 blade Catto. Max speed to date was 162KTS TAS, 2750RPM, 73% pwr, 8700 DALT

I have an oil cooler shutter that is closed on take-off and opened up as Oil temps get hotter. I've also tried to tape off a hole in the back baffling of #4 used to pipe air to the magneto.

Link to a zip file containing the two flight logs.

https://drive.google.com/file/d/15O3...ew?usp=sharing
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  #2  
Old 07-02-2021, 06:59 AM
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DanH DanH is offline
 
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Earlier flight, passing through 2000 ft:

124 true 3286 DA 2450 RPM 27.4 MP CHTs 398-366-399-401

Later flight, again passing through 2000 ft:

110 true 3591 DA 2350 RPM 27.7 MP CHTs 378-391-407-419

Difference in available dynamic pressure...

124 knots 9.45" H2O
110 knots 7.43" H2O

Given 20% less cooling mass flow, higher CHT is the expected result. Same for less RPM resulting in less fuel flow.
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  #3  
Old 07-02-2021, 08:09 PM
dbaflyer dbaflyer is offline
 
Join Date: Oct 2009
Location: Iowa USA
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That would explain the difference in those two flights. Makes sense.

My options are to accept climbing at 120/125 KTS to keep CHT in check or increase cooling flow and/or increase fuel flow for a given RPM.

If there was a way to divert some air from 1 and 2 to give 3 and 4 more cooling that might be an option. I already have the full van's dams on the front of the baffling there. Any other options to force more air to the back of the engine?

Has anyone tried to force smoke or other kind of visible air through the front two intakes to watch for leaks in the baffling? I would be interested to hear any ideas for how to do that.

I have been contemplating a larger fuel jet, but wanted to do this as a last resort and I'm not really comfortable doing that by myself. Seems there are some success stories on this lowing CHT maybe at the expense of power and fuel cost.

Are there other measurements I could take to see how efficient my incoming and outgoing air flow is?

I would really like to improve this some. Not looking for 85/90 KT climbs but climbing at 100/110 seems reasonable. It's kind of disappointing to have an RV with a Vy of 85 KTS but needing 120 to keep the engine happy.
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  #4  
Old 07-02-2021, 09:18 PM
blaplante blaplante is offline
 
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Location: Southern California
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Not to mention the "impossible turn" is less possible the flatter the climb out.

No expert, but I've heard that the carbs (if stock) vary. What exact carb part number is on this engine?

Have you tried the dark hangar and flashlight (or trouble light) trick to find leaks? Have you looked for cylinder head fin flash?

One last thing, those EGT's seem pretty high for full rich, in my opinion.
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Last edited by blaplante : 07-02-2021 at 09:21 PM. Reason: add info
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  #5  
Old 07-03-2021, 06:56 AM
lr172 lr172 is offline
 
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It is somewhat common for 3/4 to run leaner than 1/2 on the carb'ed engines and your EGTs seem to show that is the case for you. The first thing I would address is your timing. The pmag's can get pretty aggressive on advance and increasing advance is directly correlated to higher CHTs. At full rich, it is best to be running 25*, as more advance adds no power increase but does raise CHTs a good amount at full rich. This is one of the large benefits of the SDS or megasquirt/megajolt systems that allow configurability in advance. On my 540, I run 23* on both igntions when ROP.

Some folks have found that pulling that throttle back a little from WOT will help improve the mixture distribution and suggest that you try it.

Larry
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Last edited by lr172 : 07-03-2021 at 07:02 AM.
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  #6  
Old 07-03-2021, 08:16 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by dbaflyer View Post
That would explain the difference in those two flights. Makes sense.

My options are to accept climbing at 120/125 KTS to keep CHT in check or increase cooling flow and/or increase fuel flow for a given RPM.
Cooling has a number of factors, but at the fundamental level it's about the available pressure difference between the upper and lower cowl volumes.

At 85 knots and 2000 ft, standard day, available dynamic pressure is 4.44 inches of water. A really good system will convert 85% or so to increased static pressure in the upper plenum, a poor system as little as 65%. So, a fellow might have as much as 3.8" or as little as 2.9" in the upper plenum.

