Having flown my Zodiac XL about 600 hours with my Jabiru 3300 and Bing carburetor I must say it has had it good points and bad.
The short story is that initially the performance I got was quite poor. With some minor changes, performance improved to the point where it seemed acceptable. However problems remain which are perhaps serious.
Now the longer story...
First, in defense of Jabiru and the Bing I will say that the firewall forward kit I got with the engine did not properly address installation in the Zodiac. Essentially the kit seemed to encourage installation techniques that produced bad results and were in direct conflict with the installation recommendations in the Jabiru engine installation manual. Note too that the manual itself has improved over time. The most recently released manual
(dated August 2009) is the best yet.
To make matters worse, when I built my airplane I was new to the entire process, so I hired a certified A/P to install my engine. He did a mostly reasonable job but actually did not read the installation manual. I assumed he knew what he was doing. I have learned: hay, it’s your life, read the manual no matter who is on the job.
The firewall forward kit I got failed to address the following two issues mentioned in the installation manual:
1) Do not connect scat tube directly to the carburetor.
2) Avoid a 90 degree bend from the side in the intake tube coming into the carburetor.
In addition, I have learned though independent study and personal experience that scat tube in general offers significant turbulence and drag in the intake system, especially when bent. If you choose to use it, use as little as possible and avoid sharp turns.
I no longer have any scat tube in my cold air intake system. I almost never use my carburetor heat system so I am not worried about scat tube in that system. In your climate, perhaps you need carburetor heat, in my climate I don’t.
With my initial setup, which had scat tube everywhere, I found that the engine ran very rich at higher throttle settings. At full throttle the engine burned about 13 GPH – not exactly what I was expecting! Also at 10,000 feet the engine ran so rich that it shook the airplane badly at full throttle. I knew something was wrong so I started looking into it.
I learned that air does not like to turn corners. Because the air traveling along the outside of the bend has further to go than the air traveling along the inside of the bend, turbulence results. Scat tube is not smooth, especially when bent. The uneven walls of the scat tube create drag and turbulence. Inducing turbulence right at the carburetor intake creates problems, especially for the Bing carburetor which as sense ports at its opening.
I have read that A 90 degree bend in an intake system produce drag that is roughly equivalent to 3 feet of straight tube. Because of the way my intake system was put together, when I computed the total effective length of the tube in my cold air intake system it came out to about 12 feet! What a mess. No wonder the engine was performing badly.
My cold air intake system now uses only radiator hose. The hose is as short as possible with as few bends as I could manage without a complete re-work of the system.
Simply eliminating all scat tube and replacing it with radiator hose eliminated the high fuel consumption and added a very noticeable amount of power – it was not subtle. However a new problem showed up. At wide open throttle the EGTs were very uneven. To help reduce this problem I added a piece of aluminum inside the radiator hose running from the air box to the carburetor. The aluminum is centered in the hose, is vertical and is bent such that it runs down the center of the hose as it makes the 90 bend. This helps the air make the corner, reduces turbulence and has evened out the EGTs a bit.
All these modifications were made about 550 hours ago. EGT temperatures are still not as even as I would like and vary considerably with throttle and altitude. Different cylinders are hotter than others depending on throttle setting. On some days when I fly at about 1,800 feet MSL and have a certain throttle setting at least half of the EGTs get too hot. It is necessary to change altitude or throttle setting. About 150 hours ago I re-pitched my prop to make it steeper. That lowered my engine RPM by about 100 RPM. That has helped keep this annoyance to a minimum but it is still there.
I have noticed that on my engine it is the back cylinders which seem to have high EGTs. I have always suspected that this is because their intake runners connect to the intake manifold on the extreme left and right. The Bing’s main jet is in the middle and the Jabiru’s intake manifold is quite small so mixture is unevenly distributed to the cylinders. The two rear cylinders run lean as a result. Just a theory mind you…
About 80 hours ago I found a burned exhaust valve on my number 5 cylinder. It was easy to replace but got me to thinking. Perhaps that cylinder has been running too lean.
More recently oil consumption has been going way up. The engine now burns about a quart every 4 hours. When I replaced the burned valve I notice some scoring in the cylinder wall. I am not sure but at this point I am suspecting broken rings on #5 which might explain where the oil is going. In the next day or so I am going to run another compression check with the hope of learning what is wrong and where the oil might be going.
So, is it possible that in 600 hours with un-even EGTs with the tendency for the rear cylinders to run lean I have shortened the life of my engine? I don’t know – I am just wondering…
I have been considering one or more modifications.
1) Redesigning the air intake to eliminate the 90 degree bend from the side. This would be done with a custom air box intake system to replace the radiator hose.
2) Replacing the Bing carburetor with a Rotec TBI-40
In summery one of the Bing’s strengths is also one of its weaknesses: the lack of a mixture control. I like the simplicity. I don’t like the lack of control.
I am attracted to the Rotec TBI-40 because of the claim that EGTs are more even. I am not looking forward to re-running my throttle cable, turning my coke cable into a primer cable and adding a mixture control.
On the other hand I can’t say I am thrilled at the prospect of an early engine overhaul either.
I wish I had more information…