Jab Plenum Chambers

[facthunter]

The two carb set up tor a 912 is much less than ideal. When checked the mixture distribution is not good There is very uneven pulsing with the cylinders on one side with a flat four. On a six it CAN be a solution depending on the crank throws. Nev

 

 

[jetjr]

Nice work

 

I have read of a similar outcome before in a motor glider but never saw proper numbers.

 

In cold climates a "protected inlet" or using under cowl air is ok even in certified setups. Someone else might know more.

 

The air shouldnt be that hot and in the Jab you still have filtered and carb heat if you need it. Engine usage will also increase airflow through cowl helping cooling if you block off the naca inlet altogether.

 

Nice part is that is simple to try and can be undone easy too.

 

I have a perfect silicon elbow but its not doing much on the bench

 

 

[Old Koreelah]

Nice workI have read of a similar outcome before in a motor glider but never saw proper numbers.

In cold climates a "protected inlet" or using under cowl air is ok even in certified setups. Someone else might know more.

 

The air shouldnt be that hot and in the Jab you still have filtered and carb heat if you need it. Engine usage will also increase airflow through cowl helping cooling if you block off the naca inlet altogether.

 

Nice part is that is simple to try and can be undone easy too.

 

I have a perfect silicon elbow but its not doing much on the bench

A timely comment, JJ. Today I'm rebuilding my cowl, putting the oil cooler back under the spinner, and I've still not decided on the shape of my carby intake. (In its last incarnation, it shared the NACA duct with the oil cooler on the LHS). I might just extend the side patch to completely cover where the NACA duct was; the carby can pull air from the engine bay. A temp probe in the air filter box registers up to 58C when I apply carb heat, so it should tell me how hot the engine bay air is.

 

 

[seb7701]

Well, a few further figures since the deletion of the naca duct and the move to simply sucking air from inside the cowl.

 

Today at a previously iffy ambient temp of 26 degrees, I saw EGT's more even than ever before!!

 

Admittedly, these end figures were at 5100ft, but still-

 

CHT/EGT's as follows:

 

99/619

 

140/626

 

120/606

 

120/564

 

The comparison is a previous flight at 24 degrees ambient where EGT's were more like

 

106/632

 

155/686

 

116/580

 

145/679

 

Biggest gap in EGT's went from 106 to just 62!!!

 

Would love to know what jetting you guys are running, but the 2.82 needle jet is going to be swapped for maybe a 2.80 for a try, as I am (unfortunately) averaging 19LPH with a few good climbs thrown in.

 

Anyway, some of you guys may perhaps wish to check out the air box biz as I have!!

 

 

[geoffreywh]

Has there been any input from the Jab factory about removing the NACA inlet? It sounds very promising. The 24 reg people would like to know....

 

 

[Bruce Tuncks]

Congratulations Seb, while we would all like the figures to be the same, the best we can hope for is for max CHT less than 150 and max EGT less than 750.

 

You have done a great job.... Bruce

 

 

[Old Koreelah]

A timely comment, JJ. Today I'm rebuilding my cowl, putting the oil cooler back under the spinner, and I've still not decided on the shape of my carby intake. (In its last incarnation, it shared the NACA duct with the oil cooler on the LHS). I might just extend the side patch to completely cover where the NACA duct was; the carby can pull air from the engine bay. A temp probe in the air filter box registers up to 58C when I apply carb heat, so it should tell me how hot the engine bay air is.

This arvo I finally got to test fly with the carb intake inside the engine bay. I won't be doing it again. Climb out was low and slow. The intake temp (measured between the air cleaner box and carby) climbed to 68C. That's the hottest I've ever got with carby heat. (I wasted lots of time making a complicated hot air glove around the muffler, when I could have just tapped air of the same temp from anywhere in the engine bay!)

 

As the sun was setting I cut a hole in the side of the cowl, bent up a bit of al sheet from some discarded roofing panels and by dark I had a cool air intake. Not pretty, but it should work well enough to get me to Narromine tomorrow.

 

 

[seb7701]

Thanks guys - yep, very pleased!

 

Geoffrey - the Jab factory is currently playing with the NACA duct (which is where I got the idea), although the are covering it and replacing with a 2.25 inch (I think?) hole. I just covered the lot... :) As such, I dare say you will hear more.

