Jab Plenum Chambers

[Oscar]

OscarIf you are referring to my comments above it's probably not relevant to my situation as I do a 25 hr oil change approximately every 2 weeks. I personally am more concerned with oil temperatures than anything else from my experience.

 

Aldo

Aldo - my comment was not meant to be a criticism, and 25-hour oil changes is a very conscientious maintenance schedule. My comment was based on the fact that the standard Jab. oil system doesn't carry much heat away from the heat-critical areas, and also the fact that too much oil-cooler area - unless modified by the use of a TOCA (CAMit makes one) - tends to introduce overly-long warm-up times on the ground that can be very bad for the head cooling.

 

IF you have the CAMit oil-piston-cooling mod. installed ( I have it), then you certainly get a heap more cooling effectiveness from the oil and oil-cooling efficiency is very important. The oil-spray to the piston crown reduces the potential for detonation under heavy load considerably, and also reduces the likelihood of ring impaction from burnt oil.

 

 

[Bruce Tuncks]

Oscar, is it possible to install the piston oil-cooling into an older engine like mine?

 

 

[Oscar]

Bruce: we did ours at CAMit, thanks to Ian Bent and the wonderful crew there, on an engine that is very probably the OLDEST engine ever to have any CAMit mods. The only guy who can give you the answer, is Ian Bent - but I would have to say, that my co-owner and I spent nearly three weeks at CAMit in all, to strip, clean, modify and assemble our engine. Ian and his crew not just tolerated us, but helped us on every step of the way - you could not ever meet a greater bunch of blokes. It is seriously humbling to have someone stop a $m-dollar CNC machine to show the process being undertaken to create a complex part out of billet - which they would do out of pride in what they were doing!

 

We are two old retired farts time-rich but not very pecunious, and it was a wonderfully satisfying time for us, as we learned new skills and a huge appreciation of just how complex it is to make a decent aircraft engine!. Think - the ULTIMATE 'men's shed' for aero-nuts... run by people who have the humanity to tolerate and help 'peasants' become basically knowledgeable in areas that they had only ever dabbled in. We'd both built competition car engines, at the amateur level, so we didn't need to be taught how to use spanners.. but Jab.-based engines are another level entirely.

 

If you have the time and the inclination to become a 'three-week apprentice' at CAMit - and if they have the time now to accommodate you - you'd come away with knowledge and satisfaction of achievement that is almost impossible to purchase - and some new friends. Otherwise, my suggestion is that you consider a CAMit 'core-rebuild' engine incorporating all of their mods., or contact Ian Bent for any new information regarding other alternatives.

 

 

[Camel]

Thanks Oscar - love to see what goes for cowls these days and what mods people have down. Quite simply, if the increase in the intake size isn't effective, I will know to go back downstairs to the outlet and perhaps increase lip size etc or add some tab style lips as a trial. So far, anecdotal evidence indicates that intake enlargement is quite successful, so we'll see. The fact is, there are perfectly satisfactory LSA cowled Jab in existence, so I am not of the impression that drastic measures are required, just some tweaking. Next after that though, I am curious to remove that big barrel muffler from behind the trusty oil cooler, as I imagine it could be creating all sorts of complexities in terms of air output from the cooler striking it, as well as the radiated heat.This shows exactly what I have in mind. There appears to be quite a few people I know of locally who have success to the amount of 10-22 odd degrees reduction.

I am by no means an expert here but no dummy either and I know if you open the front opening you have to open up the exit as it needs to expell the hot air not just bring in more air ! Just saying, good luck and would like to hear your results. Had a friend who added a different muffler to a water cooled Subaru set up the muffler reduced the air exiting and caused overheating and after enlarging the exit the engine ran cool again.

 

 

[Oscar]

The trick is, to increase the 'exit' side so that it improves the extraction low pressure. It's not just area, but the effective venturi effect that lowers extraction air pressure.

 

Have a look at the Cessna cowl flaps for a 172: cowl flaps cessna 172 - Google Search

 

The outlet area is hardly much more than the intake area - but it works.

 

 

[Aldo]

Aldo - my comment was not meant to be a criticism, and 25-hour oil changes is a very conscientious maintenance schedule. My comment was based on the fact that the standard Jab. oil system doesn't carry much heat away from the heat-critical areas, and also the fact that too much oil-cooler area - unless modified by the use of a TOCA (CAMit makes one) - tends to introduce overly-long warm-up times on the ground that can be very bad for the head cooling.IF you have the CAMit oil-piston-cooling mod. installed ( I have it), then you certainly get a heap more cooling effectiveness from the oil and oil-cooling efficiency is very important. The oil-spray to the piston crown reduces the potential for detonation under heavy load considerably, and also reduces the likelihood of ring impaction from burnt oil.

