Hot #3 CHT - Solved!

[Toobuilder]

OK, its now six months later and I had a chance to really check this mod out today. I left Phoenix at noon with the temp passing through 105. On my normal stair step climb to the west staying clear of the class B sections, the CHT often runs right past 400 on a few cylinders within minutes, but not today. I maintained WOT and best power EGT for the entire 15 minutes it took to get out from under the PHX airspace and I never saw anything hotter than 385. Once to 8500 feet and LOP, the temps settled to 325 -350.

I know that 1 data point does not make a trend, but my first "hot" flight of the season sure went well.

Soon, I will try a climb at VY all the way to altitude and see how it behaves.

[alpinelakespilot2000]

With some help from Michael Robinson (Toobuilder) and Pete Howell, I finally got around to attacking my hot #2 cylinder. It was always the hottest in climb at full throttle, and also a bit hotter than the rest in cruise. I had already tried the washer trick on both #2 and #3, but neither seemed to help that much. Given that lack of success, I pretty much followed Michael's approach this time around...

Cut a bypass hole in my #2 baffle to allow air to get around the lack of fins:


Built a plenum to funnel air into that bypass:


The end result at 65% cruise? Averaged over a few flights before and after the mod...

Before plenum, #2 ran just slightly hotter than #3.
After plenum, #2 runs 25-35F cooler than #3.

Before plenum, #2 ran about 35F hotter than #4 (traditionally my coolest).
After plenum, #2 runs about 5F hotter than #4.

The long and short, it appears that the plenum cooled #2 by about 30F. In addition, though I don't have hard data for climb temps, #2 used to be my limiting CHT by a fair margin. Now #2 and #3 keep pace as my hottest, but it is now easier to climb WOT than before.

Now, I need to attack my #3. Michael's solution there is a bit more complicated to picture, and to fabricate, but definitely is at the top of my "to-do's".

[Toobuilder]

Glad this worked out for you Steve. I have performed this mod to my Rocket and experienced similar results. I have also developed a much simpler plenum for the rear cylinder which is just as effective as the one shown at the start of this thread. I'll send you the drawing and/or publish it here. If you have not started cutting metal yet- wait until you see my new one. It will save you a bunch of time.

[SeanB]

Michael,

All this is very interesting. I believe a lot of us would like to see and could possibly benefit from your latest rear plenum idea. Hope you decide to post it on this forum.

Steve, your data is also appreciated.

Thanks,

[BillL]

Here is what I did for the rear head. When I did all this, I had not seen this thread!



Head Side


A plot of areas show this will provide all the air needed for the lower fins.
Edit: 125 hrs and tons of temperature data over many test types say this works. 5.22.20

[Toobuilder]

Bill, that's really clean and a huge improvement over the "washer trick". Mine goes one step further however, with particular emphasis on pulling air through the upper fins. My line drawing is at the bottom, and here are the installed pictures of the new plenum. Notice the upper section which hugs the upper fins.




If you look at the above picture and compare it to Dan's line drawing in post#5 you can see where I got my inspiration for the new design. Thanks Dan!






This plenum sits on the “flat” part of the head, with extra material to make the wrap sections on top and bottom.

Keep in mind that solid lines are “cut” and dashed are “bend”. Also, the bend order that works well is:
1. Sides
2. “back kink”
3. Flanges.

[DanH]

Originally Posted by Toobuilder

Bill, that's really clean and a huge improvement over the "washer trick".

All of Bill's work looks like that.

If you look at the above picture and compare it to Dan's line drawing in post#5 you can see where I got my inspiration for the new design.

Your idea. I just do pictures.

Speaking of pictures and ideas...check out these from a certified Grumman Tiger. Yep, that's good 'ole #3. It's not like we invented any of this. It's a well known problem and fix:

[Toobuilder]

Notice that post #16 is updated with my drawing for the "new" plenum.

[Greg Arehart]

I finally got around to shamelessly stealing these ideas for my hot #3. Made a version of Bills modification and it took ~30-40 degrees off my CHTs on the test flight this morning. Now my #3 is just a few degrees cooler than the #1 & #2 and about the same as #4.

Absolutely recommend this modification for anyone with #3 problems - took me about two hours to do it, including removal and reinstallation.

Greg

[Toobuilder]

My hope is that Van will eventually come around and "shamelessly steal" this idea.

...And in one fell swoop fix most of the cooling problems these airplanes have.