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IO550G-AP Overhaul - should I switch to an `N`?


ronr

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I am planning to have my Engine overhauled in a few months and the overhaul shop offered the option to switch to the "N" cylinders.

The engine was originally an IO550G, but had the Midwest 310 hp upgrade. The current STC (I haven't reviewed the one actually in the airplane) allows for either a "G" or "N" engine. The only relevant difference between the two engines is the cylinders, so I am told.

I have read that the "N" cylinders have better cooling, due to fin design. I have also read that they are "thinner" and maybe don't hold up as well. However, the O/H shop says there's no issue with longevity. The weight is less so I'd probably gain some useful load.

Any thoughts or experience?

Thanks

Ron

Ovation2DX

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Ron,

Sounds like you are fully informed...

I went (N) cylinders based solely on weight...

The TopProp adds a Few pounds to the nose...

The (N) cylinders look pretty cool... and remove a couple of pounds each...

Where the (N) cylinders may fall down... they have experienced more cylinder cracks, but this is from other airplane’s experience...

The (N) cylinders were all about improved cooling, using a funky different fin design...  My (G) cylinders didn’t have any difficulty staying cool... @280hp....

We could ask @M20Doc because he sees a lot of those other airplanes...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Typical (g) cylinder...  all the fins look similar to each other...

 

2A46E260-CEB8-4476-B0DD-B1AADAC4D7F5.jpeg
 


Typical (n) cylinder...

Two different fins, changed every other fin...

 

Still looking for a pic...  this one is too low res to show the fin differences... and the pic requires pushing the view button to see it...

every other fin is long, short, long, short...

They are easy to tell by looking at them in real life...

:)

-a-

 

78B4D66A-C226-4976-ACE5-1DA51ED7F0B3.webp

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4 hours ago, carusoam said:

I went (N) cylinders based solely on weight...

The TopProp adds a Few pounds to the nose...

 

Thank you for your thoughts.

I'm not going to change the prop -- that was done in the past with the 310 hp STC.  And yes, extra useful load would be attractive.  Except for #5, I haven't had any heat problems.  But #5 runs about 40°F hotter than the others, and limits my rate of climb in the summer.  (We've fine tuned the baffling in that area, also).

What I read had to do with cylinder head cracks. The OH shop said he had not had any negative experiences with the conversion. I would expect if there was excess head cracking, he would have gotten complaints; but I don't know the sample size.

Also, I think the **N** is used in the Cirrus. Maybe I'll check their board and see if there are complaints about the engine that create a pattern.

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Cirrus uses the N, that’s why I invited Doc to stop by... he has a few Brand Ci customers for reference...

Could be that brand Ci drivers don’t watch their CHTs very closely... :)

As for CHT#5 being the hot one... check your cowling inlet on that side to see if there is a pixie hole... allowing some air in around the hardware mounted there...

The pixie hole improves the difference in #5s CHT, but didn’t eliminate the challenge on my bird...

Its amazing that the CHTs are so close together...   balancing cooling air flow is tough to change...

Early on... in 1994 when the IO550 was first installed in the Ovation...  there were a bunch of oil cooler leaks... Mooney had one or two... Cirrus had a number closer to 10...

The oil cooler is an engine part and comes directly from Continental...

Best regards,

-a-

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No pixie hole. And the baffling flexible baffling is as it should be.  I have to ask my IA if he also checked the inter-cylinder baffles.

No oil cooler issues of which I am aware (except the oil temp usually runs a bit on the cool side - just below the bottom of the green).

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12 hours ago, ronr said:

I am planning to have my Engine overhauled in a few months and the overhaul shop offered the option to switch to the "N" cylinders.

The engine was originally an IO550G, but had the Midwest 310 hp upgrade. The current STC (I haven't reviewed the one actually in the airplane) allows for either a "G" or "N" engine. The only relevant difference between the two engines is the cylinders, so I am told.

I have read that the "N" cylinders have better cooling, due to fin design. I have also read that they are "thinner" and maybe don't hold up as well. However, the O/H shop says there's no issue with longevity. The weight is less so I'd probably gain some useful load.

Any thoughts or experience?

10 hours ago, Little Dipper said:

Anyone have pictures showing the difference between the G & N cylinders?

 

 

6 hours ago, ronr said:

Thank you for your thoughts.

I'm not going to change the prop -- that was done in the past with the 310 hp STC.  And yes, extra useful load would be attractive.  Except for #5, I haven't had any heat problems.  But #5 runs about 40°F hotter than the others, and limits my rate of climb in the summer.  (We've fine tuned the baffling in that area, also).

