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XL Jabiru CHT's
On my flight from Idaho to Prineville, my #3 CHT was running high. I had modified the system for diverting air, the left side was running well, so I went back to my original system on the right side. Still need to do some tweaking, but I'm happy with the numbers. The original diverters were made of fiberglass and I'm trying to recreate using aluminum to increase durability.
FYI, I have custom ram air ducts and don't use deflectors. Deflectors and dams create cooling drag. I've focused on shaping the inlet air by diverting it. More later once I've tweaked.
*More info. Using the original diverters on the right side reduced the temp on #3 by 40 F, but also dropped #1 by 40 F. Next modification will be to adjust the new right side diverters to more resemble the original left side design to reduce #2 to match #1 along with #5. I've had the 4 corners running the same temps. Now that #3 is under control and matches #4, I can work to bring those temps down another 40 F. The goal is all CHT's in the 260 F range.
Oil temps are better, but still in the 200+ F range. I've made a new lower cowl bottom to increase the outlet depth, cowl length and incorporate the exhaust cuffs. Need to install this new piece and get some new numbers before tweaking to adjust other engine temps. Trying to maximize suction from the cowl outlet.
Here's a good link to some cowl ideas I found last week, basically validates the path I'm on. Some of this stuff is in the Jabiru Install manual. Not sure how much you can trust LoPresti when it comes to reducing drag ;-)
http://www.myrv10.com/tips/mods/Cowls/index.html
A nice hangar came available at the Bend airport and it was too good to pass up. This reduces my drive time from 45 minutes to 20 minutes.
Live long and prosper!
Jake
Comment
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Comment by Phillip Owens on October 5, 2011 at 6:48pm
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I also experienced high CHT's on my Jabiru 5100 8 cyl engine. when we checked the airflow we found that because the air intake design from Jabiru was angled backwards from the center outwards and also from the bottom of the opening to the top. Result, the upward movement of the prop was driving the intake air over the opening instead of into it, resulting in high CHT's on the left bank. I made an aluminum lip and rivited it to the top of the left intake and the result was to bring down the CHT temps of the left bank to almost match the right bank. We were also very careful to provide a 3.5:1 air exit ratio to intake when we made the cowl since we knew in advance that was the minimum, we also made sure the point of exit was aft of the firewall. This small detail is very important to prevent a high pressure zone inside the rear of the cowl. We have been informed that most all Jabirus run high CHT temps until at or after the 25 hr break in period, we noted lower temps as the engine was run. Hope this is of some value to you.
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Comment by Robert Emery on October 4, 2011 at 3:31pm
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I had problems initially, with CHTs (now flown 25 hrs) in Australia where we get high OAT's. I played around witrh the inlet ram air ducts, but found most change when I woorked on the outlet area.
I have increased the size and angle of the outgoing air cowling area. By creating a lower pressure here the air is drawn through as per Jab website info. I also found that sealing air intake and around air cooler had a beneficial result. I am now running CHT's of around 120 to 140 degres C (I think that equates to 250 to 285 F)
Max is on centre cylinders at around 300F, but by climbing at 90 knots I keep it under this. I will post some pics when I get a chance
Regards
Bob Emery
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Comment by Alberto Martin on October 2, 2011 at 3:49pm
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Great values !
1.3.6 Cylinder Head Temperature (CHT)
Maximum Peak Cylinder Head Temperature1 ......... 200ºC (392°F)
Maximum Continuous Temperature ........................ 180ºC (356°F)
Note: Time with CHT at between 180°C and 200°C is not to exceed 5 MinutesAt least, that is what the manual says...
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