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Surefly + Powerflow Pirep, #4 CHT mystery in M20C


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I installed a Surefly left mag and a Powerflow exhaust in fairly rapid succession on my C model, and I thought I’d share the experience and maybe get some help.

 

The good:

-Starts are ridiculously easy (Surefly benefit)

-Can lean a bit more at lower power and save fuel, even on a carb’d bird (Surefly benefit)

-Takeoff distance and climb rate improve dramatically. Yesterday I had no trouble maintaining >1000 fpm climb rates on an 80+ degree day in South Carolina, even while pitching for 120+ mph to control CHTs and reducing RPM slightly - more on that below.  Never seen that before. (Powerflow benefit)

-Clearly perceptible improvement in cruise performance. At 7000-9000ft, my previously 145-148kt plane has now become a very solid 150-152kt performer. (I think it’s mainly the Powerflow, with maybe some contribution from Surefly timing advance in cruise)

-CHTs stay lower during ground runs on a hot day (Powerflow benefit)

 

The bad:

-The Powerflow is louder for sure, but that ain’t the real problem…

 

The ugly:

- As long as I’ve had my plane, #4 has been hottest in climb, #2 hottest in cruise, with #4 never having been an issue in cruise.  On the hottest summer day, #4 might hit 430s initially but icame down quickly during the climb,. Generally CHTs had been manageable.

-After the Surefly install, there was no perceptible difference in CHTs or performance.  

-After the Powerflow install, I saw a major difference in CHTs. #4 is MUCH hotter in climb and also the hottest in cruise.  #1 and #3 run a tad cooler if anything, and #2 hasn’t changed behavior.   On a hot day, #4 rapidly rose to 450 and wanted to stay close to there despite my best efforts to limit pitch, speed up, cut rpm slightly.  When it finally stared to come down around 5000 feet to about 440, I think the timing advance on the Surefly kicked in and kept it from declining further. And in cruise at 9000ft with a 15C OAT, I was having to run >10gph and reduce rpm together to keep #4 under 400. 

(But even at 21.8” and 2300 rpm, I was still zipping along around 148kt TAS. Never seen that before!)

 

Troubleshooting to date:

-Inspected baffle carefully – I bet it’s way tighter than your average C model doghouse.

-Timing checked and fixed – it’s not that.

-I thought it might be an exhaust leak blowing on #4 CHT probe – nope. 

-No change in fuel flow- I’ve always gotten in the 17-18gph at full power, and that’s still the case.

-I doubt it’s a CHT probe issue.  I have a JPI EDM900 with all 4 CHT probes in the factory position.

 

Question:

-What’s up with my #4 cylinder now and how do I fix it??   Is it the extra power from the new exhaust?  Then why aren’t all cylinders affected?  And I thought the Powerflow exhaust was supposed to give you lower CHTs by evacuating exhaust gas from the cylinder more efficiently – I think I see some evidence of that on the ground at least.  The isolated effect on #4 seems like a baffle leak issue to me- it's similar to when I've had bigs leaks in the past, but I really don’t see a gap now to explain it.  The timing advance from the Surefly may add a little to CHTs at altitude, but it doesn’t explain #4’s behavior on initial climb, and I don’t see its effect on the other cylinder CHTs. 

 

 

 

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I have a similar problem with a very similar set up. I have a M20C and I installed a power flow exhaust and a Electroair ignition.

My CHT on #3 is similar to your #4. Around 430 on take off unless I really manage the climb proactively.

I tried everything, baffles, carburetor, cowling, etc. I don’t want to discourage you, but I was not very successful. It seems that the problem is a design issue. So the way to address the problem is through technic.


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11 hours ago, Oscar Avalle said:

I have a similar problem with a very similar set up. I have a M20C and I installed a power flow exhaust and a Electroair ignition.

My CHT on #3 is similar to your #4. Around 430 on take off unless I really manage the climb proactively.

I tried everything, baffles, carburetor, cowling, etc. I don’t want to discourage you, but I was not very successful. It seems that the problem is a design issue. So the way to address the problem is through technic.


