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Submitted By:   charles.long@gm.com
Email List:   Zenith-List
Name:   

Charles F. Long

Date:   

Oct 23 2005

Subject:   Cooling improvements on the Zenith 601HDS w/Jabiru 3300
Description:   

During my first few hours of flight, I experienced elevated cylinder head temperatures on my Zenith 601HDS with Jabiru 3300A engine. Also had issues with Exhaust Gas Temperature imbalance at full throttle. The following is a summary of changes made to improve the situation. This really just builds on the recommendations of Jabiru and Zenith to provide plenty of cooling for the 6 cylinder engine. I would like to thank Jeff Small, Fred Hulen, Stan Challgren as well as Pete Krotje at US Jabiru and Andy Sylvester at Sun Coast for their contributions. As a place to start, US Jabiru provides some good cooling suggestions at their website:

My aircraft has the following modifications:

1) Fuel Economy Carb Kit as supplied by Jabiru.

2) Gull wing baffles between cylinders as recommended by Jabiru

3) Full deflectors angled down between the spark plugs of cylinder #5 & #6 as recommended by Jabiru.

4) Smaller deflectors angled down between the spark plugs of #3 & #4. These middle cylinder deflectors usually need some tuning. Would suggest starting with 3/4" tall. On #3, I stayed at 3/4", on #4 ended up trimming down to 1/2".

5) The inlet to the ram air ducts has a gap between the duct and the cylinders at the bottom. This allows air to escape downward without doing any useful work. I added some .025" aluminum plates that butt up against cylinder's #1 & #2 to prevent this from happening. To reduce the over cooling of the front two cylinders and force more air upward to the rear cylinders, I angled the material upward about an 2-3¡¨. The exact height needs to be determined by trial end error. The new XL cowl has openings that are quite small and located high on the cylinders. My modification attempts to duplicate the XL cowl openings and seems to work quite well. At the onset, this change appears to be counter-intuitive. Just keep it mind, the goal is to get as much use out of every air molecule as possible!

6) Small baffles mounted between valve tappet covers. This prevents more air from escaping without doing any useful work.

7) There is a temptation to add an L angle to the bottom rear of each ram duct. I tried this change and it elevated my temps rather than reducing them.

8) The oil cooler inlet is keep quite small to allow more air to find it's way through the cylinders. I ended up with about a 1.75" x 4.00" opening. The Jabiru website provides a good discussion on this. I tried a much bigger opening as can be seen in the picture and it provided very little benefit.

9) Don't forget to keep your oil level midway between the Full and Add mark. Jabiru indicates that overfilling can cause high oil temps.

My #1 cylinder was running much cooler than the #2 cylinder with identical intake openings. I rotated the bottom of the carb 5-10 degrees towards #1 and that took care of it. The main jet feeds the carb at the bottom so if the carb is tilted, it can direct more fuel towards one bank of cylinders than the other.

At full throttle, EGT's from left to right bank were 150 F different. I suspected that air entering the carb through the 90 degree intake elbow was piling up on the outside of the curve. I installed a vertical divider in the elbow to keep left side air separated from the right side air. This worked beautifully. My EGT's are now balanced within roughly 50 F at all throttle settings including WOT. The divider was fabricated out of a piece of 6061T6 aluminum - 5.25" x 3" x .016". The upper and lower edges were rolled around a 1/16" cable to add stiffness and prevent cutting of the rubber elbow. Finished height is just slightly over 2.25 inches so it fits snuggly within the elbow. After rolling the edges, the part was bent 90 degrees to fit the elbow contour by wrapping around a 2" diameter plastic pipe. After it springs back, you end up with a 2" bend radius whichis equivalent to the center radius of the elbow. Once the part is finished, the cable can be removed (prevents a potential corrosion issue). The rubber elbow is quite pliable in the free state, so the flow divider installs quite easily. After installation, the divider is trapped pretty well in all directions.

With the Aid of my Engine Information System from Grand Rapids Technology, I was able to monitor my progress for each Cylinder Head Temperature: I now have the following readings at 60 F ambient temperature (my engine has 28 hours TT, so temps have fallen after break-in):


RPM   IAS   CHT1  CHT2  CHT3  CHT4  CHT5  CHT6  EGT5  EGT6

2600  110   247   228   259   247   272   245   1465  1423
2700  120   242   221   253   239   265   240   1486  1451
2800  125   242   226   252   242   265   244   1512  1498
2900  130   253   246   260   260   268   262   1482  1538
WOT   138   288   293   293   298   301   297   1463  1482

This was a quickie test and temps/speeds were not totally stabilized. Oil temps varied from 200-230F depending on Throttle setting. This is an area where a NACA inlet could improve Oil Cooler efficiency. The new XL cowl incorporates one of these. The Flight Test was run at 3500 ft MSL and an ambient of 60F. Wheel pants and gear fairings are installed with an otherwise stock airframe.

The Jabiru is running very nice. I'm happy with the installation, power and smoothness after getting through these initial teething problems. Fuel burn at lower cruise settings is estimated at a miserly 4 gph. With the fat wing, I suspect fuel burn will go way up at the higher cruise speeds.

Chuck Long
Zodiac 601HDS
N601LE, 28 hr TT

 
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