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Old 04-08-2012, 07:10 PM
RV8R999 RV8R999 is offline
Join Date: Dec 2008
Location: na
Posts: 1,457
Default Climb CHTs and CARB Heat

A few days ago I tested a theory I had about using carb heat during a Vy climb to help increase cooling efficiency: The basic premise is the engine increases exit area by pushing lower plenum air through the engine during combustion and out the exhaust pipe (which normally accounts for a decrease in exit area).

My first test didn't work out so well and I though maybe I was getting some blow-by at the FAB/cowl inlet junction. Turns out I just didn't run the test very well.

Today I ran 6 consecutive climbs from O-8000ft PA during a WOT, Full Rich, Vy (110MPH) climb with my exit flap open. The odd runs 1,3,5 were with Carb Heat Off. Runs 2,4,6 were with Carb Heat Full on. CHTs, OAT, Elapsed Time were recorded at T/O and every 1000ft PA. The data for each of the 3 different cofigurations was averaged and plotted as shown below...

Of note: OATs were within 1 deg for all runs to altitude.
The Avg CHTs at T/O for the Carb On conditions were purposefully higher (by 5-10) than the Carb Heat off conditions.

Rate Of Climb was 100 fpm less during Carb Heat On runs with RPM about 40-50 lower as well.


No only is the engine cooler with carb heat on but Average CHTs peak nearly 600 ft sooner (even though climb rate is slower). Any thoughts?

Last edited by RV8R999 : 04-08-2012 at 07:34 PM. Reason: more
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Old 04-08-2012, 07:44 PM
Kyle Boatright Kyle Boatright is offline
Join Date: Aug 2005
Location: Atlanta, GA
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What's your climb rate with the carb heat on vs off. The lower temps with carb heat on might be because the engine is developing less power due to a less dense induction charge, and has less heat to dissipate.
Kyle Boatright
Marietta, GA
2001 RV-6 N46KB
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Old 04-08-2012, 07:46 PM
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scard scard is offline
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Simply a function of producing less power? I see something very similar when I reduce prop rpm on climbout on a hot day. That is critical for me to control cooling on a long hot climb. Really just a different way to bring the power back, yet still have the throttle body wide open (carb richening circuit).
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Old 04-08-2012, 07:54 PM
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RV10inOz RV10inOz is offline
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Yep, a few and if I sat here long enough I could come up with more, but I have someone wanting to go play in the -10...and know

Firstly, lower CHT is a result of producing less power in the climb, you noted less RPM. Less RPM means less MASS AIRFLOW into the engine and on the Rich side of peak that determines HP. This is validated by the loss of Rate of Climb also.

Secondly the richer mixture and lower mass airflow result in the PPP being retarded further from TDC and the peak pressure is also lower, less disturbance to the boundary layer in the cylinder and hence less heat transfer to the head, validated by the lower CHT.

In theory you could achieve the same result by retarding your timing a fraction. But why would you.

I hope you DO NOT climb like that normally. Using full rich all the way to 8000' (apart for this trial for testing with a constant) is not a very smart thing to do. I am sure you understan Target EGT so need to explain here. If not let me know and I will expand.

For the sake of a 10 degree F difference why would you waste all that extra fuel, all that extra time...wasting even more fuel, and have no significant CHT benefit, all the time adding much greater deposits to pistons cylinder heads and valves.

Interesting experiment indeed, but I can only see negatives for doing it all the time and not one positive.

Happy to discuss further either on the board or off the boards if you prefer.


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Old 04-08-2012, 08:17 PM
RV8R999 RV8R999 is offline
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certainly the reduced power has some effect

but the change in temp is a 3% reduction at the peak with only a 1.1% reduction in power.

some folks struggle with keeping temps below 430's in the climb and really worry about it. This is an attempt to educate and find all the possible ways to aid in cooling at the worst condition - Vy. When I started flying my -8 my avg CHTs were in the 410-420 range under similar conditions. The incremental changes have culminated in an avg 55 deg reduction in my Vy climb CHTs.

no need to discuss the merits of leaning while climbing, I fully understand, to each his own, but thanks.
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