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So I have an idea that's probably wrong, but may be worth mentioning.  In addition to the two vents that blow on the mags, there's the large hole in the back of the doghouse that ducts air to the muffler shroud for cabin and carb heat.  When those are off, there's a bypass to dump all the heated air overboard, and so there's always a substantial leak from the doghouse in flight regardless.  Is it possible that the Powerflow allows much higher flow of air through that heat exchanger shroud than my old Knisely exhaust did?  The difference in design suggests that might be the case.  The only other time I've seen this same pattern of CHTs during operation was  when the spark plug cover door on the right side of my doghouse partly came off in flight.  That leak may have caused a similar airflow redistribution.  

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Nice data collection, Dev!

1) Even though you don’t have fuel injectors...

You can still do a Gami spread calculation...

This will help identify if one cylinder is working harder / burning more fuel than the others...

 

2) Reports from Surefly users are starting to indicate... the stronger spark initiates the fuel burn better than the standard mag... with a similar result as increasing the ignition more BTDC... the result... better power, hotter CHTs...

 

3) If the timing shifts... and you have hot CHTs already... it is going to be more challenging to cool things off...

 

4) Great observation on where the pressure at the top of the dog house is allowed to go...  I know a mechanic that could come up with a better solution for carb heat for the M20C... if the pressurized air is allowed to escape the way you described... it is worth harnessing in a better way...

5) With the two improvements you have added to your plane... better breathing, better ignition... it may be your only recourse to add better FF...  :) going towards 20gph is similar to what a Screamin’ Eagle has for FF at 2700 rpm and a second set of jugs...

6) Get a pic of the air hose leaving the dog house... post it so David the cowl guy can see it... there may be better places to take that air from... the generator cooling hose is no longer used... leaving an interesting hole on the front...

7) a What timing does your M20C use?

Some Mooney timing...

  • 20 DBTDC
  • 22 DBTDC
  • 25 DBTDC

PP thoughts only, not a mechanic...

Best regards,

-a-

 

 

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1 hour ago, carusoam said:

Nice data collection, Dev!

1) Even though you don’t have fuel injectors...

You can still do a Gami spread calculation...

This will help identify if one cylinder is working harder / burning more fuel than the others...

 

2) Reports from Surefly users are starting to indicate... the stronger spark initiates the fuel burn better than the standard mag... with a similar result as increasing the ignition more BTDC... the result... better power, hotter CHTs...

 

3) If the timing shifts... and you have hot CHTs already... it is going to be more challenging to cool things off...

 

4) Great observation on where the pressure at the top of the dog house is allowed to go...  I know a mechanic that could come up with a better solution for carb heat for the M20C... if the pressurized air is allowed to escape the way you described... it is worth harnessing in a better way...

5) With the two improvements you have added to your plane... better breathing, better ignition... it may be your only recourse to add better FF...  :) going towards 20gph is similar to what a Screamin’ Eagle has for FF at 2700 rpm and a second set of jugs...

6) Get a pic of the air hose leaving the dog house... post it so David the cowl guy can see it... there may be better places to take that air from... the generator cooling hose is no longer used... leaving an interesting hole on the front...

7) a What timing does your M20C use?

Some Mooney timing...

  • 20 DBTDC
  • 22 DBTDC
  • 25 DBTDC

PP thoughts only, not a mechanic...

Best regards,

-a-

 

 

The C's O-360-A1D is timed to 25 DBTDC. Retiming to 20 would help with CHTs but give up performance I just gained, and I'm not sure it's legal - at least for the Surefly the DIP switches are specific to the STC. I'm sure I could set it to fixed timing though if the issue got unmanageable at altitude. It's clearly not the cause for a hot #4 in initial climb though.  

it would be great to plug the air intake in the back of the copilot side of the doghouse and have a separate intake.  I can envision how that creates a low pressure gradient toward 1&3, preventing 2&4 from getting as much airflow pushed around the fins.  That hole for the prior generator cooler (which I've RTV'd shut) is TINY though.  