Oscar

 

Sorry if that's how it came across I wasn't taking it as criticism. Yes 25 hrs is maybe a little more than necessary (but oil and filters are cheap) and as I average 8 to 10 hrs a week it doesn't take long to come around, the main changes I made were to the exhaust system and I discussed these with Ian prior to making the modifications. 3 x 1-1/4 pipes into a collector then a single 2" pipe out to atmosphere on both sides. This mod dropped both the oil temp and the CHT, I always run straight avgas and the largest jetting on the carby. Fuel consumption averages around 25 lph and up to 28 lph if I'm fully loaded.

 

It seems to work for me but that doesn't mean it will work for all and I'm a long way from being knowledgeable on the subject. I use the Jab recommended (in my POH) warm-up 1 min at 1200 and 2 min at 1600 by the time I have taxied to the threshold oil temp is at 50C and head temps at 290-300F (run ups are completed on the run) I don't dilly/dally around and I have no traffic to wait for in most cases. Average time from start up to wheels off is about 5 min and by the time I'm at 500 feet head temps are back to 280 and falling. I fly at 2900-2950 all the time with head temps at 275 or lower, what is interesting to note is that when I pull the power back to 2800 at top of descent the head temp rises back to 280-290 (I guess this is from less fuel flow).

 

Regards

 

Aldo

 

 

 

[Oscar]

Aldo - that sound like very good engine management to me, and no wonder you are happy with the temps generally. I believe that excess ground running time is a real killer for Jab engines and your regime and mods certainly seem to be doing a great job. Is that an 11-row cooler?

 

 

[gandalph]

I average 8 to 10 hrs a week

Aldo I am consumed with envy! You are living MY dream!

 

 

[Oscar]

Aldo - just some further thought - and this is absolutely NOT criticisms, just thought of a more general nature.

 

Straight-through exhaust systems, if well-designed (and having done a few, that includes, if possible, getting the individual stacks length-tuned so there is a constant low-pressure in the joiner collector for the next exhaust impulse), are very beneficial to performance. For car engines, we used to use a pyrometer (nowadays, an IR temp. sensor would do the same thing) to determine where the exhaust pulses were slowest ( i.e. the heat spots on the exhaust stack) and cut each pipe where they were coolest. However, there is a downside to a straight-through system: the noise. It looks as if your situation is one where noise would not be a problem, but for those flying out of airfields surrounded by urban development, excessive noise will be a real problem for continued use of that facility.

 

Noise generation is a serious problem for GA certification, because GA has recognised the 'social consequences' of noisy aircraft. In the certification tests for the Sunbird Seeker, for instance, it was the noise profile ( more prop noise than engine noise!) that determined the climb-out allowable maximum revs.

 

Personally, I find the whine of leaf-blowers in the early morning in a city suburb far more irritating than a group of 50+ Harley's riding past my place, as happens every weekend. But we - as a minority sport - attract ridiculous amount of attention. People want to shut down airfields close to their homes - and local Councils eager to sell off more land and generate more rates income are only too happy to use noise as an excuse to shut down airfields.

 

Modern high-performance motorcycles - in response to 'community' complaints about noise - are so damn quite that you can't hear them coming in a still day. And they deliver of the order of 180+ horsepower from a one-litre motor - serious performance. motoGp bikes with no silencers that have something of the order of 280 hp from one litre., IDLE at around 130dB and peak at 138 dB) I used to ride a Ducati with Conti cans - 900cc's, around 100 hp, noisy enough to trigger car alarms as you rode past. Do that today and you could get arrested by the cops..

 

As a group, we need to be conscious of noise; it would be great if you could extend your research into adding some sort of silencers. Motorcycle technology - where weight is a real concern - MAY provide some answers. Carbon-fibre cans, perhaps?

 

 

[Aldo]

Aldo - that sound like very good engine management to me, and no wonder you are happy with the temps generally. I believe that excess ground running time is a real killer for Jab engines and your regime and mods certainly seem to be doing a great job. Is that an 11-row cooler?

Oscar

 

I will check on the oil cooler but it is the latest Jab 6 cylinder cooler

 

Allan

 

 

[Aldo]

Oscar

 

As a general rule I don't look at most comments as criticism and I value your far greater knowledge than mine on a lot of aviation subjects as well as your well thought out responses and comments (and I'm not P'ing in your pocket here)

 

However, there is a downside to a straight-through system: the noise

Yes and it was considered when we did the mods, it was a few hundred hours ago now but if I remember correctly I think I measured 85db (loudest), I have the correct instrumentation for noise measurements so I will measure it again in a number of configurations and let you know the results. Once I have those results if necessary I will look into some noise reduction mods, you may have some ideas.

