excessive EGT spread

·         w g wester

·         Topic Author

·         Offline

·         Posts: 8

·         

·         

·         

We have an early 3300A engine (#473) recently just put in service and now has 50 hours running well in all regards except EGT spread at peak (2650 RPM) is 800F (#2 cylinder) to 1350F (#5 cylinder)

1) How to get these EGTs more even? We have tried rotating the Carb.

2) Since this is an early production engine, is there an improved induction manifold that could improve the fuel distribution?

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

G'day.
As you say, 473 is an older engine. There have been several iterations to the manifold since then that can affect performance and tuning but I'd like to check a few simpler things first.
1. Is the engine achieving correct rpm (I.e. Does it have the right propeller for the application)? Over-propping (too much pitch or blade area) in particular can upset the tuning.
2. Is the engine air intake system per the installation manual? Tight bends on the SCAT hose, sharp corners or restrictions in the airbox or carby sense pipe installed incorrectly can all upset the tuning and how the engine runs.
3. Assuming the engine was in storage for many years, was the carby checked for cleanliness and condition? A small tear in the rubber diaphragm will affect how the carb behaves.
A few photos might help eliminate a few things :).
Regards,
Doug

Please Log in or Create an account to join the conversation.

·         w g wester

·         Topic Author

·         Offline

·         Posts: 8

·         

·         

·         

And G'day to you, Doug. Thanks for your reply. Maybe you can help us tame these pesky EGTs.

Re your questions;

1) We get 2850 RPM on take off/climb at WOT. Level flight WOT we get 3150 RPM. Prop is Prince P tip 54/60 as recommended by Prince for Sonex. Prince says due to designed prop flexing (taking bigger bites) @ level WOT, may not reach 3300. Doesn't mean we are not over propped.

2) I believe intake system is pretty good. No scat tubing and only a smooth 2" dia. 90 degree bend from the air box . I'll try to get a good photo and send it. Relocating the sense pipe is worth experimenting with. May try removing the air box (ala Rotax) and see what the EGTs do.

3) The carby is new. Threw out the Aerocarb and bought Bing 94 from Jabiru USA. Tweaked the jets; needle is 2.90 main is 255. Engine runs smoothly at all RPM. We feet good about performance. Excellent climb,160MPH at WOT level flight.

4) Please note the EIS attachment. Cyl 5 actually peaks @ !432F (not 1350 F as reported before). This is at 2650RPM. At @ 2850 rpm, EGT is in limits but spread is still there.

Some how we are not getting even fuel mixture to the cylinders (duh) and would really appreciate any suggestions you might have.
Thanks again, Walt

Attachments:

·          

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Hi Walt,
The 3300/Sonex combo must be an absolute rocket. Do you have to turn the engine off to land?
Your RPM figures sound pretty good. Getting tuning right on a low drag engine / airframe combo can be tricky but at first glance, based on those RPMs, your engine loading looks about right.
The induction system is still worth a look and a photo would be great. Ideally you want to provide an even airflow into the carby mouth and that can be affected by all sorts of small details. A smooth intake pipe might even be counter-productive in some cases as it doesn't tend to mix/blend the flow as much as a hose with a little more "texture" to it.
For the sake of trying simple things first it would be interesting to swap the smooth pipe for a SCAT hose (assuming you can get a decent radius on it and it doesn't bunch up - there's a happy medium with this "hose texture" idea). Longer term we do a "cobra head" intake moulding that might be worth a try - it was designed for applications with little space between carby and firewall and it steps from a bigger SCAT hose down to the carby size. That step down tends to compress the flow and that can help smooth it out: it's designed to be a mini plenum of sorts, at the carby mouth.
Are you running on AVGAS or MOGAS?
Have you tried any alterations to the carby settings? Are you still running the stepped needle?
D.

Please Log in or Create an account to join the conversation.

·         w g wester

·         Topic Author

·         Offline

·         Posts: 8

·         

·         

·         

Thanks Doug

The a/c actually does fly faster at idle than the 2200 did, doesn't slow down.

Do attached photos help? Filter side also shown to show air sense tube location.

I'll try some scat tubing, but if memory serves tried that before the elbow installed.

RE fuel; using avgas

Only changes on the carby are the jets I've tried. I haven't had the carb apart so not sure what needle is installed, but suspect it is the stepped needle. Carb came from JabiruUSA who i think had it left over when they sold an engine to a sonex customer who wanted to use the aerocarb (!) so it is at least a year or two old or more. Sounds as though I should order a tapered needle kit. (?)

