CHT difference between old and new engine

[digits]

I'm thinking/hoping this one might be right up your alley Colonel

I'm flying a C310 with 2 continental IO 470 VO engines.
The left one got overhauled, the right one has about 1500 hours on it. Both are running fine. What worries me a bit, is that the left engine has CHTs that are about 40 to 50F higher than the right engine when both are running 'as lean as possible before i feel vibrations'. I would have expected the new engine to run cooler, not hotter.

Initially the temp difference ran 60 to 80F, but after the oil change post break in, this went down to on average 40 degrees F. To clarify, this is a difference per cylinder. The engine monitor shows a very similar pattern left and right (eg, cyl 5 the hottest on both sides, with a diff of about 40F). It's not that I'm comparing the hotest cylinder on the left to the coolest on the right. The difference on other cylinders is also around that same 40 F mark.

When I enrich the mixture, the temperature goes down a bit, but then I'm having fuel flows of 15 GPH left with 11 GPH on the right, so that seems a bit excessive as well.

During initial climb, I need to run the left one at 18GPH to keep the CHTs at/around 400. They would easily go above 420 if I were to match it to the 14GPH on the right.

Any clues as to what might be going on? I've looked at the baffles, I don't see much difference left and right. My engine shop says they should 'tune in the fuel flows', but my AME says that's basically one screw in the fuel controller that controls flows to ALL cylinders at the same time. It's my understanding that this would have the same effect as moving the mixture control?

To add: if I run at low power/max range, the CHT difference increases in cruise (more towards the 50 - 60 degrees mark). If both are running at higher RPMs for more speed, the difference between both is closer to the 40-50F mark.

Any thoughts? Should this worry me?

[David MacRay]

During break in the cylinders are going to be nice and tight so that makes sense for the extra heat. Maybe the old one is much looser too.

More importantly what are you flying for fun? Does it need me to take it out to exercise it? Maybe not if it’s a Pitts, i’m not afraid but I’m getting old. Things I used to think were fun are less so. Driving for nineteen hours for example.

[Colonel]

It seems very possible that the baffles and seals were improperly re-installed after the overhaul.

This needs to be fixed. I presume oil temp is higher too? C310 has cooler up front so airflow through it should be good. It is probably working hard to cool the engine.

From first principles - this engine is making the same power and therefore the same waste heat. The same airflow is going through the cowling right? The air intake and exhaust apertures are the same?

Given the above is true, therefore the pressurized air on the top of the engine is not being forced past the cylinder fins - it is escaping around them because of gaps which you need to find and close. Path of least resistance will be followed by the air.

I would take a USB borescope and peek around the inside of the top of the engine with the cowlings on. Compare the good and bad engines. This may be as simple as new seals which were cut the wrong size, or folding the wrong way when the cowls go on.

Also what can cause this is small baffle pieces which were not re-installed after overhaul, leaving gaps for pressurized air to escape through.

Put bright LED lights in the bottom cowl and move them around. Any bright holes as viewed from above?

Phone Continental. They have an excellent customer service line. They likely will be suspicious of airflow but let’s see what they say.

[Colonel]

This is extremely unlikely but ….

Considerable waste heat is expelled through the exhaust. Theoretically an exhaust restriction would decrease this. These exhausts are pretty simple and if internal stuff in the exhaust is going to fail and start moving around and plugging stuff up, it happens when the exhaust is old and rusty - not new.

I don’t like mufflers on airplanes because of this. If it’s not there, it can’t break.

I am having trouble believing this engine is generating more heat than before it was overhauled. It simply isn’t being expelled out of the cowling.

Crazy idea … Before the rings seat, the engine will run hot. Have the rings seated? This should happen in the first hour but do a leak down compression test. Take a picture of the spark plugs in the tray. How do they look?

[digits]

Thank you very much, I shall investigate. Some questions I can answer already:

1) The oil temperature is an almost constant 10 degrees higher on the left vs the right. 170 ish vs 160ish usually.
Note that the VO engine is partiall oil cooled. although I'm not entirely sure how that works. Very little information about it.

2) After the first 25 hours the temp difference went down from 70 degrees to 40 ish and the oil consumption went down from 1 qt every 3 hours to 1 qt every 15 hours. I think this means the rings have seated?

[Colonel]

Your oil and cooler is doing everything it can to keep the temperature down on the hot engine. I forget what the vernatherm temperature on a Continental is, but on a Lycoming above 170F the oil is forced through the cooler. It’s just like the thermostat on your car.

It sounds like your rings have seated. It’s probably still worth doing a leakdown test and take a picture of the spark plugs in the tray, but that’s probably not going to show much. Multi-cyl engine monitors take all the guesswork out.

$20 it’s ill-fitting seals or maybe a missing baffle piece that wasn’t reinstalled after the overhaul. You need to push all the air past the cylinder fins. This problem decrease as airspeed (and pressure on top of the engine) increases?

Take pictures and measurements of the flexible seals on top of the baffles and compare the two engines. Do they fold down the correct (inward) direction when the top cowl goes on?

Is there an entirely missing piece of flexible seal that was removed but not replaced during overhaul? This stuff is kind of artsy-craftsy.