 

As a general rule yes noise it not much of a problem but I do fly into places like Archerfield, Toowoomba, Redcliffe, Caloundra (both noise sensitive areas). It's certainly not as noisy as a 210 or Navajo.

 

Regards

 

Allan

 

 

[Aldo]

Noise generation is a serious problem for GA certification, because GA has recognised the 'social consequences' of noisy aircraft. In the certification tests for the Sunbird Seeker, for instance, it was the noise profile ( more prop noise than engine noise!) that determined the climb-out allowable maximum revs.

Yes and these are the issues for the likes of the 210 and 206's

 

Allan

 

 

[Aldo]

Aldo I am consumed with envy! You are living MY dream!

Grandy

 

Yes but I just wish it was that much per week travelling round Australia but work gets in the way and the fairies don't pay the bills. But flying to work and back everyday allows me to enjoy my flying and get paid as well.

 

Aldo

 

 

[Oscar]

Aldo - I can check the GA requirements if you like, but 85dB seems very reasonable - seems like you have really done a top job there. There's a Pawnee tug working out of Camden that flies over populated areas and it absolutely cracks; if they don't quieten it down, I can foresee glider operations being shut down there (ironic, or what?).

 

 

[seb7701]

Well, here's an update following a few different strategies: simply opening up the intakes did do wonders - it dropped around 10 degrees off things and adding about a 1 inch 'fence' on the base of the duct directly in front of No. 2 dropped it a further 10ish degrees also! At this stage, 2 & 4 are my warm cylinders, whilst 1 & 3 are bordering on too cold (has this ever happened in a Jab?????)

 

On the last run, at 20 degrees celsius ambient (ground temp), cruise temps @ 2900 were as follows:

 

CHT's.............EGT's

 

No 1 -108....... No 1- 664

 

No 2-155.........No 2- 713

 

No 3-120.........No 3- 602

 

No 4-150.........No 4- 656

 

Has anyone had experience with the left side being substantially higher than the right? From a duct perspective, both sides are almost identical, so it's got me scratching my head for sure. I am waiting on a Manometer, so will check the pressure differentials, but going on 1 & 3, the cowl outlet situation looks just fine...

 

I have actually tried the 'carby tilt' technique, with nil effect, so at this stage jetting is my next curiosity. It seems I have older jets, with a 255 main and possibly 276 needle?

 

From the EGT's, I gather that 2 & 4 may not be getting as much mixture....

 

Any thoughts??

 

 

[Old Koreelah]

Very interesting work, Seb. Those CHT variations are damned hard to control. Are your numbers straight and level cruise temperatures or maxima obtained on climb out?

 

I'm on my third pair of ram air ducts and still fiddling with outlets. They are at the sides, in the low pressure zone above the leading edges of the wing. At one stage, like yours, the RHS of the engine was running much cooler than the left. I put that down to the RHS outlet being larger than the left. (I had assumed that the outlets would induce low pressure evenly across the engine bay, but that doesn't seem to be the case.)

 

Last weekend I got the max. CHT down to 139 on climb out, but there's still a variation of about 20 degrees between heads. My new oil cooler is running far too hot, so I've got some work to do there.

 

When I sort that lot out I want to install updraught cooling, which makes so much more sense to me. I've figured out how to build and fit the ducts, leaving the top of the engine totally clear.

 

 

[Bruce Tuncks]

Seb, I posted this elsewhere, but I have been through the same procedure. As Oscar says, evening the temps is an arcane science.

 

A forumite from Mildura put me on the right track by saying how nothing much before the carby did much good. I had even tried stuffing the inlet scat hose with fat straws, after seeing how this was used in a wind tunnel between the fan and the test chamber.

 

What worked the best was to put a cross-piece with vanes at the entrance to the plenum. In my case no2 was much cooler than the rest.