I could imagine partially constricting the hose that dumps warm air from the heat exchanger shroud without overheating anything - I imagine that might be the easiest way to address the issue if I'm right, without giving up carb heat and cabin heat efficacy I'm gonna call Powerflow on Monday and see what they think.  Last time I called for help after an initial sloppy install by my former shop, I got the CEO picking on the customer service line.  He was super helpful. The exhaust has FAR exceeded my  expectations for takeoff and climb performance, and the advertised speed gain in cruise is legit (at least for the C model).

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Rerouting air to the heat system...

Has a few considerations...

1) volume of air... so... if the generator cooling hole works... that is just the location... a larger hole there, may be required... similar in size (or lager) to the ram air, which is intake only...

2) there is some pre-heating of the air going across the fins... so taking air from a different place may not be pre-heated enough...

3) Final heating of the air depends on the heat muff of the Power Flow....

4) I imagined for a moment...

  • A larger hose to deliver enough air...
  • Routing below the engine.... to not interfere with airflow at the top of the dog house....
  • Made out of a metal duct to allow some heat exchange for preheating of the air... doubles as a floor to the dog house, but doesn’t interfere with the air escaping out the cowl flaps....
  • Completed using the usual scat hose to connect to the heat system and Alt carb air...
  • Then it all has to fit...

5) I think it may have enough merit to discuss with David when he starts doing the layout for the cowl for the M20C... a form of pre-research?

6) Part of the performance numbers are

  • better speed... from airflow around the outside of the cowl...
  • Better CHTs... from airflow inside the cowling....

7) Looks like some opportunity for the M20C.  Kind of copies some of the knowledge of air handling in the M20R...

8) Be extra careful probing around the back of the dog house hole... it looks like it can be the source of foreign objects entering the carb... while the carb heat is on... wouldn’t want a piece of foam to fall apart and leave pieces in the carb heat system... all sponges need to be accounted for after the surgery...   :)

PP thoughts only, not a plane designer...

Best regards,

-a-

 

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4 hours ago, tmo said:

Didn't we do a similar thread not too long ago, that got more or less solved by different jetting in the carb (for more fuel flow)?

I think in that case the fuel flow at WOT was on the low end - not true for me.  Plus I already have the higher fuel flow version of the carb, not the low flow one that can be modified to match.

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I’ve never seen the PF installation up close... Pic taken from the PF website.  The tube on the right side coming out of the muffler shroud is the cooling air exhaust?  How does that work as compared to the linkage closing the exit valve on the original installation?

tom 

418D8932-D8D0-4510-9347-CC2D77BE530C.png

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12 hours ago, DXB said:

So I have an idea that's probably wrong, but may be worth mentioning.  In addition to the two vents that blow on the mags, there's the large hole in the back of the doghouse that ducts air to the muffler shroud for cabin and carb heat.  When those are off, there's a bypass to dump all the heated air overboard, and so there's always a substantial leak from the doghouse in flight regardless.  Is it possible that the Powerflow allows much higher flow of air through that heat exchanger shroud than my old Knisely exhaust did?  The difference in design suggests that might be the case.  The only other time I've seen this same pattern of CHTs during operation was  when the spark plug cover door on the right side of my doghouse partly came off in flight.  That leak may have caused a similar airflow redistribution.  

Interesting proposition. So that get us back to the idea that we have to live with it. Or can we do something about it? I also was suspicious that this issue was not so much an issue of the ignition, but more related to the Power Flow exhaust installation. 

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Just now, Oscar Avalle said:

Interesting proposition. So that get us back to the idea that we have to live with it. Or can we do something about it? I also was suspicious that this issue was not so much an issue of the ignition, but more related to the Power Flow exhaust installation. 

Just left a message with Darren Tilman - President at Powerflow - to discuss.  I'd think creating a constriction of some kind at the end of the hose from the cabin heat valve on the firewall that dumps the air overboard could be effective.  Anything upstream of the Powerflow heat exchanger would limit efficacy of carb and cabin heat - I think the heat exchanger on the powerflow is less efficient than the old system, so that may not be desirable.  

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1 minute ago, DXB said:

Just left a message with Darren Tilman - President at Powerflow - to discuss.  I'd think creating a constriction of some kind at the end of the hose from the cabin heat valve on the firewall that dumps the air overboard could be effective.  Anything upstream of the Powerflow heat exchanger would limit efficacy of carb and cabin heat - I think the heat exchanger on the powerflow is less efficient than the old system, so that may not be desirable.  