The carb had only one number on it; 94/40/148 which I suspect is a manufacturing # not a serial #

What is entailed in ordering a Cobra Head?

Just want to say this forum is terrific and i really appreciate your help and advice.

Thx W

Attachments:

·          

·          

·          

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Hm, I suspect that 90 deg curve in from the side is a good part of the issue. The "brains" of the carby - the sense ports etc - are symmetric around the centreline so coming in from the side we're probably confusing it, introducing swirl etc. It Looks like there would be room for an inch or two of straight pipe leading into the carby which might be enough to help. Is there any form of bell-mouth where the air exits the filter box? Anything we can do to smooth / regularise the airflow will help.
Page 33 of the 3300 installation manual shows some cobra head installations. I'm not sure it will fit in your case but if you're keen to try they're available from your local agent.
From memory the numbers on the carby are the jet sizes - idle, main and needle jet (not necessarily in that order). For a slippery aircraft like yours the stepped needle may not be ideal: the "step" is tuned to go from lean cruise power settings to rich full power settings at a certain power/rpm... It's designed around a "typical" airframe... but at that it's not a bad place to start.
Glad you like the forum, I hope we're helping!
D.

Please Log in or Create an account to join the conversation.

·         w g wester

·         Topic Author

·         Offline

·         Posts: 8

·         

·         

·         

Thanks Doug

Taken enough of your time. I'll reconfigure the intake and let you know how it works out.

The forum's great; keep it up

Walt

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Cheers Walt, have fun!
D.

Please Log in or Create an account to join the conversation.

·         Rob Turk

·         

·         Offline

·         Posts: 17

·         

Hi Walt,

I have an older engine too (S/N 551). My intake system is everything Jabiru tells you not to do.. SCAT tube with a direct 90 degree angle right behind the carby. I simply do not have more room, not even for a cobra head. My EGT spread was considerable as well.

Then I built a cross divider to stop the intake air from swirling and throwing all the rich mixture into one corner of the intake manifold. Cause that's what is happing. I built one out of brass, soldered them together to form a cross and inserted into the carby throat. Immediate EGT improvements all over. Attached is a picture of my EGT spread before and after adding the cross, as well as pictures of the contraption.

Hope this helps

Rob

Attachments:

·          

·          

·          

·          

Please Log in or Create an account to join the conversation.

·         w g wester

·         Topic Author

·         Offline

·         Posts: 8

·         

·         

·         

Rob,

You have achieved what I was searching for; a way to straighten out the airflow to the carb. Your photo and drawing are going to be put to good and timely use. I have a new air box half finished and I am going to incorporate your cross divider into my changes.

Great work. You may have solved a problem that seems endemic to Jabirus. Many thanks for your input and response. I'll put the results on the forum.

Walt Wester

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

The cross type divider (and all the variations on that theme) certainly can work but there are a few issues associated with them.
First, the jabiru installation recommendations for the intake are there to optimise the overall installation - including power, fuel consumption and cooling as well as EGT spread. So if the installation doesn't follow those recommendations you might be able to use a divider to even out the EGT but still be down on power or having excess fuel consumption - it becomes a band-aid to bigger issues.
Second, there's the risk of leaning out a cylinder while tuning your divider and doing engine damage. I've seen it happen, the dividers can have strong and counter-intuitive effects so you have to be super-careful when tinkering with them.
Finally, there's the FOD risk of having a (usually) flimsy metal fabrication just upstream of the carby.
Not saying not to try it, but I'd be making sure the basics are covered first!
Regards,
Doug.

Please Log in or Create an account to join the conversation.

·         Rob Turk

·         

·         Offline

·         Posts: 17

·         

Walt,

Glad I could help. Two points to add:
1. First and foremost, take Doug's advice at heart. Design your intake as much as possible using the guidelines that Jabiru supplies. Usually you will get good results when you stick to those guidelines. I had to compromise (a lot) because I couldn't add the necessary space and resorted to this band-aid.