 

From the pics you will see there is still a fair bit of variation, but it is a lot less . I started with no4 EGT over 800,

 

 

 

 

[horsefeathers]

Well, here's an update following a few different strategies: simply opening up the intakes did do wonders - it dropped around 10 degrees off things and adding about a 1 inch 'fence' on the base of the duct directly in front of No. 2 dropped it a further 10ish degrees also! At this stage, 2 & 4 are my warm cylinders, whilst 1 & 3 are bordering on too cold (has this ever happened in a Jab?????)On the last run, at 20 degrees celsius ambient (ground temp), cruise temps @ 2900 were as follows:

 

CHT's.............EGT's

 

No 1 -108....... No 1- 664

 

No 2-155.........No 2- 713

 

No 3-120.........No 3- 602

 

No 4-150.........No 4- 656

 

Has anyone had experience with the left side being substantially higher than the right? From a duct perspective, both sides are almost identical, so it's got me scratching my head for sure. I am waiting on a Manometer, so will check the pressure differentials, but going on 1 & 3, the cowl outlet situation looks just fine...

 

I have actually tried the 'carby tilt' technique, with nil effect, so at this stage jetting is my next curiosity. It seems I have older jets, with a 255 main and possibly 276 needle?

 

From the EGT's, I gather that 2 & 4 may not be getting as much mixture....

 

Any thoughts??

Hi Seb.

Do you have any before and after pictures of your modification?

 

 

[Bruce Tuncks]

The pics are on cruise-climb, and on egt the hottest is now no1 . On cht, the left side nos 2 and 4, are a bit hotter but not by much.

 

I reckon the engine runs smoother now but that is a bit subjective .

 

If you want more details of the cross-vane setup, let me know.

 

Old Koreelah, a max cht of 139 is real good. And with the assymetry of the cooling airflow with fore and aft cylinders, 20 degrees variation is good too. But what oil temps are you seeing?

 

It will be interesting to see your updraft system one day.

 

 

[seb7701]

O.K. - sounds like you have achieved nirvana with your temps! If you have an pics, I would be curious, particularly if the top of the engine is totally clear!

 

Thanks Bruce - likewise, your temps are looking really quite good to me with no.4 not being overly outlandish. Bruce - I have read about the cross pieces in the cobra head. Did that actually lower some temps for you?

 

Just went for a lap since that last post and found no. 1 really probably too cold to be flying. With a little defending here and there (power left on as best I could...), it was actually in the 80's and 90's at times!!

 

The temps quoted are straight and level @ 2900.

 

Please don't judge me on duct tape, as the fence was strictly trial only and now to be permanent! It is possible to go too big, as I also trialled a bigger fence which covered the triangular outline of the head and instead it added about 11 degrees to no 2, but did drop the rear cylinder by 10 degrees to 140. (These figures are around 20 degrees ambient BTW)

 

Just trying to find some other pics, but you may be able to gauge somewhat how much the intake has been opened. (I only have a 'before' pic of the left side I'm afraid...)

 

 

 

 

[Old Koreelah]

Seb you're apologising for duct tape...wait till you see mine.

 

My new oil cooler is a ten-row item similar to what I've seen on Jab sixes. It should work a treat but initial oil temps were 125C. It's mounted on the LHS with an external inlet (so I could tap it's warm outflow for cabin heat).

 

A good idea at the time, but the NACA duct has been a miserable failure. It's "back around the corner" from the ideal location on the sloping front of the cowl. Even the external scoop (with the duct tape) only lowered oil temps by a couple of degrees.

 

My new cowl flaps were also a disappointment, with CHTs still too high (Max. 156C on climb out).

 

As you can see from the wool tufts, the problem is air spilling around the shallow returns on the flaps and reducing the suction.

 

Temporary plywood triangles to seal the gaps made a huge difference. Max climb out CHT 139C.

 

I'm working on some sort of folding leaf mechanism for a permanent fix.

 

The oil cooler will have to go back under the spinner. No more cabin heat!

 

 

 

 

 

[Oscar]

NACA ducts are over-rated as a means of 'scooping' airflow - they work very well for transferring high-pressure to lower pressure areas with minimum airflow drag, but they don't 'grab' passing air and force it into a new direction. If you don't have a decent p-delta between the intake and the exhaust side of a NACA duct, they look good but don't do the job as well as a scoop-type intake.

 

Getting decent air-feed to all cylinder heads in standard Jab ram-ducts is extremely fiddly; I spent, late last year, some time with a guy down at Camden who has managed it on his J160, and the amount of research (and the quality of it) that he had done to achieve really impressive results, was outstanding. He even put a borescope camera into the ram ducts and wool-tufted the baffles to fine tune them - and found that a few mm. of difference between the baffle shape between left and right sides was quite critical. He spent literally MONTHS of work fettling, testing to a tightly-controlled regime, changing one parameter slightly, re-testing..

 

Since I needed to build new cowls for my own beastie, in consultation with Dafydd Llewellyn who has a great deal of experience in this area, I've gone a completely different route - and I can't yet say that it will work!. But, I've used the work that was done on the prototype Jab engine install for a Bristell, that had good results.. so I'm hoping..