Please let me know how it goes.

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Perhaps try to partially close off the hole with speed tape for a test flight....not all the way, just some?  One other question, does the power flow change how air exits the cowl flap area?  Has the overall airflow been changed or reduced?

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5 hours ago, tmo said:

Didn't we do a similar thread not too long ago, that got more or less solved by different jetting in the carb (for more fuel flow)?

Yes, that was me and indeed I was able to get the CHTs down thanks to better fuel flow, but only down to 430 on take off (huge improvement from where it was before) on a hot day. So now I am trying to figure out what else can I do to get it down to normal parameters. 

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42 minutes ago, 47U said:

I’ve never seen the PF installation up close... Pic taken from the PF website.  The tube on the right side coming out of the muffler shroud is the cooling air exhaust?  How does that work as compared to the linkage closing the exit valve on the original installation?

tom 

 

This is how the system works in my '68C. In 63'-'67 models, excess air is dumped from the carb heat box via the butterfly valve on the pilot's side, not from the cabin heat intake at the firewall.  Notably in all the planes before '63, there was no bypass, so the hot air would just sit there in the shroud, carb heat box, and hose to the firewall.

 

 

 

image.png

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9 minutes ago, takair said:

 Perhaps try to partially close off the hole with speed tape for a test flight....not all the way, just some?  

 One other question, does the power flow change how air exits the cowl flap area?  Has the overall airflow been changed or reduced?

I think I could close off the hose that finally dumps air overboard entirely for a flight without risk.  I'd be hesitant to mess with the airflow upstream of the heat exchanger.

Yeah i considered the increased size of the Powerflow tailpipe occupying the cowl flap port on the right side (my cowl flaps are nonmovable). However increasing the pressure  in that area should reduce cooling of 1&3 and favor cooling 2&4 I would think, and I have the opposite problem.  

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@DXBAs you know, I have a similar setup.  My 2 and 4 cylinders tend to run hotter in climb, but they don't tend to go much over 400.   There haven't really been any hot days since I purchased the plane so I don't know exactly how it will fare. Here are 2 representative flight for comparison.  My FF on takeoff is pretty consistently around 18gph, typically climb at full power and start leaning around 5k feet.

https://apps.savvyaviation.com/flights/3909158/27c28cc0-6302-489d-b60c-421f05a8c6cc

https://apps.savvyaviation.com/flights/3909153/6185d1ea-ab21-4937-a243-67a5b0b3acb6

Differences are that I have a full cowl closure, no Surefly, and adjustable cowl flaps.

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1 hour ago, cferr59 said:

@DXBAs you know, I have a similar setup.  My 2 and 4 cylinders tend to run hotter in climb, but they don't tend to go much over 400.   There haven't really been any hot days since I purchased the plane so I don't know exactly how it will fare. Here are 2 representative flight for comparison.  My FF on takeoff is pretty consistently around 18gph, typically climb at full power and start leaning around 5k feet.

https://apps.savvyaviation.com/flights/3909158/27c28cc0-6302-489d-b60c-421f05a8c6cc

https://apps.savvyaviation.com/flights/3909153/6185d1ea-ab21-4937-a243-67a5b0b3acb6

Differences are that I have a full cowl closure, no Surefly, and adjustable cowl flaps.

Another difference that may be key that you have the butterfly valve bypass on your carb heat box - which is much narrower than my  2" scat hose bypass.  It's definitely not the Surefly - that didn't really change temps in climb.  Your ARI cowl may cool a bit better - I'm not sure.  I just have the LASAR cowl closure which doesn't do much.  The cowl flaps may be an overall factor in climb, but the #4 issue persists in cruise.

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  • 2 months later...

UPDATE - I've more or less solved the #4 high CHT issue.  Recap - This cylinder has always cooled the worst in climb, and the CHT problem became intolerably severe upon installing the Powerflow (easily shooting up to 440s very early in climb and having to fight to keep it from going much higher).  Interestingly, the other cylinders had no issue and if anything ran slightly cooler.  I had solid baffling, fuel flows reaching 18gph, and no mag timing issue.  

