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Exhaust turn-downs: How deep under water?

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Old 04-21-2008, 08:33 AM
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Default Exhaust turn-downs: How deep under water?

Here's the system I'm putting together. It's basically an external Captains Call system. There is a valve at the rear of the muffler that is actuated by a pneumatic cylinder, forcing the exhaust out through the pipe along the transom. This down pipe is 3" diameter. How deep should it be with 600's to keep from just blowing all the water away at idle and at idle speeds (6-8 mph)? I have heard 8" to 12" under.

Would it help to cut slots in the sides of the tubes to relieve pressure, so it doesn't all just blow out the bottom?

I also attached a shot of the current system, but it's 98 db at idle....
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Old 04-21-2008, 08:41 AM
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My guess is pretty deep if you intend on not have the water be blown all out of the way... 6 " at least.

I have turn downs and they are underwater by at max 1" at Idle, obviously when running it blows water around but it muffles the sound enough to keep me legal.

Switchables are not legal in most of the area I know we can boat.

Hope this helps

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Old 04-21-2008, 09:50 AM
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Ours are close 8" under. At 6" we were over limit.
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Old 04-22-2008, 08:09 AM
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GLH, where did you get those turndowns?
I would like to get some for my boat, but the way my tips are above my swim step I need custom ones made.
Need two that are a shorter curve off the tip and two that extend farther off the tip before the curve.
The ones I have seen, I think Gibson??, were very pricey!!!!!

I was thinking of trying to find a shop that does custom exhaust?
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Old 04-22-2008, 09:37 AM
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Check out Mercury's new exhaust system:

http://www.mercuryracing.com/news/pr...5453cfb2e55f0#

as featured in PB mag
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Old 04-23-2008, 09:45 AM
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Quote:
Originally Posted by MnFastBoat View Post
GLH, where did you get those turndowns?
I would like to get some for my boat, but the way my tips are above my swim step I need custom ones made.
Need two that are a shorter curve off the tip and two that extend farther off the tip before the curve.
The ones I have seen, I think Gibson??, were very pricey!!!!!

I was thinking of trying to find a shop that does custom exhaust?
CMI did them for me custom.

Cost around $1,100 if I recall
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Old 04-23-2008, 11:26 AM
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Quote:
Originally Posted by GLH View Post
CMI did them for me custom.

Cost around $1,100 if I recall

For all 4?
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Old 04-25-2008, 02:23 AM
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Default Downturns High performance Motors & Bad Valve springs

I installed Gibsons downtruns on my 38 Fever 650 blower
motors . I have been trying to get answers from Gibson how to prevent water to enter thru exhust into heads caused by
valve spring failure ! When I blew a crank in one motor , upon tearing down Nickerson found other motor had water steaming in head from the broken valve spring sucking up water thru my CMI e top dry tail downturned exhust system .

Now not hearing back from Gibson I am scared to replace them on my new motors without some proper modification
to them . I found out the prior motors I had built had valve springs known to have had factory defects in some parts.

Downturns work great on a functioning engine , but so far all I know my bad valve springs would have stayed dry if the downturns were not used .

Anyone have any ideas how I can use them safely on my engines ?
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Old 04-25-2008, 08:46 AM
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Quote:
Originally Posted by GLH View Post
My guess is pretty deep if you intend on not have the water be blown all out of the way... 6 " at least.

I have turn downs and they are underwater by at max 1" at Idle, obviously when running it blows water around but it muffles the sound enough to keep me legal.

Switchables are not legal in most of the area I know we can boat.

Hope this helps
Every law I've read say that cutouts are illegal. switchable exhaust is not necessarily a cutout.
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Old 04-25-2008, 09:13 AM
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Quote:
Originally Posted by MnFastBoat View Post
For all 4?
Yup
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