I have seen some pretty good exhaust leaks on aircraft engines…..pilots and pax blissfully unaware.

Standing beside it even, might be hard to hear with all the prop noise.

Cars, yes much easier, less competing noise.

i thought a rough running engine at idle would have been a decent indicator.

Looking at the exhaust it appears the damage is located in the aft/lower corner. Is that correct?

Also, you said the engine had been sitting around for a long time prior to being used.
Do these heads corrode easily?

It's interesting to note that the burn marks on the cooling fin stops at both of the bolt holes and the breach is nearly right in the middle of these holes.

These older Jabiru heads use exhaust manifold gaskets, they are a regular replacement item. Yes you can hear them when they blow out.
The burnt oil between other bolts is another sign of escaping oil past joint and pretty high temps
Id have thought an inside inspection tension of heads and valve clearances a wise move on an engine thats sat around for many years, even a new one.

Ultralights:

***
my meaning being that the exhaust gas temps leaking through into the void, would be no different to the gas temperatures in the combustion chamber if the engine was 100% fine and running normally.
***

Are you sure?

The combustion temp early in the power stroke will be 3500+dF. As the piston moves down, the temp will decrease with combustion chamber expansion until at the time of exhaust valve opening, the combustion temp and exhaust temp will be closer to the same, with the combustion temp being slightly higher. So, if the leak is occurring high in the power stroke, the temp will be MUCH higher. Why would it not leak there when pressure is highest?

i see what you mean about it being hard to describe theories online, ill have another crack at it, i am assuming, that some think that a leak such as the one imaged, will cause higher temps due to the leaking gasses, but what i am trying to say, is that the leaking gas temps will be no higher than the maximum temps that can be achieved during a normal cycle, at its highest point on the power stroke. i cant see how the escaping gasses through the leak point could increase temps from what they would already be at the maximum pressure point with combustion as well.

I think i introduced confusion by mentioning EGT, my meaning being the temps inside the cylinder at their maximum peak pressure and temp, the actual temp inside the chamber at that point, not what would be measured on a probe anywhere in the exhaust system downstream of the exhaust valve.

from my understanding from a maximum temperature and pressure in the combustion process, then any leaking gasses would loose temperature rapidly as they expand in to the area beyond the leak, but the gasses passing through the leak should? not increase beyond what would already be their maximum at peak pressure and temperature inside a non compromised cylinder? am i getting this right?

The boundary layer in the combustion chamber acts as a very good insulator to the combustion chamber wall. This is why aluminium, which normally melts at 660°C-ish can contain gas at well over 1000°C

If the boundary layer is disturbed in some way, say by violent gas motion such as knock, the heat transfer rate skyrockets. That's one reason why knock causes heat damage to pistons and cylinder heads. Heat reduces the tensile stress of the material and the high frequency pressure loads do the rest of the fatigue damage to the surface, causing pitting etc.

With a high velocity gas leak through the cylinder head joint, there will be no boundary layer to speak of and the heat transfer to the surrounding metal will be enormous! Yes, very blow-torchy!

Maybe plasma cutter ish?

Much of what Andy says here, in the hidden detail is so important to many other things…..APS graduates will understand exactly.

Great Post Andrew Whoeveryouare :ok:

The gases are not 1000 degrees.

They are 3500-3800dF and being pushed by about 800-1000 psi of combustion pressure. (on average)

Now think about it. :ok:

Alt-248 is what you want, Walter.

And our degrees are bigger than yours, even in Texas.

Looks like a classic case of a poor seal, being only recently installed would lead me straight back to how it was torqued down or how clean the mating surfaces were.

Interesting choice of placement for the cylinders head bolts, the way their holes cut into the sealing flange seems like a good way to induce distortion. But I'm not an expert.

Jabiru engines look so pretty on the outside.....

Andy:

1000dC = 1832dF if my calculations are correct. That's still about half of the combustion temp.

FWIW, and who cares?? <g>

Technically the combustion temperature varies as the pressure rises, but yeah, who cares ;) It's still enough to burn the barn down and destroy any cylinder heads in the way...