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CHT climb out

dbaflyer

Well Known Member
I have 6 hours on my newly minted RV9A. It has an O-320 (carb) with a Catto 3 blade propeller. Diameter 67 Pitch 70

#4 cylinder is consistently the hottest
#3 is the next hottest
#1 is the next hottest
#2 the coolest by far

Here is an image of some data from a flight from a couple days ago. It shows RPM reaching 2470 on climb out before I reduced down a bit to lower the CHT on #4. I also lowered the nose for a shallower climb.

What I am looking for is what to do in order to raise #2 CHT in order to give #4 more cooling. I currently have the standard vans blocking dams installed on the front of #2. You would think that deflection would give more air to #4 and less to #2 but maybe that's not the case?

The IAS during this period on the chart ranged from climbing through 110 KTS and then up to around 125 KTS.

C38F5ama0TnS6gO1J_Ajn004C5TWwsraZgqtveheTENT42vvAB7PjFuimfHPLhrMNIUJFWhGiiWIlrmRbxVyYnIHquLrcKCe9xQUFiJVFURiVeD3p3Nwe1mwjyKhrY7BWeUZ_uFqBpzrK5ukdJCwOaKFnHcUL3go1L_od27xwmJ1qyGu_wz0iOshCKjpzrIVOOTk8BvCJtMFlA_Mmlg9u8ZreWUMa4FO4tnYTcmvckco_wwCiq7jz_BAjlmcjwKkURuY511rxSwmKeIdIhQnHQCo4RPNP4y1tu_WrYu2nyD49dAXUzwT4xi_tCtitgCcixR7bgOzp-B_J8gko8zVuJnoeDYehz2INi6EEyb-VnJjoMTu7kbLYPvIL3aENU9p71f2e7LMyhC8Js1nPdriu1Gjq3NjPdHh_pfZk92LKOHJSQ0BnsoQ90bWbRm6-TopL5nQHRMAnVEgpbtyGWVM8gmDfgnuruVArYaTjNonv0RitCeDYO4p_plafKAoiIeVh73uUSoSOuXFaMfG_mDHwOJlco8F9GK1gQXp56bkF_b0aFl3cai5Cz7PfGhvjtRpIk_wfuvFoLsbVwh9dJmHRiykULymXnZ_Z9_9bmmEPMMgvlnGI3oCd48eJg0S_IqjnvEmTpT9L1ewRJaU0OQcUHkcmm1cLG90-fbzRyuCBoZSBjvBIP07Ag=w2556-h1270-no
 
Look for leaks everywhere in the baffling. Almost all of us, myself included, could lower CHTs a fair amount with a few hours of baffling work.

One other thing to consider and that I have noticed as well is that with a carbureted engine, the mixture distribution, and thus CHT, depends on throttle position. In the wide open throttle setting, the mixture in the two front cylinders seems to be a good bit richer than the rear two, resulting in cooler CHTs. Pulling back just a bit on the throttle will change the air/fuel flow path resulting in a different mixture, and CHT profile. For this reason, when up high, and WOT, a lot of us pull back just a tad bit, or add carb heat, to make the air/fuel flow a bit more turbulent and better distribute among cylinders. You might find that if you crack the throttle back just the smallest amount after initial climb out, your temps will even out as well.

Chris
 
Thanks for that info. This is from a flight today at 60% power and 3,400 MSL. The CHT and EGT spread looks good once leaned out. It's almost like you say in that the front 2 cylinders are getting less fuel at full power/climb. Leaning the mixture doesn't seem to reduce the flow to Cylinder 3/4 but it does to 1 and 2 causing the temps to increase.

Maybe it's all good, just trying to get used to what's normal so that I know when to be concerned.

QC_y_KQCCXNxMkvXw_LKCl4Np6Fp62G0sBuSYesj-i_CoiUZE4OrWcnK8fMgLihQ5JERjs9pBirppIk06J2L0mahT52haoALtv5DD4by8Yw3U12NzPcel94JdbTPj66RpTXFcMCjy24p3-OQ3tcz7rB3MCDV9OIsK03xD3nIXkVAV-WM-thEUd5qfW-z5T4XtS-9u1fgFK1UxDY37R_5fw0LYI45iermJ1ixqovHzCmNsUj3nRMewL0zsnmxENsDkpwyChNQWa8PKlOJuTrs1xd24TiibnRUcp4Dmz56qbNqQG3ejcEKGuhcwu19tKq1jwIwbVoO1A8qwZEwznqdb1jgPNe8ZHA5z1Tcw8eKSFEFeXjESjh1O1Jk-jb0zW2WKB0V09Vgk5TpwA2V3nmHphsH140PJ8BfBqPZBLsQ9K1UFgbX1Xw-_L_QjHfJ78lKc2QtqHFXRjtCRuQc5tMFrk7nKtGQwJtqXYrxI8yqrQ8AJFG8x23-3mCFdHDO5KQk5ASYB4coQ_cVTAx8fpChHdB17WV6BBU8OuMc82EQaec395OgCOCgjwXfG5H3eJcTdB_b7y23v_9GzZT_JlgA976oSG7KEz4tqBBQDrXVdQg7ovALZFy-UoHjF3KzMCluXwyfdtY0xMGqtHfozAdiwNf2bsbjX4-fio1aOZn5kDWFVjMWhyVW3Q=w328-h357-no
 
Is your main jet too small?

