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Pindan

How to balance CHT on a 6 cylinder Jab

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Hiya

 

I know this has been cover extensively but i was hoping someone might be able to point me in the right direction.

 

i had a runway incursion a couple of years ago in my 230 which damaged the couling,nose wheel and undercarriage of the aircraft. I got it all fixed and put back together but now i have major differences in 2 of my CHT's.

1 to 4 all sit in the range of about high 120's C to mid 140'sC but 5 and 6 remain at around 90C. Prior to the accident they were all within 15C of each other so i think something has changed because of the accident. there is a skirt fitted to the bottom of the couling which has been bent a little which may be causing this difference. THe fact that it is only the back 2 cylinders that have changed makes me think it is a common issue to both cylinders and not a change within the air cooling ducts.

Does anyone have any ideas where to start.

 

Thanks Paul

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As it started when you did repair work it would most likely be due to that work. You need to look at the air flow around all the engine especially for a higher flow around 5 and 6, or maybe a tightening of the flow through the finned areas.

If you have EGT you could do a check to see if there is any correlation between the cylinders. To me it seems unusual for the furthest cylinders from the intake are the coolest, unless you are in pusher configuration.

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Try interchanging the probes between #4 and #6 , and between #3 and #5 , (if lengths allow) and note temps ...... Bob 

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1 hour ago, Yenn said:

As it started when you did repair work it would most likely be due to that work. You need to look at the air flow around all the engine especially for a higher flow around 5 and 6, or maybe a tightening of the flow through the finned areas.

If you have EGT you could do a check to see if there is any correlation between the cylinders. To me it seems unusual for the furthest cylinders from the intake are the coolest, unless you are in pusher configuration.

i have EGT and they are all pretty similar as they were before the repair and its tractor in a 230 jab except when i put it  in reverse...hehe

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45 minutes ago, biggles said:

Try interchanging the probes between #4 and #6 , and between #3 and #5 , (if lengths allow) and note temps ...... Bob 

thats a good idea to check the probes, i didnt think of that, thanks

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My 230b No6 runs 248F/118C ,i  thought that was cool but 90C,i would have thought impossible for an air cooled motor,good idea to change over those probes.

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5 hours ago, Thruster88 said:

But hard to point it at the rear cylinders while it’s flying. 

 

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Posted (edited)
42 minutes ago, Jaba-who said:

But hard to point it at the rear cylinders while it’s flying. 

 

Yes it is, I would suggest doing it after a ground run, cowl on then off, two people. It is only to check the CHT probes. Is that cylinder running cool or is the probe dead.

Edited by Thruster88

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1 hour ago, Thruster88 said:

Yes it is, I would suggest doing it after a ground run, cowl on then off, two people. It is only to check the CHT probes. Is that cylinder running cool or is the probe dead.

The problem is that probes can go “out of linear” at times. Usually just before they fail properly. 

 

The probe may be close to the real temp at lower ( non operating temps) and then go significantly out as the temps rise. 

As failures tend to be high resistance failures the temp error is usually the opposite - with the temp going high rather than low. But I have had a couple over the years do the opposite and read low. Replaced the probes and got back to real temps again. 

 

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You really need to check how the probes are attatched as well.

If they are the old way, a ring around a randomly chosen spark plug, there is huge margin for error.

The new Jabiru method not much better, a ring lug attached to a screw on the surface of the head.

Proper way is thermocouple junction entirely "down the hole", immersed in heat conducting paste, covered over by silicone RTV or similar.

If not an instrumentation error, then maybe you need to increase the baffles in the air ducts per jab installation instructions.

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I have the old engine where the cht was that washer under the no4 plug.  So I made senders by getting some thermocouple wire ( careful, there are different sorts ) , twisting the ends and putting the ends into a small length of fine copper tube, along with a length of tie-wire. The tube was crimped down ( hammered) and the tie wire used to hold the sender end down at the base of the fins. A dob of silicon over the top to seal things and keep the sender at the metal temp.

So far ( 2 or 3 years)  this has all worked ok. I have an Arduino based setup ( thanks to Jab7252) which displays the 4 cht's to a degree.

I agree with the good advice you have got here.  The first thing is to check the instrumentation,  only then start doing things with the airflow.

   

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