View Full Version : Z21 Thru Hull Exhaust question.

09-03-2012, 05:02 PM
I was wondering if anybody has converted their Z21 or early Z22 to rear exit thru hulls 72832 here is a picture of the rear of my boat, I would like to put them under the swim platform to ease the sound a little at idle. they exit through the sides above the waterline now72833 and its tough to have a conversation in the boat (good thing sometimes;)). I am going to be installing a roller 350 Vortec over the winter so the engine will be out anyway, the current mag is on its last leg. thanks for your help, Andy.

09-03-2012, 06:02 PM
I know of a boat in my area, (Not a Donzi) , that has two 4" exhaust pipe outlets with rubber flappers sitting just under the molded-in swim plaform.
That boat has a single Big Block Chevy 502.
At rest in the water the bottom of the swim platform is sitting directly on, or slightly under, the water surface.
When this boat is stopped at idle, or going at very low speeds, the expanding exhaust gas bubbles comming up to the surface get somewhat trapped under the overhanging swim platform and this results in violent shaking & vibration of the entire boat.
Once you get moving faster the vibration and shaking suddenly stops.
The owner often talks about swapping to aft hull side exit exhaust outlets like you already have to solve this vibration & shaking issue..

At rest in the water is the underside of your molded-in swim platform sitting on, or slightly under, the water surface ?
If so~~~I would keep the exhaust outlets where they are now~~~on the aft hull sides.

09-03-2012, 06:18 PM
Silent choice maybe? Those motors aren't too big to use thru-hub when you wanted I think. Depending on what the layout is like inside, *might* be less trouble to snag a pair of the side-exhaust diverters and put in a y-pipe. That'd certainly avoid any concern about what SG has observed.

09-04-2012, 08:45 AM
I never thought of a vibration issue because of that hmmm. I was trying to be frugal and avoid the cost of the captains call, but maybe that would be the way to do it. I love the boat, and have no plans of getting rid of it. I love the sound 80% of the time but on a sunday morning cruise it gets a little over bearing.

09-04-2012, 10:20 AM
switchable exhaust is the way to go IMO, especially considering you are already pulling the engine.

There was a guy selling a different style of diverters on eBay a while back (not corsa). Fixxall said he had used them before and they worked well, and that they aren't an On/Off system like the corsa but rather used a setup like a power antenna. Basically they allow you to run as closed or as open as you want so you can knock the sound down but not all the way like the Corsas. They were also much cheaper... if you can find that brand that's probably what I'd go with.

09-04-2012, 10:53 AM
Or take your time, set up a search on eBay, wait until the parts you want show up at the price yuou want to pay.
It'll eventually happen.

09-06-2012, 03:57 PM
I believe the early 86 and 87 came out the back and on to the platform, but it would stain the platform. If you just went to inside the platform I don't think you would be creating a great place for the exhaust to trap underneath and bounce off of the inside of the platform.
If you are going thru the trouble of removing the engine then just add the y- pipe and exit out the side. Remember Baffles are important ot stop the water form going back up the pipe.

09-07-2012, 02:05 PM
I appreciate the input, I think I will find some switchables. I should have the motor out within the next few weeks, and I'll go from there.

09-07-2012, 10:13 PM
I have a friend with thru transom exhaust on his Fountain who also mentioned how much vibration it causes at idle due to the swim platform. That might very well be a concern if you are thinking about going that route. Bill

09-07-2012, 10:18 PM
The only caution I'd offer on switchables is that the fad of late has been for states to outlaw them, regardless of volume levels. Not sure of the particulars/what, if anything, is grandfathered/etc. But if that's not an issue for you, switchable sounds like it might be a good fit. Best of luck with it, and let us know how it goes.


09-08-2012, 09:07 AM
I have a friend with thru transom exhaust on his Fountain who also mentioned how much vibration it causes at idle due to the swim platform. That might very well be a concern if you are thinking about going that route. Bill

actually, in the case of Fountain the platform sits basically at water level and doesn't allow anything to really escape around it.... on my Z235 and my Formula it wasn't an issue because the underside of the platform was above the waterline by a comfortable distance... also, if you use the rubber flappers on the tips it forces the noise and water symphony down instead of up or straight out

09-19-2012, 03:22 PM
I have a 1988 Z21 with four inch side exhausts. Under water at idle and putting. Light it up and it gets considerably louder. Because of the rubber hose exhaust off the risers and the bent each have to make to get out the sides a captains coice won't fit. Jamie

09-19-2012, 06:28 PM
Are they the factory side exits or aftermarkets? Did you have to glass them in? Is your boat a Big block? do you have any pictures? Thanks, Andy

09-20-2012, 03:01 PM
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afd414. I thought the side exhaust were factory because I saw another Z21 and it had the same setup. I have a small block Merc 350Mag. Not sure on year of engine but I think it was put in later in the boats life. I read somewhere they original came with OMC drive/ Ford engine??? The merc is a 270 HP with a Q4 rochester carb. I run a 19" SS prop. The boat runs WOT 52MPH. It works great and fuel mileage is pretty descent.I have some pictures of the setup. Can you tell me how to post pictures to this site? Thanks, Jamie

09-21-2012, 12:11 PM
I have the factory side exits with a mag 350 and mine are above the waterline at all times. I cant see your boat sitting lower in the water to cover those unless maybe you had the big block option (Weight). to post pics you hit the insert image button at the top of the new post and attach it from your computer.