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Exhaust Advice

I checked my carb temperature rise a couple days ago. Normal cruise, 2400 RPM, carb temp 15 deg F. Pulled carb heat and max was 31 deg F. 16 deg rise. Both CAR-3 and Part 23 specify a 90 deg rise. I read them both yesterday and confirmed. How did this get STC'd?

Personal opinion, but 90 deg seems like substantial overkill. However 16 deg is clearly insufficient.

Edit, a little more data: Cruise altitude in this test was approx 1500 MSL. I did not check OAT, but probably mid to upper 30's. My carb air box is in good condition, rebuilt with excellence by Randy Rubbert, and it is properly adjusted. The scat tubing from muffler shroud to airbox is in good condition.
 
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Gordon how about another experiment? Connect a heater hose from the cabin heat shroud to the carb heat and see what it shows? Might tell the limits of that muffler's setup.

So are you seeing a 15*F drop from ambient air in the carb venturi (~30 - 15*F)...cooling through fuel vaporization? Then I wonder if CAR3 and 23 measured the 90* rise over ambient air or carb temp?

Gary
 
Yeah, I wondered the same thing about the reference temp. It is ambiguous. If it's ambient though, then 90 would be even more dramatic, and harder to attain.

And agreed, running cabin heat to the carb would establish a baseline. HOWEVER - do you really expect me to go without cabin heat when it's way down in the 30's outside? :cry:
 
For science yes. Dress warmly and pretend you're Orville.

Edit: Ok, then hook the carb heat up to the cabin inlet. Might get some comfort.

Gary
 
And as appreciated by an Engineer. Now go do the right thing for critical data and report back...Ha!

Gary
 
Odd, to me, your carb temp readings are so wildly different from mine. 20 degrees ambient and 54 degrees on carb temp my last flight. Full carb heat at cruise power for 5 minutes resulted in no change. I have never seen below freezing temperatures on my carb temp gauge, Jim
 
Gordon, look in AC-23-8C. Flight test guide. The instructions for all testing is there. As I recall there is a condition when you add 100* to the calculations.
 
I agree that it would be worth while knowing what the temperature rise in the carb would be with the cabin heat outlet hooked to the carb heat inlet. Keep in mind it is ram air your introducing. With ram hot air it will change your mixture significantly. Watch the CHT and RPMs during the experiment.

I would be be very interested in hearing the results and I appreciate your doing the tests for the benefit of all of us.

Stu
 
cruiser said:
Odd, to me, your carb temp readings are so wildly different from mine. 20 degrees ambient and 54 degrees on carb temp my last flight. Full carb heat at cruise power for 5 minutes resulted in no change. I have never seen below freezing temperatures on my carb temp gauge, Jim
Click to expand...

I wonder Jim if it's possible your carb heat box is continuously leaking some hot air to the carb even when then flapper is closed? Maybe do a ground runup with the heater hose connected then removed to look for carb temps? You could do it in the air if careful to note any icing but...

Gary
 
I will give that a try. Of course, it has been my contention right from the start on this that my carb is not getting any, or much, hot air.
 
cruiser said:
Odd, to me, your carb temp readings are so wildly different from mine. 20 degrees ambient and 54 degrees on carb temp my last flight. Full carb heat at cruise power for 5 minutes resulted in no change. I have never seen below freezing temperatures on my carb temp gauge, Jim
Click to expand...
Cruiser,
Respectfully, I think there must be something amiss with your carb temp instrumentation. Possibilities might include, connected to a different probe somewhere, wires crossed, wrong connections at the instrument, incorrect location of probe? Dunno, just making wild guesses. However - it stands to reason that the venturi temperature cannot be higher than ambient. I doubt that there is anything wrong with your carb heat box, unless the carb heat control possibly doesn't actually activate the carb heat flapper. That seems crazily unlikely.

But I can't imagine that something isn't wrong for venturi temp to indicate 30+ deg above ambient.

No criticism intended, just thinking about the mechanics of it all.
 
