I am not sure what the correct ratio is, and have heard different
recommendations. I did a lot of experimenting with my old 320 installation
before it was acceptable. Thankfully, I hit it right the first time with the
new 360 cowl.
With the 320, I started off at about 1.5 exit to inlet (with a lip, and some
other tricks). That was OK on wheels, but woefully inadequate on floats. I
then enlarged the exit a couple times until I had barely acceptable cooling.
I didn't measure it, but the exit was very large, probably at least 4 to 1.
I also smoothed the inlets, added an eyebrow on the left side, and smoothed
the lower firewall lip.
After all this, the cooling was fine in cruise (below 400F CHTs). But when
doing long climbs in the hot summer (35C+), my #3 CHT would still get up to
about 430F, and I sometimes had to back off and do a step climb. I then
changed the inlets to make them larger, and that totally solved the problem.
I probably could have reduced the exit after that, but I didn't bother.
On my 360 installation, I used the 2.5 inch prop spacer to provide more
clearance on the inlets. I had only a 1.75 spacer on the 320, which I think
was a big part of the problem, too. With the 360 cowl, I made the rear exit
larger by lowering that centre rear straight portion a few inches, which
didn't alter the overall design shape that much. My old 320 one was
downright ugly.
I think the ratio depends on the inlet size. If you have small inlets, then
you need a larger ratio exit to help suck out the air. Of course, the faster
you can fly, the smaller the inlets need to be, and probably not as large a
ratio.
I should mention that all my cooling problems had to do with CHTs not oil
cooling. I have a good sized oil cooler mounted on the rear baffle, and that
was never a problem. With the 320, my hottest cylinder was always #3, but on
the 360, my temps are more equal, with #1 being slightly hotter during
climb. As least I can control that one with the baffle air dam on the front.
Walter
-----Original Message-----
From:
mike.davis@dcsol.com [mailto:
mike.davis@dcsol.com] On Behalf Of Jeff
Micheal
Sent: November 14, 2009 9:00 PM
To:
rebel-builders@dcsol.com
Subject: Re: [rebel-builders] cowl
Bernard,
With engine cooling, the typical rule of thumb is your cowl air exit should
be 3 times larger than the air intake hole(s) being 3:1 or even 4:1.
Walter, have you ever figured out what your cowl airflow ratio is ? Would be
interesting to know if it is within this "typical" setup ratio.
And of coarse, the infamous cowl lip at the aft bottom does do wonders for
extracting air out of the cowl.
Cheers,
Jeff
On Sun, Nov 15, 2009 at 6:33 PM, Ralph Baker <
rebaker@sc.rr.com> wrote:
Bernard,
When Walter says "make the exit area much larger" it does not mean to cut
ir forward. It means looking from the rear to make it project lower and
/or
wider so there is a bigger opening when viewed from the rear. Also, bring
the cowl opening to the rear perhaps 50 cm to the rear of the firewall.
One
final suggestion is to build a small fairing for the bottom lip of the
firewall so air coming down the firewall can exit easily and not create
turbulence at the bottom lip.
Ralph Baker
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