2. Again, as Doug mentions, adding stuff in the airstream is something that can cause counter intuïtive results, and can pose a danger when not done carefully. The construction I made consists of two thin, flimsy brass plates. The engine is a massive air pump, it s*cks big time. Without structural support those plates can be sucked into the carby and cause stoppage. I added the outer ring which sits on the outside of the carby rim (adds 1mm to the diameter) and supports the brass plates on all sides. This is a solid construction that isn't going anywhere. Using just the plates by itself is recipe for disaster, thread carefully..

Rob

Please Log in or Create an account to join the conversation.

·         w g wester

·         Topic Author

·         Offline

·         Posts: 8

·         

·         

·         

Rob/Doug

Fair enough. I am half way through a new airbox and if it solves things, I will pass on the vanes. Rob your drawings and the photo are clear and i will be careful not to stuff flimsy bits down the carb. I am certainly glad it works for you . The vanes you designed are pretty robust.The EGTs have been very frustrating and i think in the long run extreme unevenness would shorten engine life.
.
Doug, the new air box will have no bends and only a relatively straight tube to the carb. Should do the job. It is fiberglass and I can keep the sharp edges and constrictions to a minimum. I can understand your concern and be assured I am not going to put something in the intake stream that could possibly come loose or substantially impede airflow. (honest!)

Thanks to you both for the feedback. The Forum is a really useful format to trade ideas,

Spring is here and I want to go flying!!

Walt

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Tinkering is all very well but flying is better.
Sounds good Walt, let us know how you go!
D.

Please Log in or Create an account to join the conversation.

·         Steve Bennett

·         

·         Offline

·         Posts: 8

·         

·         

·         

Can Jabiru clarify the updated EGT temp specs in the latest installation manuals (Oct'15) for both the 2200 & 3300. Section 1.5.6/7

It seems the celsius temperatures have been changed from 720°c to 740°c for cruise and 680°c to 700°c for above 70% power, the fahrenheit limits however remain unchanged.

Cheers

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Hi Steve, Apologies for the late reply things got a bit hectic over Easter!
My understanding is that is a conversion error: the new Celsius limits are correct. The maintenance manual JEM0002-6 shows the correct Fahrenheit values on page 19.
I'll pass that info on to the manual writing squirrels to be addressed at the next issue.
Regards,
Doug.

Please Log in or Create an account to join the conversation.

·         Steve Bennett

·         

·         Offline

·         Posts: 8

·         

·         

·         

I'd assumed as much.

Cheers for the clarification

Please Log in or Create an account to join the conversation.

·         Rob Turk

·         

·         Offline

·         Posts: 17

·         

Steve's original question remains valid though. The EGT spec went up from 720 to 740C, and the Fahrenheit values will go up once corrected. Is this because new materials were introduced (if so, for which parts, starting at which serial numbers and can they be retrofitted), or are there new insights that allow older engines to use these higher limits as well? Again, if so, starting at which serial numbers?

I looked at the history of updates list in JEM0001-8 (Appendix E) and didn't find any recent changes that would justify or explain the change in limits.

Please Log in or Create an account to join the conversation.

·         w g wester

·         Topic Author

·         Offline

·         Posts: 8

·         

·         

·         

Hi doug,

Just a status report on new air box

Working OK but still a large spread on the EGTs. 850 -1390F (worst case) but at least within working (high) limits at cruise.

Tinkering w/ the sense tube helped somewhat.

Going to enlarge the filter, I think it is impeding the airflow.

Spring Flying is terrific....

Cheers Walt

Attachments:

·          

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Hi Rob,

That the change is due mainly to the rise in popularity of digital engine monitoring systems where you can see small temperature differences at a glance and which are generally more accurate than the old steam gauges. I.e. Better resolution instruments (often with alarm functions if you exceed limits) make it safer to get closer to the true limits.

That said, 740 is getting up there and you've got to remember that a red line on the gauge at 740 does not mean you're fine and dandy at 739: you're best to treat that area as a precautionary range. It's often interesting to do a little research to see if you can find specs for the tolerance of your instruments and take that into account too (I.e. If they're +/-20 C 720 indicated could be 740 actual).

Regards,
Doug.

Please Log in or Create an account to join the conversation.

·         Doug Smith

·         

·         Offline

·         Posts: 287

·         

Hi Walt,

Thanks for the update. Good to hear you're getting results, and enjoying the flying involved. Spring is always a good time to go flying!

A bigger air filter is not a bad idea at all, we made the switch to a bigger element back in the late 90s and it made a nice little improvement in power and general niceness of engine running.

Have fun!

D.