EGT probe location on old style Jab 3300 pipe

[gregv]

Hi all

 

I'm sure this has been covered somewhere at some time in the past, so apologies for repetition.

 

I'm nearing the end of a top end overhaul of my solid lifter 3300 (#391) at 400 hr TTIS. Problem appears to be chronic overheating, evidenced by primarily by cylinder head recession on the exhaust side of all heads and stuck rings in all cylinders at tear down due to excessive burnt oil deposits.

 

I am installing better monitoring (6 cyl CHT monitor is going in this weekend) and am undecided on EGT. Currently I have one EGT probe as installed by the builders in pipe for number 6 only. I would like to add at least one more on the left, but it seems to me putting it further down the pipe where a mixture of all 3 cylinders is seen would be better. I'd obviously have to shift the right side to match to make it useful.

 

My question relates to a 6 cylinder EGT monitoring solution. How close can they go to the exhaust outlet? Obviously closer = hotter. Anyone installed 6 cyl EGT monitoring on this style of exhaust pipe? I'll have to recheck the measurements but from memory I would only be able to get 50mm away from the exhaust flange on all 6 before hitting the common part of the pipe.

 

 

Also any comments on EGT probe type? I was considering this as a easy solution (http://www.aerospacelogic.com/store/index.php?dispatch=products.view&product_id=75) as it all comes in a box and if I install it carefully, at least the measurements will be somewhat internally consistent. Any issues with the clmap type EGT probes compared to the welded on type that is currently fitted to my number 6 pipe (in photo below)?

 

 

 

[Thruster87]

Hi I have the solid lifter in my aircraft and I have an edm 700c which moniters 6 EGT's and 6 CHT's.I placed the egt probes all equal distance from the exhaust manifold flanges but as far as allowed by the jubilee clamps.Where you have the arrows seems to be close. Cheers

 

 

[gregv]

Hi I have the solid lifter in my aircraft and I have an edm 700c which moniters 6 EGT's and 6 CHT's.I placed the egt probes all equal distance from the exhaust manifold flanges but as far as allowed by the jubilee clamps.Where you have the arrows seems to be close. Cheers

Have you noted your EGT's to be higher than those specified in the Jab manual, being closer than the 100mm (or 120mm, depending on which manual you look at) specified by Jabiru?

 

Greg v

 

 

[Thruster87]

Still working on the temps [cht's] as the aircraft has only 13hrs on it.Modifing the ram air ducts etc. The egt's so far in limits [a tad high] and plug colors good.You really don't have much of a choice on the distance.Can't remember exactly but I think it was 50mm ish Cheers

 

 

[jetjr]

I think 6 x EGT is very important to see, you will get a fright with the spread accross the engine.

 

I had EGT probes installed pretty much where you have them marked, I found that you have to make your own limits as you arent where Jabiru says to put them, but they arent too far out. Even being close would be a big start.

 

I tried for a long time to get EGT to stay below 700deg when located at these places, never got 100% good result, there was always a cylinder which creeps up above this.

 

As you are doing overhaul I strongly suggest getting heads machined and exhausts modified for tapered exhausts seals, the gaskets prove a real headache, my first set went 400hrs then every 100 after this needed replacing which isnt an easy job

 

 

[Jaba-who]

Unfortunately this discussion is a bit moot.

 

1. Experience across a large number of Jabiru owners & internet discussion groups and Jabiru themselves (and my own experience matches) is that you need all 6 EGT probes.

 

The spread of temps (and thus fuel:air mix ) is so variable that if you don't have all six you have no idea what is going on. The cost of getting them all is far less than the cost of an overhaul every 400 hours!

 

2. You MUST put the probe at 100 mm - 120 mm from the flange (The manuals now seem to imply that anything in this range is acceptable) .

