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I know of it being used on a nitro aircooled speed attempt and steam was used.  It's a pretty rare situation where that would  need to be used. High specific horsepower / Litre. The Jabiru is well inside the power density aircooled engine practical ceiling of around 50 HP/litre. The Jab lacks the controlled airflow around the cylinder fins that most other similar aero engines almost universally use (Baffles and ducting shields). They are not hard to fit and don't run it for long with the cowl off. Nev

 

 

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Looking back at post No 1 Is see the max temp was 161 degrees.

 

My Jab manual recommends a max of 170 deg or 150 continuous, so there is no real problem.

 

Lower the nose for more speed as soon as practicable.

 

If you really want to get the temps down, try more baffling and ducting.

 

 

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I don't think a  bit of spread matters much I don't like putting sensors under the plugs. Most P&W BRW's run to 235 Cdegrees as a max. I'm not suggesting that is suitable for a Jab. You have NO idea what the piston crown temps are and they have just as much effect on detonation as the head temp does. The most likely things to cause detonation are hot carbon build ups hot exhaust valves and spark plugs crook fuel and a momentarily stuck valve which can force high pressure into another cylinder. like what happens to LPG engines when they backfire into the inlet manifold.  That even shortens conrods. so would easily stretch the long Jabiru cylinder studs. Nev

 

 

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The Jab lacks the controlled airflow around the cylinder fins that most other similar aero engines almost universally use (Baffles and ducting shields). They are not hard to fit and don't run it for long with the cowl off.

 

 

This above statement is the best advice you can get. All air-cooled engines should have tight-fitting, effective baffling and ducting, to ensure the airflow is getting to the fins, where it's most needed.

 

 

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The accuracy of your method of measuring is obviously a problem if it's not reliable. Surely you would transpose the senders if you suspected this. Where an engine type is sensitive to heat problems  of course colder is better, but it gets harder to achieve and the less  the temp , the more reactive it is to ambient as heat flow is proportional to temp difference. That's one of the advantages of air cooling. The exhaust port area will be the hottest area of the head and that's where the bolts take up the most and who knows how hot it is there? There used to be paints that changed colour when a certain temp is reached.  and they were applied to the Wright cylinder heads when their excellent J5 engines were being developed. Nev

 

 

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You can still readily acquire temperature-indicating crayons, they are called Tempilstiks and they are still widely used in industry, for measuring heat levels in alloy and low-alloy steels, when heat-treating or welding on them. 

 

They melt when the preset temperature they are designed to melt at is reached. You can still also buy temperature-indication paints, pellets and temperature-indication cards.

 

All these products are merely useful indicators, and for precise accuracy, a thermocouple is the most desirable temperature indicator.

 

https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-a-tempil-stick

 

 

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I don't want to give the impression my Jabiru is malfunctioning. On the contrary, is is 18 years since its first flight and it hasn't had any problem in the air at all. I'd buy another Jabiru for sure. ( see how the lost valve-seat from ground-running at high power is  not considered a fault of Jabiru's making) 

 

But as a tinkerer, trying to get those CHT's all the same is quite a challenge, and the LHS ones do run hotter than I like. On a warm day, 34C for example, with 2 people on board, the climb is a lot slower than I would like it to be, on account of the need to keep the max temp below 170C.

 

While it would be better if the presently hottest cylinder could be kept back to the coolest by ducting and skirting alone, I would settle for a control which seemed to magically cool the engine almost instantly. That would be the water spray system.

 

To keep things in perspective, a Lancair I know well has a max temp of 190C, and it often runs over 170. But the Jabiru heads on the old engines like mine  were made from quite soft alloy and I reckon I've seen one which distorted without the pilots being aware that they were overheating it.

 

Part of the blame for this lies with the original CHT gauge which was a thermocouple with its cold junction under the cowl. It under-read on hot days for sure.

 

All of this is history now as the latest Jabiru suffers from none of this.

 

 

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The  steel cylinders on the old motors are not good conductors of heat. It's common to see distinct bluing of the area which is adjacent to the exhaust port. This causes bore and piston distortion and wouldn't help ring sealing. The main path for heat out of the piston and rings is the cylinder (not the Head). The extra fins on the upper sides of the heads doesn't do much for the other side of it as it's a fair distance away. so there's going to be a large temp variation across it.. As to material we don't know a lot but you can be sure there are better alloys. Heat treated heads  of aero quality can run to 235 Degrees C MAX  and I presume the heat treatment process will suffer if they go above that. The best ones of them are forged and then CNC'd and a semi permanent fit to the cylinder. Screwed and shrunk  eliminating the joint problem.. Not cheap when good quality control is maintained. Often the bores are nitrided. (hardened on the surface). ALL steel bores can rust.  Nev

 

 

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"The Jab lacks the controlled airflow around the cylinder fins that most other similar aero engines almost universally use (Baffles and ducting shields). They are not hard to fit and don't run it for long with the cowl off."

