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My early model SK has the leads on the outside but later model 4 cyl engines have bigger cooling ducts and the leads on the inside. I reckon your idea is worth trying.  You might have to do something extra where the spark plug connectors go in.

 

 

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Paul this topic came up recently on the Jab/CAMit forum. [email protected]

 

Someone suggested possible problems from the plug leads getting hotter inside the air ducts than they would be outside, like the earlier Jab designs.

 

I've heard of good results with Jabs that have had their head shrouds replaced in favour of whole-engine plenum as per Lycomings, etc.

 

 

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My Jab3300 engine has the original plenums with the leads on the outside. This was mainly due to the fact that the later larger ones would not fit under my cowl as they were too tall. I do have custom made intakes but my engine has always run cool. I did spend considerable time on cooling (read the NASA Research report CR3405, 152 pages on Aerodynamics and cooling of an air cooled horizontally opposed aircraft engine installation) and the air exhaust at the bottom is over a metre wide with a lip to provide suction. The minimum rule of thumb is 3 x the area of exhaust to intake. Intake area must also include the oil cooler unless it has a separate intake NACA duct and exhaust as mine has.

 

The early Jabs had a really small cooling air exhaust & Jabiru came up with a Lip kit to improve suction for those aircraft

 

 

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The old style with the leads outside the ducts work well for me, but I doubt that having the leads inside would heat them up much, as the air passing them is on its way to the hot parts, not having passed them.

 

One the air starts its downward journey, past the cylinder head fins it is picking up heat.

 

 

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The old style with the leads outside the ducts work well for me, but I doubt that having the leads inside would heat them up much, as the air passing them is on its way to the hot parts, not having passed them.

 

One the air starts its downward journey, past the cylinder head fins it is picking up heat.

 

Yenn you're right about the in-flight air keeping the leads cool, but after shut-down their temperature would rise rapidly. The top of my cowl is very hot for ages after I park my plane and it amazes me that the epoxy hasn't distorted as a result. I dimly remember adding a few layers of vinyl ester for its heat-tolerance, so maybe that's why it's kept its shape.

 

 

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The ducted air is pushed down when engine is operating,

 

BUT

 

An idle engine has the hot air rising, against the ducting.

 

add a battery blower to cool the engine to a better temperature.

 

spacesailor

 

 

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I've just done the opposite on my early ducts,cruise temps were 310F,so after some discussion decided to do some mods,now leads are 

 

inside like the later D model and cruise temps immediately dropped to 245F and been like that for 6 months.Interesting comments about the

 

leads sitting on the hot fins,will take a look and see if they can be held up out of the way.

 

cheers

 

colin

 

 

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I’m not so sure the leads being in the ram air ducts  is as big a negative deal as you might initially think. 

there’s two potential issues - effect on the leads and effect on engine cooling. 

 

 

Obviously there’s less heat to the leads  during flight compared to on the ground due to airflow cooling them but I guess they heat up after shutdown for a while. But the practical experience is that by the time you descend and land and taxi at low power the engine temps are pretty low unless you are stuck on the ground for long periods I guess. 

 

 

As for engine cooling effects. If you look at the positioning of the leads inside the ram air ducts they are to the sides well out of the way of the actual (preferred) air path. 

the preferred air path is through the fins from top to bottom ( well inside the path of the leads). 

if anything you actually don’t want the air to travel down the sides where the leads are but to be forced to track inside and down through the fins. 

 

I’d guess they might serve a positive purpose being in there as evidenced by Coinz experience. 

 

 

 

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It's one way of making sure they don't droop and melt onto the fins. Keeping leads clear of the cylinders is essential.  After shutdown  it's a pity the heat can't be let out of the cowl better. Nev

 

 

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It's one way of making sure they don't droop and melt onto the fins. Keeping leads clear of the cylinders is essential.  After shutdown  it's a pity the heat can't be let out of the cowl better. Nev

 

Never seen an ignition lead melt. But that could be because I have mine zip tied together and they stay in place and away from the metalwork. 

 

 

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If I didn't have such a backlog of projects I'd be redesigning my airflow to come in under the spinner and rise up thru the fins. Updraught cooling works well for pushers like Rutans.

 

Would this mean having the outflow going up through the top cowl or turning over and going downward again after the engine? 

 

 

just wondering what evil effect, if any, would hot air etc have in blowing across the windscreen. 

 

 

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If I didn't have such a backlog of projects I'd be redesigning my airflow to come in under the spinner and rise up thru the fins. Updraught cooling works well for pushers like Rutans.

 

Getting the baffles done below the cylinders would be a challenge, exhust, engine mount, induction tubes (most engines) all in the way. The Cirrus above makes it look simple. The Rutan pushers may have no choice due to lack of pressure on the top side ?  

 

 

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One of the major issues is springing an oil leak and covering the windscreen, obscuring  visibility.

 

Pushers obviously have no problem with this.

 

Probably with a fire, you don't want it coming out the top a also. 

 

 

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I’ve always made a practice of opening the dipstick access flap after shutting engine down . It’s surprising the amount of hot air that’s evacuated via the opening, and how quickly the engine bay cools...... Bob 

 

 

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Would this mean having the outflow going up through the top cowl or turning over and going downward again after the engine? 

 

 

just wondering what evil effect, if any, would hot air etc have in blowing across the windscreen. 

 

The second option, Jaba. My cooling air already exits out the sides, into the low pressure zone over the leading edges, so it should make its way out without too much trouble.

 

Updraught cooling is probably only a good ideas for pushers.

 

I agree it's not a good idea to have any vents in front of the windscreen. One oil leak and you're blind. 

 

 

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Getting the baffles done below the cylinders would be a challenge, exhust, engine mount, induction tubes (most engines) all in the way. The Cirrus above makes it look simple. The Rutan pushers may have no choice due to lack of pressure on the top side ?  

 

A good point, T88, but I've put a lot of thought into how to make the ducts. Probably no harder than the above-engine ducts (the third ones I've made). Two pieces each side, with overlaps around the pipes. Sealing around the heads so that all cooling air must go up via the fins.

 

I've had quite a bit of interest in the idea from a couple of industry gurus. I believe one advantage would be improved cooling for the hottest bits, because they get the cooling air first. 

 

Just wish I had the time and energy to do it...

 

 

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 You only have a few inches of hot air effect. It's almost inconsequential if one considers the pressure drop across a cowl for normal flow. It would help after shut down to have an exit but you would have to consider any downside of another catch to be left unsecured. Parking into wind will help where you have a choice. Nev

 

 

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My problem is that one side runs 20 degrees cooler than the other. The right-hand cylinders 1 and 3 are cooler than the left-hand 2 and 4.  I have tried an extender lip on the left-hand ones, to try and divert more of the prop air into the duct, but this had no effect. The hot cylinders have the upgoing prop, and they are further back. 

 

Maybe discarding the ram-air ducts and putting in a plenum chamber would solve the problem.  But apart from the uneven cooling, those ram air ducts sure do make for a tidy setup.

 

The engine does not run real hot, on climbout the max ( no 4 ) can be kept under 160 C by being careful.

 

Maybe another scoop just feeding into the hot side duct? Maybe an electric fan helping the cooling flow?

 

 

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