Gordon Misch said:
Cruiser,
Respectfully, I think there must be something amiss with your carb temp instrumentation. Possibilities might include, connected to a different probe somewhere, wires crossed, wrong connections at the instrument, incorrect location of probe? Dunno, just making wild guesses. However - it stands to reason that the venturi temperature cannot be higher than ambient. I doubt that there is anything wrong with your carb heat box, unless the carb heat control possibly doesn't actually activate the carb heat flapper. That seems crazily unlikely.

But I can't imagine that something isn't wrong for venturi temp to indicate 30+ deg above ambient.

No criticism intended, just thinking about the mechanics of it all.
Click to expand...

SIMPLE to check... before start OAT should be close to carb temp... providing not in hot sun or such...
 
skywagon8a said:
Gordon, look in AC-23-8C. Flight test guide. The instructions for all testing is there. As I recall there is a condition when you add 100* to the calculations.
Click to expand...
Thanks, I did. I did not find any reference to induction icing other than a referral to FAR 23.1093, which reads the same, at least for "sea level engines" as CAR-3. That is 90 deg F rise at 75% power.
 
Thanks Gordon, the temps I am referring to are when the engine is warmed up well and running. The reading is ambient temp prior to start. I believe I have read on here that the carb absorbs heat from the oil pan. I assumed that was the cause of the temps I an seeing
 
OK sorry, That Flight Test Guide is nowhere near as concise as the one for CAR 3. I'll have to see if I can find my old dog eared copy with some pages missing and hope that page is still there. This may take a little time. Every page is set up with step by step instructions with lines to place your data and the equations to make the calculations.
 
The CAR 3 guide came out prior to ACs. I can not find it on line. The following may give you some insight while I am looking in the attic.

Carburetor Ice Test MethodologyEvaluation Final Report
https://crcao.org/reports/recentstu...Carb_Ice_Revised _Final Report_2015_12_21.pdf

Also this is not what I'm looking for but is the procedure:
From
FLIGHT TEST GUIDE FOR ASSESSMENT OF AMATEUR-BUILT AIRCRAFT ACCEPTED UNDER AN ABAA3. Procedures
All tests should be conducted in air free from visible moisture.
The temperature sensing probe should be installed in the induction system downstream of the heater and upstream of the venturi. It may be necessary to make a tapping into the intake duct.
Heat rise requirements should be met at an outside air temperature (OAT) of -10C at an altitude where the engine can develop 75 percent MCP. If it is not possible to obtain these conditions tests should be conducted at the required power setting at the lowest OAT achievable and reduced to the required condition.
At the test altitude stabilise the aircraft in level flight at 75% MCP with the carburettor heat control in the cold position. Allow all parameters to stabilise then record pressure altitude, OAT, RPM, manifold pressure and carburettor inlet air temperature. Apply full carburettor heat and allow temperatures to stabilise before recording the new carburettor inlet air temperature. Repeat this procedure two or three times to ensure consistent results.
Air temperatures should be measured with calibrated temperature probes. All other quantities may be recorded from the standard aircraft instruments.

And this from Canada:
https://www.tc.gc.ca/eng/civilaviation/regserv/cars/part5-standards-525-sub-e-291.htm
[h=5]525.1093 Induction System De-icing and Anti-icing Provisions[/h]
 
Well, here are a couple of things - -

I flew again today, with the cabin heat hose connected to the carb airbox. 1200 ft MSL, OAT 40, normal cruise at 2400 RPM, carb temp 18 without heat, and 41 with heat. 23 deg rise, as compared to 16 deg rise from the carb heat side of the shroud. I was surprised; I expected more temperature gain. But there's the data.

From the information Skywagon8a provided, the regulatory temperature rise is to be measured at the heat source discharge, upstream of the venturi. My carb heat transducer is downstream of the venturi, and upstream of the throttle plate. I think that port is standard on the Marvel Shebler carbs. So I am a little bit uncertain about how to compare that regulatory rise to the rise observed at the carb temp sensor. I think, emphasizing 'think', that the delta t between carb heat and no carb heat should be the same at either location, i.e. upstream or downstream of the venturi. But it's been too long since studying thermodynamics of dynamic fluids, so I'm uncertain. Any of your Thermo / Fluids guys able to shed some light on that?

