M20J cylinder temps and baffling

[Kirch56H]

So I’ve monitored my M20J- IO-360 A3B6D cylinder head temperatures for quite a while. I consistently notice that the number two cylinder runs cooler than the other three. I understand number two is out in front and gets quite a bit of air, but I know the baffling is designed to properly flow the air over all four cylinders. I’ve inspected my baffling, and it all appears to be correct. No obvious signs of air leakage around the soft baffles or air escaping between the cylinders is present as best I can tell. 
 

I have a JPI EDM900 so I know the temps are probably pretty accurate. #2 seems to run about 30-40 degrees cooler than the rest. For example- last night I flew it quick to check the temps after checking all the baffling. The OAT was 32 degrees and I flew it LOP at about 68-70% power and in level flight it stabilized out as follows:

1- 333

2- 303

3- 334

4- 336

oil temps and everything else are normal and where they should be.
 

I can’t help but notice that on the number two cylinder more of that cylinder is exposed with the way that the metal baffling is designed from the factory. The number one cylinder has a taller piece of tin in front to deflect air up and through the cowling. Has anybody else experienced this type of temp spread? If so, What did you do to fix the issue? 
 

 

[EricJ]

Mine runs that way, too.    Been that way for a long time and it runs fine, so I figure it's just what it does.    I had considered fiddling around with the baffling, but decided to just let it be that way.

[Ragsf15e]

48 minutes ago, Kirch56H said:

So I’ve monitored my M20J- IO-360 A3B6D cylinder head temperatures for quite a while. I consistently notice that the number two cylinder runs cooler than the other three. I understand number two is out in front and gets quite a bit of air, but I know the baffling is designed to properly flow the air over all four cylinders. I’ve inspected my baffling, and it all appears to be correct. No obvious signs of air leakage around the soft baffles or air escaping between the cylinders is present as best I can tell. 
 

I have a JPI EDM900 so I know the temps are probably pretty accurate. #2 seems to run about 30-40 degrees cooler than the rest. For example- last night I flew it quick to check the temps after checking all the baffling. The OAT was 32 degrees and I flew it LOP at about 68-70% power and in level flight it stabilized out as follows:

1- 333

2- 303

3- 334

4- 336

oil temps and everything else are normal and where they should be.
 

I can’t help but notice that on the number two cylinder more of that cylinder is exposed with the way that the metal baffling is designed from the factory. The number one cylinder has a taller piece of tin in front to deflect air up and through the cowling. Has anybody else experienced this type of temp spread? If so, What did you do to fix the issue? 
 

 

What “issue”?  Those are great temps and won’t hurt anything! 
 

I have an F with much sloppier airflow and see much more spread than that.  As long as they are staying cool enough, you’ll be fine.

[mike_elliott]

A J tends to have the rear baffle seal droop right in the middle allowing some air to escape without getting to cool rear cyl's. Shine a light from the mag area and check, also look at baffles for evidence of contact with cowl there

Probes should be calibrated with some boiling water prior to relying on their reading as gospel. little things like a missing or incorrect star washer installation cause reading errors.

But your IA knows all this...

[PT20J]

I would first check the instrumentation by swapping connections between probes to see if the problem follows the channel.  I know we think that the new “digital” instruments are more accurate, but it’s really only the display that is digital - it’s still an analog measurement.

I had a EDM 700 and #2 CHT consistently read about 20-30 degrees higher than the others. When the G3X EIS replaced the EDM we used the exact same probes and wiring and suddenly the #2 fell in line with the others. 

 Currently my M20J CHTs are all about the same except for #3 which reads 20 degrees hotter (all have the same JPI bayonet probes.)

[A64Pilot]

On 1/13/2024 at 1:37 PM, Ragsf15e said:

What “issue”?  Those are great temps and won’t hurt anything! 
 

I have to agree with this, an issue with getting modern digital instrumentation is we think it’s dead nuts accurate and we want precision out of a very old design engine.

I’ve been guilty myself, chasing high oil temps that weren’t really high temps, but higher than what I wanted, after spending a not insignificant amount of money and lots of time I gave up and of course it wasn’t hurting anything.

I think those are stellar temps.

Remember for ever it seems aircraft only showed you one heads temp and the gauge wasn’t even really marked very well, but it worked just fine.