THE ANSWER - it turns out the leaner mixture distribution to #4 became intolerable upon the Powerflow allowing it to make more power.  The solution was to keep out the throttle pulled back  ~1/2 an inch on the takeoff roll and initial climb.  This change does not appear to cut out the enrichment circuit, and doing so still allows me to get the same MP and fuel flow along with benefit to takeoff distance and climb rate.  However #4 CHT now runs MUCH cooler - still hottest, but more like what I'd seen before with the old exhaust.  Cocking the throttle plate slightly seems to offer a big improvement for mixture distribution in this condition.  I have to credit @kortopates for making the suggestion.

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3 hours ago, DXB said:

UPDATE - I've more or less solved the #4 high CHT issue.  Recap - This cylinder has always cooled the worst in climb, and the CHT problem became intolerably severe upon installing the Powerflow (easily shooting up to 440s very early in climb and having to fight to keep it from going much higher).  Interestingly, the other cylinders had no issue and if anything ran slightly cooler.  I had solid baffling, fuel flows reaching 18gph, and no mag timing issue.  

THE ANSWER - it turns out the leaner mixture distribution to #4 became intolerable upon the Powerflow allowing it to make more power.  The solution was to keep out the throttle pulled back  ~1/2 an inch on the takeoff roll and initial climb.  This change does not appear to cut out the enrichment circuit, and doing so still allows me to get the same MP and fuel flow along with benefit to takeoff distance and climb rate.  However #4 CHT now runs MUCH cooler - still hottest, but more like what I'd seen before with the old exhaust.  Cocking the throttle plate slightly seems to offer a big improvement for mixture distribution in this condition.  I have to credit @kortopates for making the suggestion.

Hot starts be damned, this makes me thankful for my relatively balanced fuel injection!

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Sounds like the airflow has increased...

...and with it, a slightly different airflow pattern...

Did Paul mention anything about possibly adding more fuel through the second fuel jet?  Possibly a larger jet..?

If you wanted to keep all that air flowing in... and cooling it with the tradition ROP cooling mechanism...

Great results, Dev!
 

Thanks for sharing them...

Got to see where David’s cowl performance is with updated exhaust and timing updates.

Best regards,

-a-

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This is why I don't fly my C at WOT, but instead pull the throttle back until the MP needle wiggles. Cocking the throttle plate creates turbulent flow through the carb inlet, resulting in better fuel atomization and more thorough (likely still not homogeneous) mixing and distribution. Yours is now more sensitive, requiring the turbulent mixing all of the time instead of only at cruise, because you are pushing more air through with Power Flow, and the Surefly is changing your timing.

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2 hours ago, Hank said:

This is why I don't fly my C at WOT, but instead pull the throttle back until the MP needle wiggles. Cocking the throttle plate creates turbulent flow through the carb inlet, resulting in better fuel atomization and more thorough (likely still not homogeneous) mixing and distribution. Yours is now more sensitive, requiring the turbulent mixing all of the time instead of only at cruise, because you are pushing more air through with Power Flow, and the Surefly is changing your timing.

Accurate, except that the Surefly does not advance timing at high MP/rpm combinations, so it was not part of my CHT issue on takeoff. In principle it may contribute to CHTs in cruise up high, but then the extra power dividend from advancing the spark should let you pull throttle back for better cooling while getting similar performance as with fixed timing at higher MP (this seems to fit my experience but I haven't tried to quantify carefully).  

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9 hours ago, carusoam said:

Did Paul mention anything about possibly adding more fuel through the second fuel jet?  Possibly a larger jet..?

Yeah I discussed this option with him as well as the Powerflow folks - Both recommended the same shop that can do this mod, but since my fuel flows are already pretty good pulling the carb seemed like the very last resort.  18gph is good even for the IO360s with 20 extra hp.  It would be upsetting to do the mod and still get no benefit because the mixture distribution to #4 was so bad.  I also imagine that at some point the massive extra fuel to the other cylinders would also start to rob power.  I'll reconsider the carb modification further if the engine comes out for overhaul.

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