EGT and CHT questions

[CamTom12]

I think your just showing off all those blanking plates........LOL....HA...HAHAHA

Hahaha, not on purpose! Though I do need to paint it and install the rest of the fasteners for a cleaner look. That’s just one chunk of sheet metal back there.

[bluejeepdad]

I noticed that after replacing my Superior Millenium cylinders with Continental (ex ECI) my CHT’s are 40 to 50 degrees cooler. Better cooling was immediately noticeable with absolutely no change to the baffling. Going from the previous low compression to the high compression cylinders I expected hotter CHT’s due to the higher heat of compression.

At least some of the lower CHT would be due to the increase in compression ratio. When you raise compression ratio the amount of air and fuel being burned stays (essentially) the same, but the power output increases. This happens because, along with the increased compression ratio, you get an increased expansion ratio. The extra expansion converts more of the energy from the air/fuel in to useful work. Same energy in but more energy out means less energy wasted through the cooling fins and out the exhaust.

Tim

[CamTom12]

One additional question: I know that actual EGT values are largely meaningless, but do my takeoff EGTs indicate a possible slightly lean condition? Just wondering if I need to increase the main jet size (new/rebuilt carb, not doing that one on my own).

For reference:
#1: 1358
#2: 1413
#3: 1248
#4: 1337

[Gilbert Pierce]

What carburetor do you have? The usual -12 or -34 carb has a economizer valve (mis-nomer) that adds extra fuel at wide open throttle for additional cooling. Lean the engine at full throttle and pull the throttle back to about 2400 rpm.
When you do this the EGT’s should quickly show a noticeable rise indicating you closed the economizer valve, actually a power valve.

[CamTom12]

I've got the -32 carb, I climb out at full throttle and mixture full in. My climb speed is usually set by CHT on #1 and #2. I speed up as necessary to keep them around 400 and usually end up near 110 mph.

[Steve Pierce]

At least some of the lower CHT would be due to the increase in compression ratio. When you raise compression ratio the amount of air and fuel being burned stays (essentially) the same, but the power output increases. This happens because, along with the increased compression ratio, you get an increased expansion ratio. The extra expansion converts more of the energy from the air/fuel in to useful work. Same energy in but more energy out means less energy wasted through the cooling fins and out the exhaust.

Tim

I figured more power, more heat. I know my oil temp went up when I put the 160 cylinders on my engine.

[Gilbert Pierce]

If you creat more energy in the cylinder you will have more heat radiated to the cylinder wall and head while percentage of loss remains constant. More heat more power.
As I said earlier, the heat of compression alone is higher so heat transfer to head and cylinder walls will be higher. An extreme example is a compression ignition engine where the heat of compression is enough to ignite the fuel. Examples are a Diesel engine or preignition in a gasoline engine. In the case of a gasoline engine the results are catastrophic.

[bluejeepdad]

Gilbert,
What I'm pointing out is that overall you have the same energy going in as before (same fuel and air flow). The mixture is heated to a higher temperature during the compression stroke; I agree. And that may change the distribution of heat; like Steve was pointing out with higher oil temp.

But the higher compression also has an affect during the power stroke, the piston takes more work out of the combustion gasses. That is where the increased power output comes from. It's not from burning more fuel and air; it's from using the fuel more efficiently. Because more energy goes in to moving the piston during the power stroke, by the time we get to the bottom of the stroke there is lower pressure and temperature. This means less heat going in to the head at this point which offsets some of the additional heat added during compression. And the cooler exhaust gasses transfer less heat into the valve, guide, seat, etc, than there would be with lower compression.

So overall, if we are putting in the same energy to the engine but getting more out of the crankshaft, there must be less energy going somewhere else. And that shows up as lower EGTs and that tends to lower CHTs.

I'm not saying that the difference in cylinders from Superior to Continental isn't helping too. Just pointing out that higher compression ratio is more efficient so there will be less wasted heat, particularly around the exhaust area.

Tim

[CamTom12]

I've been thinking about it and here's my plan of action. I'll definitely do the first 2 at once, since time I get at the hangar to work is scarce these days. If I have the time I'll do all three at once. Not good for scientific experimentation, but good for scheduling around a toddler who's growing up too quickly!

1) retard engine timing by a 2-3 degrees and see if I get any difference in takeoff CHTs (as noted here).

2) smooth out the "dings" in the induction tubes at the hose/clamp connection (I'll take before/after pics).

3) perform the #3 baffle mod I posted about here.

I'm also seriously contemplating replacing/overhauling my carb. My engine has been "dieseling" for a second or two after I pull mixture to ICO, and the one I have on there is pretty old. I know it doesn't have the blue floats or the improved nozzle. That's not happening soon though, more of a "want" to get it updated than I think an actual need.

[Stephen]

I've been thinking about it and here's my plan of action. I'll definitely do the first 2 at once, since time I get at the hangar to work is scarce these days. If I have the time I'll do all three at once. Not good for scientific experimentation, but good for scheduling around a toddler who's growing up too quickly!

1) retard engine timing by a 2-3 degrees and see if I get any difference in takeoff CHTs (as noted here).

2) smooth out the "dings" in the induction tubes at the hose/clamp connection (I'll take before/after pics).

3) perform the #3 baffle mod I posted about here.

I'm also seriously contemplating replacing/overhauling my carb. My engine has been "dieseling" for a second or two after I pull mixture to ICO, and the one I have on there is pretty old. I know it doesn't have the blue floats or the improved nozzle. That's not happening soon though, more of a "want" to get it updated than I think an actual need.

Last summer I had a dieseling problem a couple of times. I sent my carburetor in and have not had a problem since. Extra fuel was passing into the venturi.