Let's assume the lower plenum has an exit so large that it is at freestream pressure, no restriction at all.

Now look at the Lycoming cooling chart I've attached below. Assume a standard day, 60F for our purposes. Go up (red) to the 435F at 75% power line, then across to the sea level and 5000 ft curves, then down to fiind the required differential pressures. Surprise....it takes about 5" to maintain 435, and that's not at full power.

Go back and run a line (green) for an extrapolated (guessed) 400F at full throttle, and you'll find it takes about 8".

See the problem? Although the airframe does indeed have a Vy of 85 knots, there is simply not enough available freestream dynamic pressure to cool the Lycoming design at full power/low speed on a continuous basis. Sure, improvements are possible, but unless physical laws are different in your universe, Vy can only be used short term. Dynamic pressure is a function of velocity squared.

Caveat; the above considers only freestream velocity. A cowl which does a good job of harvesting the velocity of propeller outflow will result in higher upper plenum pressure at high power, low speed. That mostly means moving the enlarged inlets outboard, forward, and upward.

Ok, fueling. First thing, determine how far rich of peak EGT it is at full throttle. If 250F or more rich of peak, drilling a jet for more fuel flow is just a dirty way to make a power reduction.

Quote:
Are there other measurements I could take to see how efficient my incoming and outgoing air flow is?
Yes, if you're serious and willing to fly an accurate test program. You'll record pressures in the upper and lower plenums at three or more true airspeeds.

Quote:
I would really like to improve this some. Not looking for 85/90 KT climbs but climbing at 100/110 seems reasonable. It's kind of disappointing to have an RV with a Vy of 85 KTS but needing 120 to keep the engine happy.
If you want to climb at 100 and stay under 400, it can probably be done, but there is going to be a lot of mods.
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  #7  
Old 07-03-2021, 09:06 AM
dbaflyer dbaflyer is offline
 
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Thanks, Dan I will work on some of these suggestions, but this is usually a slow process so I'll post updates pretty slow. My goal is to be able to climb a short period at a relatively high VS and then bring the speed up. No reason to sustain 85.

One poster asked about my carburetor. I have a Marvel Schebler 10-3678-032 I came across a service bulletin SB-22 for this exact model. I contact them and this was the reply:
Service bulletin SB-22 was written specifically for 10-3678-32 carburetors installed on CubCrafters CC11-160 Carbon Cub aircraft equipped with CC340 model engines.
The 47-862 nozzle introduced in this bulletin has not been tested or authorized for use in any other aircraft.

That being said, the 47-862 nozzle does not provide any more fuel at full throttle than the 47-564 nozzle currently installed in the 10-3678-32 carburetors. The intention with the design change was to provide better vaporization and distribution of the fuel/air mixture to the cylinders.
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  #8  
Old 07-04-2021, 09:04 AM
dbaflyer dbaflyer is offline
 
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Has anyone covered this area with positive results. Seems like a pretty large area for air to bypass the fins
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  #9  
Old 07-04-2021, 09:08 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by dbaflyer View Post
Has anyone covered this area with positive results. Seems like a pretty large area for air to bypass the fins
If you have properly sealed up the gaps between the inter-cyl baffles and the cyl's / case, there will be no air flowing in that area; It will be stagnant air. Many mistakenly skip this step.

Larry
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  #10  
Old 07-05-2021, 11:08 AM
dbaflyer dbaflyer is offline
 
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Properly sealed inner-cylinder baffles, well mine weren't sealed very well. I was able to RTV the front cylinders pretty well. Can't really get to the rear ones.

I also spent about 2 hours filing the cooling fins on #4 with a file. Someone should invent a tool to do this and you'll make big money. Could probably get them a bit better, but not with the files I have.

I also RTV'd the sides of the Van's supplied baffling that wraps around the cylinder fins on the front two cylinders but again the back two are not easy to get at on the side nearest the center of the engine. This keeps the air flowing round the fins instead of leaking out the side.

This is just one run but it seems to have helped. OAT 25.5C

112 true 3585 DA 2350 RPM 27.6 MP CHTs 382-372-399-403
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