[Kirch56H]

14 hours ago, A64Pilot said:

I have to agree with this, an issue with getting modern digital instrumentation is we think it’s dead nuts accurate and we want precision out of a very old design engine.

I’ve been guilty myself, chasing high oil temps that weren’t really high temps, but higher than what I wanted, after spending a not insignificant amount of money and lots of time I gave up and of course it wasn’t hurting anything.

I think those are stellar temps.

Remember for ever it seems aircraft only showed you one heads temp and the gauge wasn’t even really marked very well, but it worked just fine.

I guess it’s me “overthinking” the temps! I have a bit of OCD so I watch the numbers constantly. I know the numbers are good for a 43 y/o plane, I was just hoping I could even them out(wishful thinking). I also know it’s running great as it is currently so I’m not overly worried. 
 

modern instrumentation can certainly make you chase a problem that doesn’t really exist. 
 

I have checked all the baffling and the rubber is good and hasn’t flipped allowing air leaks and the flashlight trick shows no light leaking through indicating a source of air flow leakage. 
 

I’ll just keep flying it like this and continue to monitor for irregularities. Thanks guys! 

[A64Pilot]

1 hour ago, Kirch56H said:

I guess it’s me “overthinking” the temps! I have a bit of OCD so I watch the numbers constantly. 

We all do especially soon after installing a monitor, it’s normal.

But all motors, especially these old designs have a significant amount of variability in them, the matching injectors to even out when they all peak is a band aid way of addressing that, I say band aid because the best way is to blueprint the engine so that all cylinders are identical, but who’s got $50,000 to fix something that arguably isn’t  broken, so matching injectors is acceptable $ wise and gives an acceptable result, best bang for the buck.

See all of our cylinders aren’t making the same amount of power, some have higher compression that’s a function of combustion chamber volume and some flow better than others, so therefore even if cooling were identical there would be some spread in cyl head temps with the ones making more power running slightly hotter than the others. Heat of course follows power production if all other variables are the same.

The issue is how much difference is too much? I’d ask the engine manufacturer for an answer myself, I won’t hazard an answer myself because I think the honest answer is it depends like most things when you try to drill down to specifics.

We seemingly accept the differences in EGT as normal, I guess maybe because you can’t do anything about that, but the differences in EGT give a good clue as to the variability between cylinders. I suspect what they show is the differences in compression from one cylinder to the next, because I don’t know what else it could be, assuming ignition system is in good shape of course. it’s not mixture if we are talking about differences at peak.

Interestingly the higher the EGT, the lower the compression, and most likely the higher the EGT the lower the power output which is just opposite to what seems to be logical, all this is getting into the weeds because other than noticing it you can’t do anything about it.

 

[Fly Boomer]

1 hour ago, Kirch56H said:

I know the numbers are good for a 43 y/o plane, I was just hoping I could even them out(wishful thinking).

Mike Busch or the GAMI guys (I get confused) would say you really have 4 single-cylinder engines flying in loose formation.  It's probably impossible for them all to behave identically.

[Stephan Kablitz]

On 1/13/2024 at 6:46 PM, Kirch56H said:

I have a JPI EDM900 so I know the temps are probably pretty accurate. #2 seems to run about 30-40 degrees cooler than the rest.

Since you didn't mention it, you are positive that all you 4 JPI probes are the primaries and the probably previously existing ex-factory single CHT probe was removed? If your installer for some reason did not remove it but "piggybacked" the #2 JPI probe in the way it is done with the non-primary installations that could explain exactly the 30° that you see.

[Kirch56H]

Yeah, I checked and all the probes got swapped out for new JPI probes. 

[A64Pilot]

You can get a clue as to function when you turn on the system after the airplane has sat for a couple of days in a hangar.

I used to have an aircraft with an MVP-50, when I turned on the battery I would have 14 temp indicators showing the identical temp.

[Kirch56H]

6 hours ago, A64Pilot said:

You can get a clue as to function when you turn on the system after the airplane has sat for a couple of days in a hangar.

I used to have an aircraft with an MVP-50, when I turned on the battery I would have 14 temp indicators showing the identical temp.

Yeah, I noticed that they’re all within a degree or two when it’s cold in the morning sitting in the hangar. Based on that, I figured they are pretty accurate