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Can you lean a carb'ed engine using CHT?

AndyRV7

Well Known Member
My RV-7 has a carb, with CHT and EGT information for each cylinder, in the panel. I learned to fly it using the Skyhawk method of pulling the lever until the engine runs rough and then pushing the lever back a certain distance. It has worked as well in the RV as it did in the Skyhawk, however much that is...

I've never relied on the EGT because they vary by 100-200 degrees maybe. My cylinder head temps are spread over a smaller range though and I never have a need to climb-out much hotter than 380 degrees (maybe 400 degrees in a short climb on a 90 degree day). Typically my cruise temps are below 350 on the hottest summer days, maybe lower.

If I remember correctly, the problem (or one of the problems) with high CHT's is the pressure inside the cylinder. So every time I lean for cruise and look down and see 320 or so on the CHT's I always wondered if I was wasting fuel and could lean further while watching the CHT's for a more accurate but safe level of leaning.

Any thoughts??

Thanks! Andy

PS. I'm talking about leaning on the ROP side of things.
 
Andy,

I use the EGT for initial leaning to roughness then enriched for a smooth running engine. My EGT spread also has a 100-200 degree difference. That doesn?t matter as much as they should be increasing/decreasing at the same rate. Placement of each EGT prob has an effect on the temperature reading.

Once I?ve found the sweet spot, which is around 14 gph in the 195, I then make fine mixture adjustments based on the CHTs. The issues with leaning initially by CHT is they are slow to react, where the EGT is instantaneous. Just like our flat engines I try to keep the CHTs on the Jacobs at 380 or slightly less.

Hope this helps.

Jeff
 
LOP?

Andy
I have the same setup and until recently leaned the same way as you based on flying my Pacer which had no useful CHT/EGT monitoring. Never had an issue with the Pacer or the RV and plugs have looked fine every year.

Recently I have been trying the lean mode on my Dynon and I am generally able to run 0.2 to 1.0 gph lean of peak smoothly with no issues. CHT's for the two hottest cylinders (3&4) generally run in the 360 deg range.

From what I have read on here (and Mike Busch's YouTube presentations) the EGT is faster responding but is only relative with respect to actual temperature so trying to get to the actual power setting you want is not exact science.

At the end of the day I figure as long as the engine is running smooth and the CHT's are at a reasonable temperature life is good:D
Figs
 
Andy
I have the same setup and until recently leaned the same way as you based on flying my Pacer which had no useful CHT/EGT monitoring. Never had an issue with the Pacer or the RV and plugs have looked fine every year.

Recently I have been trying the lean mode on my Dynon and I am generally able to run 0.2 to 1.0 gph lean of peak smoothly with no issues. CHT's for the two hottest cylinders (3&4) generally run in the 360 deg range.

From what I have read on here (and Mike Busch's YouTube presentations) the EGT is faster responding but is only relative with respect to actual temperature so trying to get to the actual power setting you want is not exact science.

At the end of the day I figure as long as the engine is running smooth and the CHT's are at a reasonable temperature life is good:D
Figs

Figs - thanks for sharing this info. I'm wondering if I can convince you to expand a bit on how you got an engine that has 100-200 degree EGT spread to run smoothly enough LOP?

I ask because our engine (O-360-A1F6) will react very well to the "lean 'til rough, then two twists to smooth" method. I've tried to get LOP, both by doing the "big pull" and then enrichening after the engine has nearly sputtered to a stop, as well as very cautiously approaching LOP from the more dangerous ROP side. I can get EGT's within 50-75 degrees spread but not with smooth running. The leanest I can go still produces smooth running and reduced power output so I know it's lean of Best HP setting. Maybe that IS LOP for this engine, or as LOP as it will go. Certainly can't seem to get all cylinders leaner than peak without roughness.

Your thoughts would be welcome.
 
The relative temps shown on the EGT probes do not need to be close. What matters is when they reach peak egt relative to each other. In a well balanced engine they will all peak close together. You don?t really care what temp they peak at just that they peak as close together as possible.
 

Dates from 1994, including the recommendation not to run LOP. Much has been learned since then. Still, that old "lean 'til rough, then rich it up a bit" technique works!

SAILVI - your comment regarding absolute EGT makes perfect sense. In our case the EGT's peak sequentially rather than at nearly the same mixture setting, hence the challenge in getting to a more economical fuel burn. By the time the last cylinder is at peak EGT the first cylinder has stopped firing. I've tried part-throttle, full throttle, a little carb heat, a lot of carb heat... Still a work in progress.
 
Dates from 1994, including the recommendation not to run LOP. Much has been learned since then. Still, that old "lean 'til rough, then rich it up a bit" technique works!