What I read had to do with cylinder head cracks. The OH shop said he had not had any negative experiences with the conversion. I would expect if there was excess head cracking, he would have gotten complaints; but I don't know the sample size.

Also, I think the **N** is used in the Cirrus. Maybe I'll check their board and see if there are complaints about the engine that create a pattern.

You're entitled to install the IO550N because you own the STC.  The engine is actually flat-rated to 335HP, but derated to 310.  It has a 2700RPM limit as opposed to 2500 in the "G".  You'd need the IO550-N8B specific model variant, as your airplane doesn't have air conditioning.  Currently, a factory reman of this type is just north of $48k, with a lead time of about 4 to 5 weeks.  I've had mine since 2014, and with operating it LOP, the only entries in my engine log have been oil changes every 25-30 hours, and annual inspections.  Zero issues, cracks...nothing.  There were 708 hours on this engine when these pictures were taken a few months ago at annual.

To @Little Dipper 's question...my IO550N below.  Bob Minnis - who worked closely with Brian Kendrick and I through the removal of my "G" and installation of my "N" sent me the sheet metal pieces to fill in the gap on cyls. #5 and #6 left by the removal of the straight-barrel cylinders.  Red circled areas show the fin taper and the added metal pieces.

Regarding #5 running hotter than the others...this is normal, and mitigated by the proper baffle seals and drilling a pixie hole.  Even during the summer, I typically find that my #5 runs about 20-25 degrees hotter than the others with the below configuration.

For @ronr, I'm in Eastport somewhat frequently, so if interested, happy to connect and we can talk more about it, including your path to engine replacement if you want.  Sent me a PM if you want.

image.thumb.png.c0259b9865a6b5d0851be41c1abee299.pngimage.png.d016a4326aa33340c0dbabed9a343755.pngimage.png.b5559012d63e4f94790e32853a29cf6a.png

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7 hours ago, ronr said:

Thank you for your thoughts.

I'm not going to change the prop -- that was done in the past with the 310 hp STC.  And yes, extra useful load would be attractive.  Except for #5, I haven't had any heat problems.  But #5 runs about 40°F hotter than the others, and limits my rate of climb in the summer.  (We've fine tuned the baffling in that area, also).

What I read had to do with cylinder head cracks. The OH shop said he had not had any negative experiences with the conversion. I would expect if there was excess head cracking, he would have gotten complaints; but I don't know the sample size.

Also, I think the **N** is used in the Cirrus. Maybe I'll check their board and see if there are complaints about the engine that create a pattern.

I found on four of my five io550’s and tsio550’s, if my number 5 got to 380, all I had to do was reduce climb until temps reduced and were trending down below 370, then resume climb and they wouldn’t exceed 380 again. 
I guess purging the heat from taxi and initial climb with reduced airflow was enough to control the temps, but that’s just a guess. 
 

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I have the IO-550-N in my M20L.  There's 1100 hrs on the engine and the cylinders are all still in good shape.  When I bought the airplane a few years ago I had the typical issue with cylinder #5.  I added a pixie hole which helped a little bit, but the main issue on mine was that the lower forward cylinder baffle wrap (the head portion, not the barrel) was not the proper geometry.  It's the same part no. as the Ovation so I'm assuming this is the issue for most of these instances as well.   There was about a 3/8 inch gap between the fins and the baffle, and the baffle did not extend down as far as the rest of the lower baffle wraps.  There's a formula for how much you should wrap the cylinder based on cylinder fin depth, and the rest of the lower baffles meet this spec precisely, just number 5 was out of spec.  I fabricated a new section of baffle to match the other cylinders, and that coupled with the pixie hole, #5 is now the second coldest cylinder.  

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12 hours ago, Schllc said:

I found on four of my five io550’s and tsio550’s, if my number 5 got to 380, all I had to do was reduce climb until temps reduced and were trending down below 370, then resume climb and they wouldn’t exceed 380 again. 
I guess purging the heat from taxi and initial climb with reduced airflow was enough to control the temps, but that’s just a guess. 
 

I'm guessing my procedure is similar

  • Monitor #5 CHT on JPI
  • If/when it gets close to 400°, nose trim down to increase IAS to around 160K
  • Once it has fallen back below about 380° or so, I can trim more nose up to an IAS around 120K, and increase the climb rate (depending on my mission profile).
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Early 550N cylinders used on Cirrus SR22 airframes were prone to cracking between the fuel injector and upper spark plug.  That problem seems to have gone away with current production cylinders.

Clarence

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