Firewall forward questions

[Mike Kimball]

Hello everyone,

I'm getting pretty excited as my instrument panel is starting to come
together and engine parts continue to arrive. To further the excitement I
was able to meet with Chris Gill in Anchorage this weekend and saw his plane
on the Montana floats he recently installed. There is no question in my
mind now. My plane will be using Montana floats! Everything I hear about
them, and the physical evidence of the beautiful floats makes it a no
brainer. Chris told me that he mentioned the $3500 price tag to ship the
floats to Anchorage and the folks at Montana floats said, "Send us your
shipping money and we'll bring the floats to you ourselves and install
them." And that's exactly what they did. Including installation of a nice,
aluminum ventral fin. I hope I can arrange the same deal. They sound like
wonderful people to deal with. I cried all the way back to Fairbanks
because our schedules didn't mesh well enough while I was in Anchorage to
allow time to fly. But Chris let me crawl all over taking notes about the
float installation and his electrical installation on the instrument panel.
Very helpful. Thanks Chris!

Couple of questions about your opinions/experience with firewall forward
stuff. When building a custom engine mount, I'm assuming you should have
all your accessories attached first, such as the alternator. What about the
exhaust? Build the mount first, then build the exhaust to dodge the mount?
I'd like to use off the shelf headers if possible. If I do, I assume I
should have them mounted before designing the mount, then design the rest of
the exhaust around the mount. It is my opinion that the major sources of
noise in an airplane are the prop, airflow past the airframe, and the engine
in that order. Maybe wind and the engine are tied for noise. Do you think
it would be worth it to add some glass-pak mufflers? I mention glass-pak in
particular because of their small, cylindrical size and shape. Maybe there
wouldn't be room unless I take my exhaust down the fuselage a ways. Hey,
dual chrome exhaust along the sides like early corvettes would be cool! OK,
what about four short stacks on each side sticking out like a Mustang? What
does that do to engine performance with the low back-pressure? Might be
noisy, but it would be a good noise! Increased drag though. Last question
for now: Is it bad to attach the radiator(s) to the engine mount?
Vibration and such?
Mike Kimball
SR #044




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[klehman]

Mike Kimball wrote:

Hello everyone,
snip

Couple of questions about your opinions/experience with firewall forward
stuff. When building a custom engine mount, I'm assuming you should have
all your accessories attached first, such as the alternator. What about the
exhaust? Build the mount first, then build the exhaust to dodge the mount?
I'd like to use off the shelf headers if possible. If I do, I assume I
should have them mounted before designing the mount, then design the rest of
the exhaust around the mount. It is my opinion that the major sources of
noise in an airplane are the prop, airflow past the airframe, and the engine
in that order. Maybe wind and the engine are tied for noise. Do you think
it would be worth it to add some glass-pak mufflers? I mention glass-pak in
particular because of their small, cylindrical size and shape. Maybe there
wouldn't be room unless I take my exhaust down the fuselage a ways. Hey,
dual chrome exhaust along the sides like early corvettes would be cool! OK,
what about four short stacks on each side sticking out like a Mustang? What
does that do to engine performance with the low back-pressure? Might be
noisy, but it would be a good noise! Increased drag though. Last question
for now: Is it bad to attach the radiator(s) to the engine mount?
Vibration and such?
Mike Kimball
SR #044

I would not hesitate to mount the rad to the engine mount. Some guys
have had short rad life with it mounted on the engine and shaking it to
death but that's quite different than on the engine mount on the
airframe side of the rubber isolators. I like to use a bit of rubber in
the rad mount itself as well (like most cars do) to allow for some
structural flexing without stressing the rad.

If I could have bought headers for my Soob I'd have tried very hard to
adapt the engine mount to use them. However I'd suggest you try to keep
the mount triangulated and not compromise it too much as it is not that
difficult to modify exhaust systems once you decide to do it. Street
headers may not be optimum tuning for you aircraft anyway. Keep your
radiator location in mind too. Airflow through your rad will be one of
your most important issues. On the Soob sometimes things like alternator
mounts are integrated into the engine pickup points for the engine mount
if all else fails.

As you know headers will give you the most power. The difference is
minor on my Soob with minimal valve overlap but it's probably more
significant on your engine. I'd suggest you try to find someone who has
used short stacks on that engine for awhile before going that route.
Some engines do have valve life issues with short stacks. I suspect
short stacks might move engine noise up a little higher on your list of
noise sources though ;) . Most glass packs will disintegrate quickly as
they can't take the heat from high power airplane use. Stainless packing
is reported to last longer.

cheers
Ken




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