The advice about baffling is accurate, but you may also be running lean because your main jet is too small. There are several different variants of carbs supplied with an O-320. Often the main jet is set up for a rather restrictive airbox. That means the engine is running too lean with the free-flowing Van's airbox. It is possible to buy very slightly larger main jets, or to ream/drill out what you have. There are threads on here with the details.

It is several years since I was dealing with this problem, but IIRC the standard jet size is 0.097", and some have used up to 0.104", I used 0.101". Get up to an altitude where leaning won't cause a problem, say above 6000'. Lean out watching the EGTs. Ideally there should be a 150* to 200*F rise in EGT from max rich to peak EGT. If there is no rise, or only a small rise, you may be running too lean. Be aware that absolute EGT is not important, only difference from peak. Also it is not unusual with a carbed engine for each cylinder to reach peak EGT at a different fuel flow and for the CHTs to have a spread, say 40*. If you want to bring these differences together then it may require installing fuel injection...

Overall your CHTs will reduce significantly over the first 10 to 15 hours of operation. What is your oil temperature? I eventually fitted louvres in the bottom cowl that dropped the CHTs by 40* and oil by 20*. First make sure your baffling is good and the inboard ends of the top cowl inlet lips are blocked up.

Pete

PS No images coming through for me
 
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Everything posted here is good advise. In particular I would advise no action (except the throttle adjustment) until you have 20 hours or so on the installation. At that point things should be settled in to what they will be.

An additional factor is the location of the oil cooler. On my set up (as are most) my cooler is fed air off of the back of #4 cylinder. This set up “bleeds” air flow from that cylinder. I have a door that blocks airflow to the cooler in the winter to get oil temps up. When I close the door, #4 CHT goes down by about 20 degrees. Similarly the heat muff is typically fed air off of number 3 cylinder, again bleeding airflow from that area. Once things settle out, you may keep these two factors in mind as you fine tune your CHT balance.
 
The point about jet size was something I forgot to mention. I ended up having to drill mine out Two steps to get acceptable fuel flow on takeoff. If you can log your takeoff fuel flow as well as engine power, you can compare that to the lycoming chart and make sure you are getting enough fuel.

Chris
 
I have some more data that might help. This is from a flight done at 8,500 feet flying at 2750 RPM (Full throttle), for a short period of time. This is with 39 flight hours on the airplane.

Starting values
#1 EGT=1155 CHT=377
#2 EGT=1275 CHT=382
#3 EGT=1195 CHT=377
#4 EGT=1225 CHT=390

Ending values (leaned to engine stumble then back to max RPM)

#1 EGT=1405 CHT=386
#2 EGT=1445 CHT=383
#3 EGT=1425 CHT=379
#4 EGT=1435 CHT=389

I have an oil cooler shutter that I keep closed on take off and open slightly as the oil temp starts to go over 200. Oil temps have never really been a problem

Fuel flow during this process went from 13.2 GPH to 8.3 GPH


I have been tweaking my baffling and plugging more holes which is helping, so on this flight CHT on take-off reached 416, 382, 411, 415 so now 1,3,and 4 are much more even on heat. At this point reduced power somewhat to lower the temps.
 
What is the problem?

I have some more data that might help. This is from a flight done at 8,500 feet flying at 2750 RPM (Full throttle), for a short period of time. This is with 39 flight hours on the airplane.

Starting values
#1 EGT=1155 CHT=377
#2 EGT=1275 CHT=382
#3 EGT=1195 CHT=377
#4 EGT=1225 CHT=390

Ending values (leaned to engine stumble then back to max RPM)

#1 EGT=1405 CHT=386
#2 EGT=1445 CHT=383
#3 EGT=1425 CHT=379
#4 EGT=1435 CHT=389

I have an oil cooler shutter that I keep closed on take off and open slightly as the oil temp starts to go over 200. Oil temps have never really been a problem

Fuel flow during this process went from 13.2 GPH to 8.3 GPH


I have been tweaking my baffling and plugging more holes which is helping, so on this flight CHT on take-off reached 416, 382, 411, 415 so now 1,3,and 4 are much more even on heat. At this point reduced power somewhat to lower the temps.

Isn't this a pretty good temp. range? I don't see a problem.
 
CHT on take-off reached 416, 382, 411, 415

so 3 cylinders over 400 and increasing during initial climb out. Without backing off power the temps continue to climb.

The other data at altitude is good and was presented because someone asked about EGT spread between mix fill rich and after leaned to best power.
 
CHT on take-off reached 416, 382, 411, 415

so 3 cylinders over 400 and increasing during initial climb out. Without backing off power the temps continue to climb..

What airspeed are you using for climb? Many switch to a cruise/climb once at pattern altitude.
 
CHT on take-off reached 416, 382, 411, 415

Worth the work to improve, but not a horror story. As Kyle asked, at what airspeed?

Without backing off power the temps continue to climb.

One of golden age NACA papers stated cooling demand was proportional to mass flow. Engines are air pumps, so for us dumb pilots mass flow means RPM. Constant speed guys can pull the blue knob a few notches, but fixed pitch should already be at reduced RPM in climb.

The other data at altitude is good and was presented because someone asked about EGT spread between mix fill rich and after leaned to best power.

It's plenty rich enough.
 
This was at 125 kts for first 2,500 feet with the auto pilot flying the rest at 115 kts to the 8,500 target.

I am much happier about the CHT now than I was. #2 and #4 were by far the hottest before. Now the temps are more even, but lower.

Thanks for the help all.
 
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