 

Thanks Bruce - No.2 CHT at a more regular 3500 ft was around 152, but would probably have stabilised out a bit more if I wasn't busy step climbing to 5500 as it was easily doing 1000ft increments while reaching max CHT of 165 on the hottest. Therefore hopefully sub 150 will be fine. Just sucks mind you that no.1 was like...99 degrees!!

 

Anyhow, will wind back the thirst via the jetting, hopefully at the detriment of the EGT's rather than the CHT's... Also going to check the inlet static pressure now that I have the trusty manometer. Might give some more clues.

 

 

[jetjr]

Yes, spoke to Rod today, they have several attempts using straight hole rather than naca, exact locTion I see difficult needing zero positive pressure

 

There's evidence it works well

 

 

[jetjr]

 

Front naca blocked off, fair bit of work went into finding that location for the new intake

 

I have pic of 4 cyl Jab too somewhere

 

All credit to Jabiru for being very open and discussing these advancements

 

 

[horsefeathers]

Hi Guys.

 

Been following this discussion with great interest, and would love to have your thoughts on CHTs on a 2200

 

I recently installed a gauge with a 4 CHT readout, and all CHTs are within 5 degrees or so of each other, EXCEPT for #4.

 

This consistently reads 35 deg F higher than the other cylinders.

 

Cruising temps are approx 230F on 1 through 3, and 4 will be approx 275F

 

Also, when taxying, the CHT on #4 climbs much quicker than the other heads.

 

The probes for the new gauge sit under the head bolt closest to the exhaust valve, and the original CHT sender (on 4) is still operational, and is under the spark plug. The original and new gauge are in reasonable agreement on #4 head temps.

 

Oil temp sit about 90 degreesC. Standard Jab ram air ducts

 

So, my question is, is there a way of getting the CHT of #4 down to the level of the other heads?

 

 

[seb7701]

Horsefeathers - for starters, great temperatures, which I could only dream of!! Oil temp is great also. have been very interested to know what sort of difference I would see between the speak plug probe vs screw type, so that's interesting to hear that you tried it and that they are in fact similar.

 

I am gathering that you don't have EGT's which are a good indicator of mixtures, but nonetheless, I would be thinking that no. 4 is running a little leaner. Not sure how it works if you have a pusher installation, as per your avatar, but the change in intake for me clearly richened up no.4 specifically, so who knows...

 

If you don't mind me asking, how many hours on the engine? What sort of jetting and what sort of ambient temps are you operating in?

 

 

[horsefeathers]

Horsefeathers - for starters, great temperatures, which I could only dream of!! Oil temp is great also. have been very interested to know what sort of difference I would see between the speak plug probe vs screw type, so that's interesting to hear that you tried it and that they are in fact similar.I am gathering that you don't have EGT's which are a good indicator of mixtures, but nonetheless, I would be thinking that no. 4 is running a little leaner. Not sure how it works if you have a pusher installation, as per your avatar, but the change in intake for me clearly richened up no.4 specifically, so who knows...

If you don't mind me asking, how many hours on the engine? What sort of jetting and what sort of ambient temps are you operating in?

Hi Seb.

 

Just to clear up some confusion, I'm flying a Jab 160 (the avatar is my old Streak Shadow, with a rotax 582).

 

I have 300 hours on the engine (and airframe), and the last leakdown was 80/78 on all cylinders, so no probs there. I have put the Jabiru recommended jetting in about 100 hours ago (except the idle jet which from memory is a "35" instead of a "45").

 

Ambient temps are about 20 C (I use a simple digital OAT thermometer).

 

The new CHT probes are from Sonex, and designed for a Jab 2200 (old thick fin heads only) Sonex Web Store - AeroConversions CHT Probes (set of 2) AeroConversions CHT Probes (set of 2)

 

About 10 engine hours ago, I added a flat aluminium plate BEHIND the oil cooler- it is attached to the shield in front of the muffler, and extends about 12 inches parallel to the oil cooler. The thinking here is to direct the oil cooler air "exhaust" down towards the firewall, and prevent/minimise it mixing with the low pressure air from the heads. I have seen an observable decrease in all CHTs of about 25 F. The original gauge (head #4) NEVER goes over 300 F now, and that figure is backed up by the new gauge. A quick and dirty installation, but after this success, I am going to try to enclose and capture that exhaust air more completely.

 

However, that #4 CHT is annoying.