Bottom line though, increasing the carb heat contact with the muffler certainly did increase the carb heat effectiveness. So I'm thinking about the possibility of adding some baffling to the muffler on the carb heat side, to increase that contact. As STC'd, the carb heat air has a very direct path in the muffler shroud from inlet to outlet. The contact with the muffler surface is minimal.

Edit: Another aspect of the info Skywagon 8a posted, is the OAT when measuring the temperature rise across the carb heat system, is to be -10 deg C. That is 14 deg F. At that lower ambient temperature, the temperature rise across the carb heat system would be greater than with an OAT of 40 deg F. Reason being, greater delta t between muffler surface and air. So the temperature rise I should expect at 40 F, would be less than the regulatory rise. But not hugely less because the EGT and muffler surface temp is so much greater than the ambient temp. I did not check EGT's today, however.
 
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Wrapped door springs can help mix the airflow around the stacks and increase the output heat. I flew several winters with a '78 7GCBC exhaust very similar to the Sutton with it wrapped, and then a PA-18A stock front and rear cans with that setup but never measured the change. Never had a carb temp probe just cockpit air that got the flexible defroster hose in both too hot to hold. It was cold then and I flew to -40F while marten trapping. But it does take lots of surface area of the stack to provide the basis. And I suspect slower airflow results in a hotter output at lower volume. That's what I saw flying 185's by shutting off the fresh air mixer to the heat box.

Edit:
Citabria exhaust: https://www.acornwelding.com/products/pdf/American Champion/American Champion 7-GCBC Citabria.pdf
Sutton exhaust: https://www.propilotsinc.com/exhaust-conversion/

Compare the two and the inlet/outlet setups.

Gary
 
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Gordon Misch said:
Well, here are a couple of things - -

I flew again today, with the cabin heat hose connected to the carb airbox. 1200 ft MSL, OAT 40, normal cruise at 2400 RPM, carb temp 18 without heat, and 41 with heat. 23 deg rise, as compared to 16 deg rise from the carb heat side of the shroud. I was surprised; I expected more temperature gain. But there's the data.

From the information Skywagon8a provided, the regulatory temperature rise is to be measured at the heat source discharge, upstream of the venturi. My carb heat transducer is downstream of the venturi, and upstream of the throttle plate. I think that port is standard on the Marvel Shebler carbs. So I am a little bit uncertain about how to compare that regulatory rise to the rise observed at the carb temp sensor. I think, emphasizing 'think', that the delta t between carb heat and no carb heat should be the same at either location, i.e. upstream or downstream of the venturi. But it's been too long since studying thermodynamics of dynamic fluids, so I'm uncertain. Any of your Thermo / Fluids guys able to shed some light on that?

Bottom line though, increasing the carb heat contact with the muffler certainly did increase the carb heat effectiveness. So I'm thinking about the possibility of adding some baffling to the muffler on the carb heat side, to increase that contact. As STC'd, the carb heat air has a very direct path in the muffler shroud from inlet to outlet. The contact with the muffler surface is minimal.

Edit: Another aspect of the info Skywagon 8a posted, is the OAT when measuring the temperature rise across the carb heat system, is to be -10 deg C. That is 14 deg F. At that lower ambient temperature, the temperature rise across the carb heat system would be greater than with an OAT of 40 deg F. Reason being, greater delta t between muffler surface and air. So the temperature rise I should expect at 40 F, would be less than the regulatory rise. But not hugely less because the EGT and muffler surface temp is so much greater than the ambient temp. I did not check EGT's today, however.
Click to expand...

soooooooo..... what sort of EGT temp you got to rob the heat from??? maybe not much to rob from???
 
Gordon Misch said:
It's hard to tell detail from the drawings, but it seems clear that the inlet/outlet ports are arranged differently on the shroud. It appears the Citabria shroud would provide more hot-surface contact for the carb air.
Click to expand...