 

First up, I wanted to get the oil cooler air circuit clear of the cowl so it didn't reduce the cowl low pressure side p-delta. A completely separate duct system was needed for that: and this is what I came up with, to accommodate a seven-row Aero-classics cooler:

 

 

That makes better sense in the context of the lower cowl:

 

 

(Those 'quarter-donut' shaped lower inlets have been modified since that piccy was taken to a pair of circular intakes inserted in the holes; the oil cooler inlet remains the same. I have added a larger-base diameter spinner, roughly the same as on the newer Jabs, though from my own mould.).

 

Because this work has a significant duty to perform as research into the requirement for head AND barrel cooling, I've completely separated the cooling for both - though eventually, it may prove to be unnecessary. The upper-cowl intakes supply ONLY the head cooling, while the lower ones supply the barrel cooling.

 

For the head cooling, I want to get the intakes to operate with best efficiency for prop blast effect - which means getting the maximum intake area out into the area where the prop actually has effective blade shape:

 

 

The bump on the top of the cowl had to be added to clear the starter motor, and the oil check plate allows for a CAMIT barrel -inhibitor system, not shown on the next piccy)

 

So - what does all of that feed?

 

 

All the baffles there, are templates for fitting cut from old scrap aluminium sheet that's been hanging around my workshop for nearly 30 years- not the real ones. And there's more, but I don't have useful piccies of them. The barrel cooling is fed by a pair of 2.25"tubes from those lower intakes,and will exit through under-barrel baffles designed to meet - as best I can manage - the NASA research for cooling design for in-line air-cooled engines. I'll probably have the final baffles, complete with MacFarlane 'Cowl-saver' baffle seal attached, in a few weeks - if Winter would sod off.

 

It's taken more than six months of part-time work to develop the cowl moulds, the baffles etc. - only a complete nutter like me would DO that. AND - I don't know if it will work! But, it keeps me from making a fool of myself by going 'clubbing', or being reduced to golf for a hobby. I DO recognise that I may need professional help here, to get me over my compulsive tinkering obsession...

 

 

[seb7701]

Nice work Oscar! I will be particularly interested to see the results of the plenums depicted in the last piccie. I am fascinated by the effects of oil cooler 'exhaust' on the happenings inside the lower cowl, so your duct makes some sense. I did see a similar setup on a U.S website, so must be something in it.

 

In the mean time, given that I have been cruising around this afternoon with 108/120 on the right bank of my engine, I would say that success hopefully won't require me renting the NASA wind tunnel just yet, but we'll see... If that is what is achievable with my fairly standard setup, there is scope for success yet.

 

After I check pressure differentials, I would say it might be time to enlarge the left intake yet again, as my mod, whilst effective, seems modest compared to some I've seen, so should leave some room to stuff some more air in there! The results from the right cylinders indicates that the cowl lip is working just fine, so if I can get it in there, I reckon it'll come out!

 

Some more shots from this arvo- the only thing I didn't mention, is that I added CAMIT 'cooling plates' to the sides of the cylinders. They just make sense.

 

 

 

 

 

 

[jetjr]

I too have too cool cht one side of a 3300, rhs too

 

Started looking at installing a fence at the inlet to restrict air to RHS, if suction is whats driving it all then this should decrease LHS or at least bring RHS up to similar levels.

 

Havent gone far enough yet but in my mind even is better than too low temps

 

Oil cooler inlet sure stuffs up cowl airflow, partially blocking during winter effects cht

 

 

[Oscar]

Thanks for the nice words, Seb - and yours looks very schmick, I have to say. I had to build new cowls - mine were originally for the 1600 engine (that aircraft was used as one of the factory hacks to develop the 2200) and were altogether a nightmare - cut-and shut inlets roughly done, and the lower cowl practically touching the muffler with large, badly scorched areas in it. The join flange between the bottom and top halves of the cowls at the #1 head had been cut out to clear the fins but was still rubbing on them at the front..

 

 

(That's a cardboard mailing tube doing duty as a fake muffler on my dummy engine, as mine was still up at near Toowoomba being used to develop the engine test running cell when I started building the new ones). Similar scorching on the rhs as well..

 

An interesting point I picked up from that guy I mentioned at Camden who has done such a great job: too much exit lip at too steep an angle can cause the flow to stall off its back edge in steeper climbs and that will badly effect the outlet efficiency; when I last talked to him he was judiciously pruning it back a bit, with his usual methodical approach.