The Murphy Builders Site

Posted: **Sun Feb 19, 2012 11:34 am**

I wouldn't worry too much about the shark gills. They were more to make room
for the heat muffs and his thought was the heat may escape there as well.
They probably screw up the positive pressure situation more than anything.

That said..YES his dimensions are still working just fine all these years
later.

We also opened Bob White's O-360 Elite cowling up to be basically the same
size this spring and his is working great hauling Clamar 2200 amphibs around
the back country of Ontario.

Wayne

----- Original Message -----
From: "Gary Gustafson" <gargus@adelphia.net>
To: <rebel-builders@dcsol.com>
Sent: Tuesday, January 16, 2007 5:23 PM
Subject: [rebel-builders] Speed cowl exit air

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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Posted: **Sun Feb 19, 2012 11:34 am**

hi gary I have a rebel on floats here in the parry sound area one of
the problems I had was high temps in the summmer your numbers look good --
about 150% greater exit than intake (70 sq in) but i found one important
thing on a 160hp installation

The right hand cowl intake opening must be cut back, actually cut forward,
as much as posible the number 1 cylinder is very close to the back edge of
the fiberglass and the air can not get in properly the other side is
ok because the cylinder is further back

i found this by accident when i couldn`t get my finger between the glass and
the fins once i was done the oil temps dropped at least 6 degrees

the little opening on the sides 5in dia helped a couple degrees also

keep in touch bob fisher rebel 342
----- Original Message -----
From: "Gary Gustafson" <gargus@adelphia.net>
To: <rebel-builders@dcsol.com>
Sent: Tuesday, January 16, 2007 5:23 PM
Subject: [rebel-builders] Speed cowl exit air

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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Posted: **Sun Feb 19, 2012 11:34 am**

First I want to thank all for the numerous responses that I received in
response to this question which has been addressed several times in the
past. All comments have been helpful and assisted me in arriving at
what I am going to do.

What I intend to do is modify the speed cowl in the following way. First,
I intend cut the speed cowl so that I can lower the entire bottom of the
cowl an additional 1 3/4 inches and extend the entire bottom trailing edge
so that it overlaps the bottom of the fuselage by 3 inches. This is a lot of
work fairing in the inserted mod piece to the conform to the original
aerodynamic shape but I feel worth it. I will put a round fairing along the
entire bottom of the firewall to encourage smooth air flow out the exit
across the entire bottom of the cowl. The exit air space should be 1 3/4
x 44 inches (77 sq in) plus current center drop down of 1 1/2 x 14 inches
(21 sq in) plus area around each exhaust pipe of 5 in each (10 sq in). This
results in a total exit area of 108 sq in. which should be sufficient. I
probably will require some bracing support along the bottom of the cowl
to maintain a uniform 1 3/4 in space which ahould not be a problem.

I note that there is not much effective area on the inlet side of the cowling
with the lead cylinder and was concerned whether sufficient air was
entering to cool that side if the engine. I have read other comments and
looked at pictures where people have opened up the inlet towards the
spinner to get more intake air. In fact I have to bend the top lead ignition
wire a bit so that it does not rub on the cowling, so there is not much
clearance above the lead piston. I was going to open up the inlet toward
the spinner more. But after reading your email, I will start by cutting the
lead side upper lip forward and fair it up to the top of the cowl. If that does
not work I will widen the inlet on both sides at the top of each to get more
air on both sides. (Looks like the cowl was designed for a smaller engine -
Rotax 912?)

Does this sound like a good approach? If so, I will post pictures on the
builders web site as I get things compleated,

Thanks again for all the comments and pictures posted.

Gary Gustafson Rebel #242

---- Bob Fisher <rgfisher@zeuter.com> wrote:

hi gary I have a rebel on floats here in the parry sound area one of
the problems I had was high temps in the summmer your numbers look good --
about 150% greater exit than intake (70 sq in) but i found one important
thing on a 160hp installation

The right hand cowl intake opening must be cut back, actually cut forward,
as much as posible the number 1 cylinder is very close to the back edge of
the fiberglass and the air can not get in properly the other side is
ok because the cylinder is further back

i found this by accident when i couldn`t get my finger between the glass and
the fins once i was done the oil temps dropped at least 6 degrees

the little opening on the sides 5in dia helped a couple degrees also

keep in touch bob fisher rebel 342
----- Original Message -----
From: "Gary Gustafson" <gargus@adelphia.net>
To: <rebel-builders@dcsol.com>
Sent: Tuesday, January 16, 2007 5:23 PM
Subject: [rebel-builders] Speed cowl exit air

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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Posted: **Sun Feb 19, 2012 11:34 am**

Hi Gary !

Sounds VERY good to me !!! If you can get more than 1 3/4"
drop at the rear, the more the better. ...

One builder used rectangular sections of tubing across
the bottom of the fuse to support the cowl. They were open
to the airflow, but added rigid support ... The tubing was
about 2" deep, and approx. 3"+ wide opening, and about 1" wide, IIRC ....

You are definitely on the right track. Maybe you could just
fill the triangular lower side opening with aluminum to mount the hinge ... ??
:-)
--
......bobp
http://www.prosumers.ca
http://bpatterson.qhealthbeauty.com
http://apatterson2.qhealthzone.com

-------------------------------orig.-------------------------
On Monday 22 January 2007 03:20, gargus@adelphia.net wrote:

First I want to thank all for the numerous responses that I received in
response to this question which has been addressed several times in the
past. All comments have been helpful and assisted me in arriving at
what I am going to do.

What I intend to do is modify the speed cowl in the following way. First,
I intend cut the speed cowl so that I can lower the entire bottom of the
cowl an additional 1 3/4 inches and extend the entire bottom trailing edge
so that it overlaps the bottom of the fuselage by 3 inches. This is a lot of
work fairing in the inserted mod piece to the conform to the original
aerodynamic shape but I feel worth it. I will put a round fairing along the
entire bottom of the firewall to encourage smooth air flow out the exit
across the entire bottom of the cowl. The exit air space should be 1 3/4
x 44 inches (77 sq in) plus current center drop down of 1 1/2 x 14 inches
(21 sq in) plus area around each exhaust pipe of 5 in each (10 sq in). This
results in a total exit area of 108 sq in. which should be sufficient. I
probably will require some bracing support along the bottom of the cowl
to maintain a uniform 1 3/4 in space which ahould not be a problem.

I note that there is not much effective area on the inlet side of the

cowling

with the lead cylinder and was concerned whether sufficient air was
entering to cool that side if the engine. I have read other comments and
looked at pictures where people have opened up the inlet towards the
spinner to get more intake air. In fact I have to bend the top lead ignition
wire a bit so that it does not rub on the cowling, so there is not much
clearance above the lead piston. I was going to open up the inlet toward
the spinner more. But after reading your email, I will start by cutting the
lead side upper lip forward and fair it up to the top of the cowl. If that

does

not work I will widen the inlet on both sides at the top of each to get more
air on both sides. (Looks like the cowl was designed for a smaller engine -
Rotax 912?)

Does this sound like a good approach? If so, I will post pictures on the
builders web site as I get things compleated,

Thanks again for all the comments and pictures posted.

Gary Gustafson Rebel #242

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Posted: **Sun Feb 19, 2012 11:34 am**

It's all very fine having the exit air 150% of the inlet, but if the
inlet isn't big enough in the first place, it's still going to run hot.
At least with the O360, you have to enlarge the inlets substantially (a
good 30% in my case), and the outlets correspondingly. Now, even on hot
days, I don't see much above 104C.

Al
Elite 587

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