Looking for a fix- high CHTs on SQ2

I have a Backcountry cubs SQ2 with a 180 HP O-360. I have had high CHT issues since day 1. I've tried everything - spent hours fiddling with the baffling. Front cylinders are fine, the rear 2 are the problem. Has anyone tried scoops on top of the cowling to push colder air right down on top of the two rear cylinders? I hate to cut into the fiberglass cowling unless I'm fairly sure this will fix things.

Plenty of room near the back of those rear cyls for hear to get out?

pictures!!!

What kind of mags? Pictures of baffling/cowling top, bottom. front, sides. What are the temps on all cylinders?
DENNY

Some pictures would help - do you have any ramps leading up to the front two cylinders to help push the air up over to the back?

Hello Doug,

I changed my cowl with “cheek” like a std Cub. Sealed every tiny hole/area in the baffling. Added fiberglass to smooth inside lip of entry top and bottom cowl. Opened up bottom rear exit area. Significantly larger front opening. Call me if you want to discuss.

Ted has a 16-18 page document with tons of pictures and explanations on cooling that I bet he could send you. That’s where I’d start given he has a similar cub.

I’ll update and post a link to my cowl mods pdf in a day or two.

Update on progress. I decided to go with pressure measurements as they are more direct than temperature (which is a result of air pressure). I'm using a 2 probe electronic manometer. A couple of things I found - first I can change the open areas (areas not sealed on top) by 4 sq inches and about zero difference. That lead me to look at pressures below the engine. What I found is that I had 1.5+" (i.e. higher than ambient) of pressure below the engine. Not much point in trying to get higher pressures above the engine until I stop the bottom from fighting movement. I added gills on the bottom (used the same geometry as the gills on my Cessna 206). That helped some, but still positive pressure. So I started shrinking the opening on the bottom. My thinking is that the large vertical gap on the bottom of the cowling is causing the air to become turbulent in front of the firewall and is pushing air into the cowling instead of pulling it out. The more I closed up the bottom the better things got. I have a few more changes to make but my goal now is to get at least some negative pressure (at least not positive) below the engine. It is clear that the pressure below the engine is the problem, not the lack of ram air pressure on top. The SQ may have a unique issue here because they lowered the engine to improve visibility over the engine. Thus, the vertical gap between the bottom of the cowl and the boot cowl is quite large.

d_kandle said:

Update on progress. I decided to go with pressure measurements as they are more direct than temperature (which is a result of air pressure). I'm using a 2 probe electronic manometer. A couple of things I found - first I can change the open areas (areas not sealed on top) by 4 sq inches and about zero difference. That lead me to look at pressures below the engine. What I found is that I had 1.5+" (i.e. higher than ambient) of pressure below the engine. Not much point in trying to get higher pressures above the engine until I stop the bottom from fighting movement. I added gills on the bottom (used the same geometry as the gills on my Cessna 206). That helped some, but still positive pressure. So I started shrinking the opening on the bottom. My thinking is that the large vertical gap on the bottom of the cowling is causing the air to become turbulent in front of the firewall and is pushing air into the cowling instead of pulling it out. The more I closed up the bottom the better things got. I have a few more changes to make but my goal now is to get at least some negative pressure (at least not positive) below the engine. It is clear that the pressure below the engine is the problem, not the lack of ram air pressure on top. The SQ may have a unique issue here because they lowered the engine to improve visibility over the engine. Thus, the vertical gap between the bottom of the cowl and the boot cowl is quite large.

Click to expand...

Excellent. Just starting install of one. And was looking at that huge bottom opening and thinking it’s to big.... now I need to reread this thread. Perfect timing.


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can you tape yarn on down there on things, and a camera on gear? and go fly?

a picture of the vents & blocked of rear stuff would be nice also

tedwaltman1 said:

I’ll update and post a link to my cowl mods pdf in a day or two.

Click to expand...

that would be useful

pictures of your rear baffling? you make your own or use the rv stuff?

I did take a video. Not very useful to have the yarn at the back end of the cowl. Just flaps around. I think the yarn video works when you are looking at air flow over a surface and near the boundary layer. I thought about putting some up just inside the rear opening to see where the air was flowing back into the cowl but that requires the camera to be directly behind the cowl looking forward. Since I was borrowing the Gopro, I din't want to return it with an oil mist on it.

I've got at least two more tests to do. I take some pictures soon and post. Nothing fancy. I'm making changes one step at a time to be sure that the numbers are always moving in the right direction.

In some cases ram air hitting the lower firewall up through the air outlet creates a swirling motion which in turn makes an air dam blocking the out flow. On some airplanes a curved piece of sheet metal attached to the lower corner of the firewall extending up and around to the firewall a few inches up on the fire wall stops the reverse swirling effect. When the swirling is stopped, the outflow is improved.

I did this on my 180. I was looking for a little lower on #1.