 

I spent ages and constructed a full plenum setup similar to Lycoming, only to find it ran way hotter than the fibreglass ducts. A lot of tinkering later I went back to the originals...

 

 

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While it would be better if the presently hottest cylinder could be kept back to the coolest by ducting and skirting alone, I would settle for a control which seemed to magically cool the engine almost instantly. That would be the water spray system.

 

To keep things in perspective, a Lancair I know well has a max temp of 190C, and it often runs over 170. 

 

The water spray system would be interesting experiment fitted to the hot side only.

 

Lycoming's have a max CHT of 260C and a recommendation of max 225C for long engine life. I think the numbers are so much higher than the jab because of the Lyc probe being located on the underside between the ports.  

 

 

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 Yes, the location of the sensor is a major factor on what temp you read. (as you would expect) The Lyc and such use the heat treatment factor  Limit 235 C which is a different matter from having  detonation etc issues. Looking up the Temper chart for steel, the blue I often see in the bores near the exhaust valve is equal to 282 C This is clearly far too hot. and it's not even around the bore so there's distortion too. Nev

 

 

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Point taken guys. If I do try it , it will be a last resort and I will proceed very carefully, probably with the hot side only as thruster suggested. And with a very small amount to begin with. But if you have ever flown through a rain shower, you will have done much more than I am contemplating.

 

We used to crack Lycoming cylinders from shock cooling at the gliding club on our Pawnee tugs. Our tugmaster, a great guy, tried to save us money by buying " zero timed, factory reconditioned " cylinders. Alas, we found, you can zero time the paperwork but not the metallurgy.

 

 

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Spacey, if you fitted cowlings that wrapped around the rear of the cylinder and heads and which directed cooling air to the rear of the cylinders and heads, you'd find your engine cooling would improve.

 

 

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This has worked for me.  Move the oil cooler to the rear of the engine and duct air to it via a side naca duct (like carby inlet), reduce the size of the lower air intake that was previously there for the engine mounted oil cooler, built a close fitting duct from this reduced inlet to the sump. Max CHT less than 140 on the ground which quickly reduces to 110 for climb at 80 knots (700-1000 fpm), cruise CHT at 100 knots 105 degrees C.  I had a similar setup on my Corby Starlet and replicated it on the Jabiru. No need for cowl skirt or big mods to inlets.  I did add a bit to the cooling ducts to prevent air from going under the front cylinders.

 

 

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 Higher speed planes seem to be much less of a problem . That's pretty understandable  If you do the cooling thing well, you will use less air more efficiently.  You work on a pressure differential through the cowl, but it may not flow in a pattern where it removes the heat from the required parts. It will go the easiest way through the cowl  and the finer(closelyspaced) the fins are the more trouble you must go to to get the flow right and stop air leaking through other areas. Nev

 

 

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This has worked for me.  Move the oil cooler to the rear of the engine and duct air to it via a side naca duct...

 

NACA inlets are not a panacea; they might work if placed well forward, where the cowl is still "sloping" into the airflow.

 

I wasted lots of effort with a separate oil cooler NACA intake- it was "around the corner" from the front of the cowl and led to oil temps of 130C.

 

 

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NACA inlets are not a panacea; they might work if placed well forward, where the cowl is still "sloping" into the airflow.

 

I wasted lots of effort with a separate oil cooler NACA intake- it was "around the corner" from the front of the cowl and led to oil temps of 130C.

 

Bruce’s SK has the same cowls as mine and probably similar performance. He is trying to resolve high CHT’s. What I have done works. 

 

 

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Keenaviator. What you have done seems to me to be a lowering of pressure beneath the engine, combined with ducting air around the sump for an oil cooling effect. Would that be correct?

 

If so the lowering of the CHTs would be due to greater cooling air from top to bottom of the heads.

 

 

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Keenaviator. What you have done seems to me to be a lowering of pressure beneath the engine, combined with ducting air around the sump for an oil cooling effect. Would that be correct?

 

If so the lowering of the CHTs would be due to greater cooling air from top to bottom of the heads.

 

Absolutely correct.  That was the aim and it appears to work very well.

 

 

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