Speed cowl exit air

[Legeorgen]

Wayne,

What rule of thumb do you use for the exit air on the speed cowling? I've heard one and a half times the inlet. The speed cowl does not meet this requirement as shipped from MAM, but they make no reference to enlarging it in the manual. Listening to some of the comments on the speed cowl and high CHT's has me wondering if I made my exit big enough.

Bruce G 357R

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[Wayne G. O'Shea]

Rule of thumb!? Cut 'till the temps are right! But yes I have read in many places one and a half times the inlet. Didn't do any calcs when we did Howard's. We just slit the MAM scoop down the sides and kept putting in larger pieces of aluminum until the temps dropped to an acceptable limit. It was then glassed in at this size and the aluminum removed.

Howard's main outlet is 4 1/4 x 14 1/2 = ~62 sq inches and then there is about 12 square inches around each exhaust pipe, plus he added the two 1 1/2" x 5" shark gills for another 15 sq inches for a total of ~ 101 square inches. Don't have a speed cowling here to measure the inlet size, but I'll guess at about 5" x 7" each side for a total of 70 sq inches. 101/70 = 1.44 ratio. If you have the measurements, what is the actual inlet square inch number and the out/in ratio.

On Bob's I just cut it out from one exhaust pipe to the other and forward to about an inch behind the carb heat dump pipe. This is most likely a good 200 square inches of outlet and seems to do the trick overly well.

Wayne

----- Original Message -----
From: Legeorgen@cs.com (Legeorgen@cs.com)
To: murphy-rebel@dcsol.com (murphy-rebel@dcsol.com)
Sent: Wednesday, July 10, 2002 10:27 PM
Subject: Re: Speed cowl exit air


Wayne,

What rule of thumb do you use for the exit air on the speed cowling? I've heard one and a half times the inlet. The speed cowl does not meet this requirement as shipped from MAM, but they make no reference to enlarging it in the manual. Listening to some of the comments on the speed cowl and high CHT's has me wondering if I made my exit big enough.

Bruce G 357R

[LisaFly99]

In a message dated 7/10/02 9:28:30 PM Central Daylight Time, Legeorgen@cs.com writes:

What rule of thumb do you use for the exit air on the speed cowling? I've heard one and a half times the inlet.

BRUCE
I've heard 1.7 X inlet air. And my exit is slightly larger than that formula, with a lip.
Also if you fair out the bottom lip on you're firewall it helps air flow. That little lip for the rivet line can cause a lot of turbulance.
Phil&Lisa Smith
N414D
#460 R

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[Rick DeCiero]

Wayne,
Now you have put the pressure on me to provide some pictures of my modified speed cowl. I had to add bumps for the heat cuffs and when I cut out the cowl to clear the pipes it was obvious that more modification would be required. I basically cut the whole bottom out from approx 1" back from the carb heat/ram air deflection pipe. I cojoined the two bumps with the center of the drop about 5" below the fuse and the width is 25" wide with a smooth curve from bump to bump. The pipes have a nice clean exit. I have not computed the exit area but over the next week I'll try to come up with the numbers. It is substantially greater than what you get with the original set up. Probably something around 100 sq in. The inlet area is about 7 x 5 = 35 x 2 = 70 sq in. For 1.5 x (according to several publications) the inlet you will need 105. Next week my friend will be over with his digital camera and I will post some pics for others to compare.
Good luck, you'll need it,
Rick D.
"Wayne G. O'Shea" wrote:

Rule of thumb!? Cut 'till the temps are right! But yes I have read in many places one and a half times the inlet. Didn't do any calcs when we did Howard's. We just slit the MAM scoop down the sides and kept putting in larger pieces of aluminum until the temps dropped to an acceptable limit. It was then glassed in at this size and the aluminum removed. Howard's main outlet is 4 1/4 x 14 1/2 = ~62 sq inches and then there is about 12 square inches around each exhaust pipe, plus he added the two 1 1/2" x 5" shark gills for another 15 sq inches for a total of ~ 101 square inches. Don't have a speed cowling here to measure the inlet size, but I'll guess at about 5" x 7" each side for a total of 70 sq inches. 101/70 = 1.44 ratio. If you have the measurements, what is the actual inlet square inch number and the out/in ratio. On Bob's I just cut it out from one exhaust pipe to the other and forward to about an inch behind the carb heat dump pipe. This is most likely a good 200 square inches of outlet and seems to do the trick overly well. Wayne

----- Original Message -----
From: Legeorgen@cs.com (Legeorgen@cs.com)
To: murphy-rebel@dcsol.com (murphy-rebel@dcsol.com)
Sent: Wednesday, July 10, 2002 10:27 PM
Subject: Re: Speed cowl exit air
Wayne,
What rule of thumb do you use for the exit air on the speed cowling? I've heard one and a half times the inlet. The speed cowl does not meet this requirement as shipped from MAM, but they make no reference to enlarging it in the manual. Listening to some of the comments on the speed cowl and high CHT's has me wondering if I made my exit big enough.
Bruce G 357R

[Gary Gustafson]

I am currently working on my speed cowl and modifying it to increase
the amount of exhaust air. I have an O320 (160 HP) and it will be on wheels,
but would like to plan for eventual floats. In that regard, back in July
2002, Wayne O'Shea wrote the following in an email:

"Rule of thumb!? Cut 'till the temps are right! But yes I have read in many
places one and a half times the inlet. Didn't do any calcs when we did
Howard's. We just slit the MAM scoop down the sides and kept putting in
larger pieces of aluminum until the temps dropped to an acceptable limit. It
was then glassed in at this size and the aluminum removed."

"Howard's main outlet is 4 1/4 x 14 1/2 = ~62 sq inches and then there is
about 12 square inches around each exhaust pipe, plus he added the two 1
1/2" x 5" shark gills for another 15 sq inches for a total of ~ 101 square
inches. Don't have a speed cowling here to measure the inlet size, but I'll
guess at about 5" x 7" each side for a total of 70 sq inches. 101/70 = 1.44
ratio. If you have the measurements, what is the actual inlet square inch
number and the out/in ratio." (I measured and it is about 68 sq in.)

"On Bob's I just cut it out from one exhaust pipe to the other and forward
to about an inch behind the carb heat dump pipe. This is most likely a good
200 square inches of outlet and seems to do the trick overly well.."

My questions are:
1) Does anyone know where to get ready made "shark" gills? It
would make increasing the exhaust flow a lot easier without making a big
exhaust plenum hanging down underneath the cowling as I have seen in some
pictures.
2) Did Howard's configuration provide sufficient cooling for his flying?
3) Has anyone had any over temp problems with about 100 sq in of exhaust
area?
4) Is it necessary to have an external carb heat dump pipe or am I
misreading the quote?

Thanking any and all for your assistance.




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