Does the Sutton pull heat from the main muffler shroud, or rather what looks like an add-on cover over the left collector outside of the main shroud? The Citabria I think has a separator ring between the cabin and carb heat sources inside the shroud with two forward air intakes. We drill holes in the front of the cowl to get more air flow and pressure vs flat sources on horizontal front cylinder baffles.

There's not much stack surface area under the Sutton carb heat pickup. Baffling both heat sources externally should raise the heat similar to what Cessna does with their pins that extend from some of their muffler cores.

Edit: This answers my questions. Not much surface area for carb heat compared with cockpit: http://www.supercub.org/forum/showthread.php?37409-sutton-exhaust-system

Gary
 
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Gordon Misch said:
....the regulatory temperature rise is to be measured at the heat source discharge, upstream of the venturi. My carb heat transducer is downstream of the venturi, and upstream of the throttle plate. I think that port is standard on the Marvel Shebler carbs. So I am a little bit uncertain about how to compare that regulatory rise to the rise observed at the carb temp sensor. I think, emphasizing 'think', that the delta t between carb heat and no carb heat should be the same at either location, i.e. upstream or downstream of the venturi. But it's been too long since studying thermodynamics of dynamic fluids, so I'm uncertain. Any of your Thermo / Fluids guys able to shed some light on that?

Bottom line though, increasing the carb heat contact with the muffler certainly did increase the carb heat effectiveness. So I'm thinking about the possibility of adding some baffling to the muffler on the carb heat side, to increase that contact. As STC'd, the carb heat air has a very direct path in the muffler shroud from inlet to outlet. The contact with the muffler surface is minimal.
Click to expand...
The temperature probe in your carburetor is measuring the temperature of the fuel/air mixture after the cooling effect of vaporization. This probe is only telling you of the potential of ice in the section of the carb where the ice accumulates restricting air flow. The heat source must raise the temperature above the vapor cooling temperature plus enough to melt any formed ice.

What is the distance between the cool air inlet and the hot air outlet on the muffler? Less than 180* apart? If so there ought to be a baffle on the inside of the shroud forcing the cool air to travel the long route around the hot muffler giving it more time to heat up. You could have a series of such baffles which are staggered forcing the air to travel over a longer zig-zag route thus absorbing more heat.
 
Distance? Look at my previously posted pictures. The distance is zilch. The hose to the carb heat box is directly adjacent to the air inlet. The air flow assumption is that it all sort of swirls around the #2 and 4 pipes and the end of the muffler can to absorb heat and then comes back to the hose to merrily flow to the carb heat box. I picture the air flow going to the carb heat box directly from the opening to the hose.
 

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cruiser said:
Distance? Look at my previously posted pictures. The distance is zilch. The hose to the carb heat box is directly adjacent to the air inlet. The air flow assumption is that it all sort of swirls around the #2 and 4 pipes and the end of the muffler can to absorb heat and then comes back to the hose to merrily flow to the carb heat box. I picture the air flow going to the carb heat box directly from the opening to the hose.
Click to expand...
That is the answer. Bend up a Z section of stainless, spot-weld (preferred) it to the shroud to force all of the air the long way around. It can be riveted but be cautious that they do not work loose enough to be ingested into the engine. Perhaps there is some high temp adhesive which will prevent rivets from working loose? Screws and lock nuts could also be used.
 
The distance from shroud inlet to exit on the carb heat side is probably about 3 or 4 inches (max), with little angular displacement.

I agree that internal baffling should help, however it should be welded to the muffler, not the shroud. The shroud needs to flex. Also, some sort of turbulence generators, such as Atlee Dodge uses on their hotrod muffler should help. I've already discussed doing this with my IA (certified plane). His opinion is minor alteration.

I am unwilling to attach anything inside the shroud in any way other than welding by a competent shop. That includes wrapped springs, which are sometimes suggested. If any piece breaks loose from corrosion or vibration it could potentially go right into the engine.
 
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