Thank you very much for the updates Doug. Great information! I wonder if anyone at BackCountry Cubs is paying attention to your hard work and taking notes regarding your productive results?

tedwaltman1 said:

Thank you very much for the updates Doug. Great information! I wonder if anyone at BackCountry Cubs is paying attention to your hard work and taking notes regarding your productive results?

Click to expand...

Can you post your notes you mentioned above?


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I see other cub clones using this cowling now. Is it lighter than a standard cub cowling with aluminum and cowl cheeks? They look nice and assume you have a little more visibility. Think they will fit a 4 place airframes fuselage?

vj88 said:

I see other cub clones using this cowling now. Is it lighter than a standard cub cowling with aluminum and cowl cheeks? They look nice and assume you have a little more visibility. Think they will fit a 4 place airframes fuselage?

Click to expand...

Engine mount is different width at least on 4 place....


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I updated the PDF where I document all of the myriad changes to my SQ-2 cowl, airbox, oil cooler location, etc. Here is the latest version: http://eaerofab.com/docs/CHTprojectV3.pdf

tedwaltman1 said:

I updated the PDF where I document all of the myriad changes to my SQ-2 cowl, airbox, oil cooler location, etc. Here is the latest version: http://eaerofab.com/docs/CHTprojectV3.pdf

Click to expand...

thank you, great information...

one question, (I will add 2 pictures in a minute to this post.)

I see you also have the cylinders with the fins cut down/tapered.... did you fix inter cylinder baffles on bottom to account for that? I can't see them installed in your pictures in the PDF

Yes, I have those same inter-cyl baffles, and sealed every other possible micro-opening, with a LOT of RTV. I put a very bright light above, or below--depending on the area--and made sure by the time I was done with liberal RTV that there was no light shining anywhere. i'll get more pictures next time I'm at the hangar (but that may be a while as I have to go out of town).

tedwaltman1 said:

I updated the PDF where I document all of the myriad changes to my SQ-2 cowl, airbox, oil cooler location, etc. Here is the latest version: http://eaerofab.com/docs/CHTprojectV3.pdf

Click to expand...

great info Ted,
thanks for sharing.
curious if you mapped your leaning authority at different power settings while addressing high cyl temps?

I’m not quite sure what “mapping my leaning authority” means. If I get to say 50 degrees LOP or leaner my CHT’s go down. But 99% of the time I run about 50 ROP. Last flight, 70 degrees OAT, 2050 & 20.5”, 7900’ density altitude, running at about 390 CHT on all 4 cyl.

I went thru a similar situation after install a new Lycon engine in a pa 18, difference being, I was flying behind a proven cowl design that was more than adequate at cooling previous engine.
Maiden flight #3 cyl was flashing 450° (JPI) and climbing as I passed thru pattern altitude. I immediately returned to the field thinking I had left a rag or something on top of the engine. Thorough inspection revealed nothing obviously amiss so I went back up, same thing, all cylinders temps were Climbing thru 450°, it was impossible to push the engine hard enough to insure good break in.
landed, thoroughly went over the engine, checked mag timing, fuel flow at carb, etc.. nothing amiss.
I had sent the engine to Lycon for their hot rod treatment and was now embarking on a long arduous journey to figure out how to make it flyable.
Fortunately, this occurred during “winter” (so cal),
waking up at predawn i was able to take advantage of 30° mornings and fly enough to successfully break cylinders in.
Recreational flying was pretty much out of the question.
Thru a tip I picked up on the vans chat board, started to focus on carb mixture and “leaning authority” - how many deg rop the engine was running at various power settings with mixture knob full rich.
pen and notebook in hand, I embarked on a test flight recording temp spread between full rich and peak while leaning, beginning at 2100 rpm thru 2700 rpm on hottest cyl.(#3)
At 2500rpm, full rich, I was 20 rop.
that will destroy a motor in pretty short order. (Ideally looking for 150° rop at 2500)
long story short, solution was to drill main “jet” in carb incrementally until I was able to run 100° rop full rich at 2500
this made the plane flyable, and combined with several cooling mods similar to what Ted has described, I’m now happily climbing out on 80° days at 380° hottest cyl.

Horsepower creates heat, cub type aircraft fly too slow to get rid of heat, adding a long prop makes it worse.
For what it’s worth, this was the solution to my high cht issues, SQ2 is a different animal..

disclaimer: “drilling” the fuel nozzle requires proper tooling - lathe, drill followed by reamer followed by de burring, I’m not encouraging anyone to take this operation lightly.

Ask what procedure determines the main nozzle rate of fuel flow via air pressure differential...diameter of main jet hole, the position of its outlet in an air pressure reducing venturi, or airflow via volumetric efficiency at various power settings that creates a low pressure zone through the carb. There's no universal fix so .....

Gary

Thank you for the description of “leaning authority.” I have an injected engine, so no carb to fiddle with.

i will confuse to experiment w airflow changes, though the internal vane mentioned and pictured in my PDF is my last, current at least, idea.

As you say, high Hp and slow speeds are ultimately a problem. Couple that with starting density altitudes exceeding 7,500’ even at dawn and one has a hot engine!