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Go Back   VAF Forums > Model Specific > RV-9/9A
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  #1  
Old 10-30-2022, 07:26 PM
Flyer2017 Flyer2017 is offline
 
Join Date: Mar 2017
Location: Providence, UT
Posts: 62
Default 0-320 overheating on first take off

We have a 9A made in 2003. We bought it in 2017. It has always had CHTs in the low 400 range on first take off at our 4600 ft airport. We added a Surefly mag replacing the right mag last year. Now, instead of CHTs in the 400-410 range, we are getting CHTs in the 415 - 430 range.
I know part of that rise is the Surefly mag that has a hotter and longer spark and at WOT has the spark timing advanced. The CHTs seem to come down into the 370 - 390 range after we pull back the throttle and/or after the vernatherm lets the oil into the oil cooler. The vernatherm seems to open at about 180 - 185 F. We are getting the higher CHTs at 60 F ambient temps. In summer, it's worse.

I plan to replace the 20 year old baffling around the engine and can see on the inside of the upper cowling the dirt marks where air is escaping.

Is there a lower temperature vernatherm that would allow oil through the cooler at a lower temperature? Or is there an adjustable one? I only see one for Lycoming engines online.

I discussed going to a different jet on the MS MA4-SPA 10-5217 carb which maxes out at 12 - 13 gallons/hour. My A&P thinks if I go to the next larger size jet (actually a rebuilt carb) I could get 13.5 to 15 gallons/hour at WOT and use the extra gas for cooling.

I've read most of the discussions here, including the ones that advise to modify the metal baffling to force air over fins that may be starved for cooling air now.

Any advice on what to do first?
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  #2  
Old 10-30-2022, 07:49 PM
D-Dubya D-Dubya is offline
 
Join Date: May 2009
Location: Beaumont, Texas
Posts: 237
Default

It sounds like the vernatherm is doing what it's supposed to do. The oil IS cooler when the engine isn't warmed up, so don't fix it if it isn't broken.

I don't know enough about the Surefly system to comment on what that might be doing, but advanced timing will raise CHTs.

Regarding the baffling & seals, you've acknowledged that there's opportunity for improvement--so I would first work that out before you start changing other components. If it's twenty years old, the seals have probably lost a lot of flexibility and you could be make significant gains with relatively little cost and effort.

As others have said on this forum many times, every little crack, gap, or leak that's not pushing air past a cooling fin is wasted air. I thought I originally had my cylinders sealed up pretty well, but my EAA Tech Counselor and other RV owners have proved otherwise. Get a flashlight and see what you can see from top to bottom. Don't assume the original owner had it perfect. If you're not sure what to look for, get someone who does. An hour or so with a tube of red RTV can make a big difference.
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  #3  
Old 10-30-2022, 08:05 PM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 6,078
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Hotter/longer spark duration will not affect CHT's. Spark timing (when it occurs, not how long) WILL affect CHT's, as will mixture.
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  #4  
Old 10-31-2022, 12:40 AM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
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Suggest you try one change at a time & test to gauge each result.

1- You mention CHT temps are slightly higher after Surefly was installed. I suggest you start here. Retard the Surefly base timing by 2 degrees & test. Repeat with another 2 degrees Retart & again test. Stop adding Retard when you notice your ‘mag drop rpm’ is equal between the mag & Surefly during your run up check.

2- Tackle the obvious baffle seal issues next. There is a recent thread that suggests ideas on improving cooling effectiveness, as well many other threads on this subject. This should have a definite effect on CHT in both climb & cruise.

3- I wouldn’t get into carb jetting until you have exhausted all avenues of the 2 points above.

4- Last thing I’d look at would be the oil system, unless it is running near 250 continuously during climb & cruise. Fix 1 & 2 first
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Last edited by Ralph Inkster : 10-31-2022 at 12:51 AM.
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  #5  
Old 10-31-2022, 12:52 AM
gasman gasman is online now
 
Join Date: Mar 2007
Location: Sonoma County
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"An hour or so with a tube of red RTV can make a big difference."

But, it's soo ugly..... Use clear RTV. Easy to find for this application.
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  #6  
Old 10-31-2022, 01:28 AM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 8,672
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Iíd suggest first turning the timing advance off, back to fixed timing. If CHTís return to normal then you know the issue is the timing advance, and you can decide what to do next. Run richer (are you already taking off full rich?), use a higher airspeed on climb to aid cooling, etc.
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  #7  
Old 10-31-2022, 07:33 AM
sailvi767 sailvi767 is offline
 
Join Date: Sep 2010
Location: Charlotte NC
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The Surefly should not be advancing the timing at all during takeoff and climb at full power until you reach a altitude where the MAP drops below 25. I would as mentioned try the 23 degree basemap. Changing it however requires removing the Surefly to access the dip switches.
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  #8  
Old 10-31-2022, 07:49 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 8,034
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Much has been advised. If the surefly was set up for a flat 25*, like the mag, there is a good chance there was an error in setting it's base timing and the increased advance is causing the excess heat.

Before enlarging the jet, go see how far ROP you are. Go to 1000' AGL and WOT with full rich and observe highest EGT. Then lean to peak EGT and stop test/compare EGTs. You shouldn't need more than around 200-250* difference. With my 320, 12.5 GPH is plenty of fuel for 4500 MSL. When I have taken off from Denver, I had to lean a good bit and bet I was around 12, probably less. Probably maxing at 85% power there.

It seems clear that the added CHT is from the advance added with the surefly and 400 at TO is not that bad. If you want better and have a good ROP EGT spread, I would start with remediation of the baffling instead of throwing more fuel at it to cool. With a 25* advance, I can climb well under 400 CHT and I do not climb at full rich. I lean to around 100-150* ROP. Baffling flaws can make significant reductions in cooling efficiency.

Larry
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Last edited by lr172 : 10-31-2022 at 08:03 AM.
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  #9  
Old 10-31-2022, 10:28 AM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
Posts: 551
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You don't mention the outside air temp when your CHT's are 400+
That makes a difference.
There was a guy on here getting 400+ CHTs on a 75F day. Clearly has big issues. We gave a bunch of advice... and never heard from him again.

But if your CHTs are 400+ on climb out on a 105F day... that's not so unusual. Could certainly be improved, but it doesn't indicate really big issues.

FACTS folks! Give us the data!
And if something helps... help others by sharing what you figured out.

Thank you.
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  #10  
Old 10-31-2022, 11:01 AM
Southern Pete Southern Pete is offline
 
Join Date: Jul 2015
Location: England
Posts: 231
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Agree you should figure out where your engine is mixture wise first. I would go to 6000 ft and slowly lean off. What is the EGT rise from full rich to max EGT? If it is less than 250F then you are running too lean and a richer jet may well help. If the EGT spread is already 250F then something else is the problem. I would look at baffling, especially the gap at the ends of the ramps at the front of the cowl.
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