 

You are not just trying to get "some numbers, any numbers." You are trying see what YOUR cylinder is doing compared to a standardized cylinder on the test bed in the Jabiru factory. To compare your engine you must set it up the same as the test engine. Only then are you able to decide if each of your cylinders is running leaner, richer or same as that ideal engine. If Jabiru have their probes at 100 - 120 mm and then produce a set of figures for you to use - you must take your temps from the same place.

 

If you place them either closer or further then the temps you get will be meaningless, unless you have the capability of then converting those temps to what they would be if the probe was at the same position as the Jabiru test figures. Which is probably too difficult to do, especially when you can just put the probes in the right place to start with.

 

With probes at variable distances you are just deriving a bunch of useless numbers. Putting them in a mixed /common part of the exhaust line would be totally useless. You have no idea which cylinder is contributing what amount of heat to the mix.

 

The reason you need both is threefold:

 

1. EGT tells you about heat production. Heat is generated by the richness/leaness of the burn. The only way to tell how rich or not the cylinder is by measuring the EGT. (Not CHT)

 

2. CHT tells you about the final balance between heat production and cylinder cooling.

 

3. AND If you know about heat production (from EGT) then from CHT you can also derive how effective your cooling is.

 

(Why do you need 6 and not just 1?

 

Jabirus are notorious for highly variable distribution of fuel/air after the carburettor which is caused a known problem of swirling/spiralling air through the carb throat. Jabiru now recommend a baffle in the precarb induction tube ("Cobrahead") to make the flow more smooth. A group of us have done extensive testing with crossed baffles, grid baffles and single planar baffles with results that are definite but slightly variable for different engines - you have to try each in an individual engine to see what works best)

 

The second component is individual cooling of the cylinder (by whatever means - air flow, cooling fins etc) This is then reflected by the CHT. You can have a hot burning cylinder that has great cooling and the final CHT will be good. But conversely you can have a cool burning engine (good EGTs) but with poor external cooling and the final CHT will be high.

 

So in summary What I hope I have said is:

 

You need all 6 EGT and 6 CHT

 

You have to place them where Jabiru tell you to place them.

 

 

[jetjr]

Problem is Jaba-who, you simply cant with these exhausts - When placing them at max possible locations you get numbers which reflect this variation and you have no choice but to set your own limits. My experience is the readings where the arrows are indicated arent very different to Jab limits.

 

@ $1800 for new exhaust system its a bit steep to change over. You need new muffler can and machine heads to make the new setup fit too.

 

A full reco engine comes with new sort though?

 

Im interested in your baffle experimentation - Jabiru say (Thursday) a cross baffle in the aircleaner outlet may help, no mention of the new plenum.

 

I agree you need all 6 EGT and 6 CHT monitored.

 

 

[Jaba-who]

I see your problem.

 

I guess you should talk to the engine shop at Jabiru and see if they can give you some idea just how far out the temps will be if you put them closer than the correct distance.

 

Ask for Don Richter he has the most experience with the engine. But I admit Don can be a bit blase about the figures at times. He told me moving the CHT sensor to the hole in the ridge between the plugs gave exactly the same figures as the ring under plug probe. But when I tried it they were fine for temps below 85 but then they diverged once the temp was above that. At operating temps they read 20 degrees low. All he could say was I had cured my high temp problem! Discussing this with a number of people who have followed the Jabiru advice they found the same thing. So maybe need to be circumspect about what he says.

 

But anyway maybe they do have some figures for EGTs taken at other distances and they just don't publish them.

 

 

[Jaba-who]

I have just added some stuff to my webpage about the baffle experiments.

 

Try looking at http://www.jabiru430.com/EngineCooling.html

 

I have pages of figures for the various combinations that I have graphed in Open Office. But I have to work out how to show them on a webpage. At least the final graphs that show the better combinations that is. Basically they show:

 

The best combination seems to be:

 

A crossed baffle in the carby intake

 

Tilt the carby away from the hot side (if one side is hotter than the other)

 

Run at higher RPMs