 

Cheers

 

Gerry

 

 

[geoffreywh]

fotos please to show flat plate. Please

 

 

[horsefeathers]

fotos please to show flat plate. Please

No probs. ( i KNEW i should have taken photos before I put the cowl back on )

 

Next time I'm down the hangar, I'll be sure to take a photo or two.

 

 

[jetjr]

Jab USA has a good document of how to go about setting up deflectors to even out cht

 

 

[seb7701]

H.F. - I KNEW I should have checked what plane, before making assumptions about the streak!!!!! :)

 

Ok, well I am intrigued, as I have often wondered about some of the setups I have seen with ducted oil coolers. I am curious to know how that works, but am guessing it just send the hot air nicely 'south' towards the outlet? I too would like to see some shots if that's OK...

 

So the 160 will have bigger cowl inlets than me and presumable hydraulic lifter donk, so the jetting is going to be different anyway i guess, otherwise I'd be asking a needle jet size to try out. (I am having interesting results with jetting trials...) In any case, they are very enviable temps at 20 degrees OAT!

 

I have just removed both baffles from my muffler, so will be interesting to see if there is a difference.

 

 

[jetjr]

Been discussed before, can't really compare temps from different CHT probes, instruments mounted differently. Could be cold junction errors at a minimum

 

Work was done seeing 20-30 deg higher readings when probes shielded from air flow

 

The members seem great, by the way but needs to be measured same way and same probes to be comparable to others or the Jabiru/CAE specs

 

 

[horsefeathers]

Been discussed before, can't really compare temps from different CHT probes, instruments mounted differently. Could be cold junction errors at a minimumWork was done seeing 20-30 deg higher readings when probes shielded from air flow

The members seem great, by the way but needs to be measured same way and same probes to be comparable to others or the Jabiru/CAE specs

Hi Jet,

 

Yep, I'm sorta aware of the problems with comparing the 2 probe types, but I'm not really after absolute numbers, more the delta, either over time, or due to changes in configurations.

 

The way I look at it, if I see the temps changing for no apparent reason, I MAY be able to find and or prevent damage to the engine.

 

And, even though they are in different locations and use different gauges, its nice to see a correlation between the 2 different probes on #4.

 

I really am confused as to why head 3 runs about the same as 1 and 2, while head 4 runs hotter - the probes are in the same relative position re airflow - I'm wondering if I can slightly increase the internal baffle size above #4 to push more air onto the head, but with my existing temps, I'm just focusing on airflow around the oil cooler atm.

 

 

[seb7701]

Not sure if I asked HF, but have you got EGT's also? My problem child cylinder appeared to be mixture rather than airflow, as both my ducts are identical as can be and have extremely similar pressure differentials compared to the outlet. As such, blowing more air over my problem cyl head would only get me so far. Mixture first... The upstream of the carb (I am now convinced) is everything!!!!

 

 

[jetjr]

Yes

 

Within your setup it's all good, just can't compare to

 

Others numbers really

 

Read Jabiru USA PDF on tuning ducts, bit fiddle and trial and test. Make small adjustments

 

 

[seb7701]

Oh, I have read about everything it is possible to find on the net I assure you!!!! As you have said, it's not the figures entirely, but moreso the differences between cylinders.

 

 

[horsefeathers]

Not sure if I asked HF, but have you got EGT's also? My problem child cylinder appeared to be mixture rather than airflow, as both my ducts are identical as can be and have extremely similar pressure differentials compared to the outlet. As such, blowing more air over my problem cyl head would only get me so far. Mixture first... The upstream of the carb (I am now convinced) is everything!!!!

No EGTs, Seb.

 

Like you, my ducts are almost identical, and now hearing from you that the pressure diffs are similar has confused me.

 

I'm going to concentrate on the oil cooler airflow now, and see how that progresses....

 

 

[seb7701]

Roger- will be interested to hear about the oil cooler, as that might be my next port of call.

 

Chats with some trusted names has convinced me that uneven mixtures are the cause of odd CHT's when ducting etc look fine. My intake mods certainly confirmed it, so you simply keep that in the back of your mind.

 

On a side note, I last week had the two baffles removed from my muffler to minimise back pressure, as Oscar's experience has indicated strongly that exhaust mods can do wonders for temps, in particular oil. Haven't had a chance to see what happens, but their appearance tells me it may do a little something....hopefully!!

 

 

[jetjr]

CAE had some less restrictive exhausts, some said big performance upgrade, others say not

 

I did hear a fair bit or work went into standard Jabiru exhaust can but who knows