View Full Version : High cylinder head temps - up to 450F

05-15-2010, 06:19 PM
I recently added an engine monitor to my Cessna 172M with Lycoming O-320-E2D carbureted engine. There was a Power Flow Systems exhaust (classic, not short stack) already installed when I bought the plane, so I do not have any comparison data.

I recently added a JPI EDM-730 engine monitor, and the cylinder head temperatures seem higher than I have read about. On full power climb out, cylinders 1 and 4 get to about 425 and 2 and 3 get to about 450 in about one minute. About that time I pull the power back a little to get them all down to about 400.

The heat pattern seems a little strange to me, in addition being too hot. I have swapped probes, so it is not a probe issue. Several folks have looked a things like baffles and don't see anything wrong in that area.

Engines with PFS exhaust generate additional power. Are the readings I am getting reasonable? Does anyone have any hints for cooling this engine, or do I back off the power sooner all the time?

05-31-2010, 02:38 PM
So, poking around with this issue, I opened an issue on cessna.org. The fellow there suggested that the mixture was too lean on climb, and to check the linkage on the mixture control. If that was OK, check the carburetor on a flow bench.

Well, the linkage was fine, so I (actually my AP guy) removed the carburetor and I shuttled it over to a carb shop with a flow bench recommended by my AP. First impression from the carb guy was "Boy, that's an old carb," but he put it on the flow bench anyway. Results were that the carb was OK everywhere except full power, where it was really lean. Spec for the carburetor was 80 pounds of fuel per hour and carb was only flowing 72. So rather than overhauling the old carb, they had an overhauled equivalent on the shelf, so, for the same dollars, I bought that, took it back to my AP and he installed it.

Temps are better, but not really good, especially after some more research.

On the old carb, I was getting about 12 gallons per hour on full power climb. With the new carb, I was getting 13.5 GPH. So, it's easy to see why temps are down. But one guru indicates rather strongly that's not enough. I should have about 16 GPH on climb, and that will reduce temperatures so that I can do an extended maximum performance full power climb on an August afternoon in North Carolina at sea level without damaging the engine.

Now, I have only one question: Given that a PFS exhaust will cause more air to flow through an engine (compared to stock exhaust, this is a lycoming O-320-E2D), is there an "official" specification for the carburetor that I can take back to the carb shop and modify it in some way (bigger main jet?) to provide enough fuel to cool this engine? If not, is there something less "official" that I can do for my certificated engine? Can I just go back to the carb shop and say "Please change the main jet on this carb and run it through the flow test so it passes 96-100 pounds of fuel per hour"?

10-23-2010, 02:49 PM
Note: we worked with the poster and here is a brief summary for everyone's benefit.

It appears that the "official" specification for how much fuel should be flowing on a particular engine/carburetor model has changed over time. We have been in contact with the new manufacturer of Marvel Schebler Carburetors and they have been successful at flow testing a suspected lean carburetor and adjusting it to flow more - up to the higher end of the specification for that particular carburetor. In this case, the addition of about 3.0 more gallons per hour reduced cylinder head temperatures by about 50 degrees!

We use a rule of thumb developed by engine shops, performance modifiers and others in the industry that the engine should be flowing about 1.0 GPH for every 10 horsepower it is putting out for adequate cooling, detonation margin, etc.

For a 160 HP C172 at static RPM (2350) at sea level, standard conditions, the engine is roughly putting out 125-135 HP. So, ideally, this engine should be flowing about 12.5-13.5 GPH at static runup. During a Vy Climb, (normally 2450 RPM or so) we expect to see a fuel flow of 13.5-14.5. At sea level and 2700 RPM, the same engine should be close to the 16.0 GPH number you reference.

If someone reading this suspects they have a lean running carburetor, I strongly urge them to call me and I can take them through a troubleshooting process. We don't recommend just taking your carburetor off and then sending it out for overhaul. Experience has shown that you won't likely fix your issue and you'll spend a bunch of time and money and get back a carburetor that may or may not be richer.