SAILVI - your comment regarding absolute EGT makes perfect sense. In our case the EGT's peak sequentially rather than at nearly the same mixture setting, hence the challenge in getting to a more economical fuel burn. By the time the last cylinder is at peak EGT the first cylinder has stopped firing. I've tried part-throttle, full throttle, a little carb heat, a lot of carb heat... Still a work in progress.

He did say he had a carb. LOP operations date back to WWII. And, Lycoming's recommendations haven't changed.
 
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My RV-7 has a carb, with CHT and EGT information for each cylinder, in the panel.
Consider someday calibrating your probes. Also location of EGT probe from the exhaust port makes a difference. I assume they are equivalent. The closer to exhaust port within reason the better. However the closer they are the shorter life they will have.

Your guide to leaning https://www.cocotier.org/engines/ly...lyer/Lycoming Flyer Key ReprintsOPERATION.htm

I've never relied on the EGT because they vary by 100-200 degrees maybe. My cylinder head temps are spread over a smaller range though and I never have a need to climb-out much hotter than 380 degrees (maybe 400 degrees in a short climb on a 90 degree day). Typically my cruise temps are below 350 on the hottest summer days, maybe lower.
350F CHT is low but OT (oil temp) is really important. You need at minimum 180F. The temps in the valve area are about 30F higher. You need at least 212F oil to burn off the moisture and acids. [NOTE: To balance out EGT's consider just closing the throttle just a tiny bit to create more turbulence and/or use a little carb heat. The Carb heat door slightly closed will also create turbulence in the air box... this can balance out the cylinders. AGAIN calibrate your EGT probes and assure they are equal distance from exhaust port.]

If I remember correctly, the problem (or one of the problems) with high CHT's is the pressure inside the cylinder. So every time I lean for cruise and look down and see 320 or so on the CHT's I always wondered if I was wasting fuel and could lean further while watching the CHT's for a more accurate but safe level of leaning.
Pressure has to do with detonation which can cause catastrophic damage to the piston and valves with out much indication to pilot. CHT really does not indicate cylinder pressure. The only thing that indicates max combustion pressure is very sophisticated pressure transducers, typically only used in test cells. It is not easy even then to get combustion pressure. The key is not to lean above 75% power to avoid detonation. If CHT is too cold (assuming it is calibrated) then operate at max power mixture. EGT absolute value does not matter. EGT is only relevant to difference or delta from peak EGT. [Note: cars have detonation sensors which are microphones listening for the knock. Aircraft engines being air cooled and loud make hearing detonation near impossible. Cars also have ECU mapping that knows RPM, Torque (thus power) and Fuel, so it can keep you out of danger. For airplanes the pilot is the ECU.]

As you know you lean until the first cylinder peaks and starts to go back down (all other cylinders are cooler on ROP side). If you have EGT probes way down the pipe it may take some time for them to respond, take your time. You can run PEAK if you want, but you are not at Max power (and max CHT). Max CHT is around +30-50F ROP and max power power is about 100F ROP. The more ROP +150F the cooloer. This will give you the hottest CHT (approx). If you want to cool it 50F ROP is good. Anything more than 100F is wasting fuel (in my opinion). Keep in mind some cylinder EGT's will be cooler. You can operate right at ROP or 40F ROP, if CHT's are not above 380F (my personal limit) and OT are not above 220F. Can you have detonation issues? Not if under 75% power and using 100LL (assuming all your temps are normal CHT<400F, OT<220F). I would not get aggressive with leaning (less than 75F ROP) until 65% power. Again it depends on CHT and OT. Again calibrate your probes.
 
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I lean aggressively in the O-540 in my -10. I have a 6 cylinder Garmin engine monitor and seldom ask for more than 60% power. Since I cruise up to 17,500? on longer legs, it sure helps to have the instrumentation to see that reducing the throttle about an inch and modulating the carb heat can let me get the EGT?s balanced better and run more efficiently if only a little lean of peak.
 
Carb heat trick

Another trick with a carb is to add a bit of carb heat in cruise to bring the EGT?s closer together and lean further. I believe, the carb heat baffle in the air intake box when partially closed (not much) disrupts the airflow enough to better mix the fuel air and deflect it more evenly to each cylinder. You will also notice that EGT/CHT varies based on throttle position and how the butterfly in the carb is deflecting air towards different cylinders. Like mentioned above, pull the throttle off the stop just enough to barely lower, or not even lower the manifold pressure. In my 0-320, I can run LOP at 65% power or less, with about 1? of carb heat pulled out and the throttle off the stop.
 
Another trick with a carb is to add a bit of carb heat in cruise to bring the EGT?s closer together and lean further.

This is good advice! I have a carb heat temp gauge and after leaning the mixture bring the carb temp up to 80*-100*. Engine runs smooth and you can "hear/feel" the slight increase in power.
 
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