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AQ225D thru-hull exhaust outlet dimension?

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Walleye4Days

Contributing Member
I'm trying to determine the best overall outlet dimension for my thru hull exhaust. I know that it's 3.75" (95mm) coming off of the risers, but meeting in the current y-pipe into one smaller outlet (for back pressure I'm assuming?)

So, do I run smaller exhaust sizes around 2.5" or so from each riser until exit, to keep some decent backpressure? Or, do I just run it straight through at 3.5" all the way out and rely on a muffler to create the back pressure needed?

I'm not the most versed on exhaust systems and the metrics of it all. So, just trying to get a good understanding of what's needed sizewise for the whole system. (It'll be dual exhaust when said and done, instead of meeting up with a y-pipe like current).

*The only reason I'm doing this is because when putting the exhaust bellow in place, the y-pipe outlet crumbled on 2 corner areas of the outlet, eliminating the bellow from being able to successfully be attached. (How this engine didn't blow up over the past 2 years with virtually no exhaust bellow is beyond me! Lol. But, I'm a lot more anal in the care of my boat VS previous owner, so I'm definitely fixing this unforeseen issue.)

**I didn't want to simply replace the y-pipe because I JUST got done pulling the engine, rebuilding the outdrive, and getting it all back together and the crumbling if this y-pipe was completely unforeseen/unplanned. If I can build a nice thru you'll exhaust without having to pull that engine again, I'd much rather take that route. At least, just to get through thru season until Nov 20th that is. In the winter time I'll go back through and really do it all out to basically a new boat again.
 
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Walleye4Days said:
I'm trying to determine the best overall outlet dimension for my thru hull exhaust. I know that it's 3.75" (95mm) coming off of the risers,
You mean ELBOWS!
Spacers/Risers are not used with the AQ series exhaust system.
I'm not aware of any parts that are available for that!


but meeting in the current y-pipe into one smaller outlet (for back pressure I'm assuming?)
I'm not following you on that one!

So, do I run smaller exhaust sizes around 2.5" or so from each riser until exit, to keep some decent backpressure? Or, do I just run it straight through at 3.5" all the way out and rely on a muffler to create the back pressure needed?
The AQ 225D exhaust Elbow-to-Y-pipe hose (aka couplers) ID is 95mm.
Most any marine exhaust hose company can bring it in for you.

Make sure that you have enough 95mm hose to push on to the elbow up and past the scalloped areas.
In other words, DO NOT clamp the hose over the scalloped areas.


I'm not the most versed on exhaust systems and the metrics of it all. So, just trying to get a good understanding of what's needed sizewise for the whole system. (It'll be dual exhaust when said and done, instead of meeting up with a y-pipe like current).

*The only reason I'm doing this is because when putting the exhaust bellow in place, the y-pipe outlet crumbled on 2 corner areas of the outlet, eliminating the bellow from being able to successfully be attached. (How this engine didn't blow up over the past 2 years with virtually no exhaust bellow is beyond me! Lol. But, I'm a lot more anal in the care of my boat VS previous owner, so I'm definitely fixing this unforeseen issue.)

**I didn't want to simply replace the y-pipe because I JUST got done pulling the engine, rebuilding the outdrive, and getting it all back together and the crumbling if this y-pipe was completely unforeseen/unplanned. If I can build a nice thru you'll exhaust without having to pull that engine again, I'd much rather take that route. At least, just to get through thru season until Nov 20th that is. In the winter time I'll go back through and really do it all out to basically a new boat again.
Click to expand...
 
So am I able to just run 95mm straight from the elbows (I was referring to those as risers and didn't know the difference) straight out the back at 95mm being the exit size diameter as well, kinda like straight piping a car?

I didn't know if I needed to create some type of back pressure when running straight exhaust, or if the back pressure from the mufflers I'd use would be enough.
 
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Walleye4Days said:
So am I able to just run 95mm straight from the elbows (I was referring to those as risers and didn't know the difference) straight out the back at 95mm being the exit size diameter as well, kinda like straight piping a car?
The Thru-Transom exhaust outlets will also need to be 95mm capable.

I didn't know if I needed to create some type of back pressure when running straight exhaust, or if the back pressure from the mufflers I'd use would be enough.
Best to acquire as little restriction as possible.
Click to expand...
 
Thank you very much for clearing that up. Definitely helps out to clear some of the confusion I had. Now, going ahead and ordering my thru hull tips!
 
Walleye4Days said:
Thank you very much for clearing that up. Definitely helps out to clear some of the confusion I had. Now, going ahead and ordering my thru hull tips!
Click to expand...

Warning.... it will be LOUD if you do this and in some jurisdictions, illegal. Don't forget flaps on the exhaust outlets.
 
Yeah, SandKicker, think I'm gonna use those slip in mufflers to bring it down around 18-28db for the sake of other's on the water. Granted I'm in lake Erie and very far from one another, I still have to go in and out of my marina, so better to be courteous of others.

Is it better to go straight 4" exhaust from the 95mm on back using a coupler? Or should I be looking around the 2.5-3" realm?

The reason I ask, is because its currently 3.75 coming out if each manifold, but then goes into one single 4"ish exit port via the y-pipe. So, with a 2 to 1 setup with 3.75", would I still have just as much free flow with individual 2.5" or 3" individual outlets for each one? (Just trying to figure out the best options and plan of attack using yours and Rick's advice). Not to mention, quite a cost difference from 3" to 4" exhaust systems hehe.
 
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Walleye4Days said:
Yeah, SandKicker, think I'm gonna use those slip in mufflers to bring it down around 18-28db for the sake of other's on the water. Granted I'm in lake Erie and very far from one another, I still have to go in and out of my marina, so better to be courteous of others.
I like it that you are considerate of others.
However, if you run straight out without any mufflers, as you enter the marina, keep your engine RPM below 5,000 and you'll be OK!
(just kidding)

Is it better to go straight 4" exhaust from the 95mm on back using a coupler? Or should I be looking around the 2.5-3" realm?
As said, the OEM exhaust elbow outlets are 95mm, very close to 3.750".

The reason I ask, is because its currently 3.75 coming out if each elbow, but then goes into one single 4"ish exit port via the y-pipe.
The Y-pipe inlet OD should also be 95mm.

So, with a 2 to 1 setup with 3.75", would I still have just as much free flow with individual 2.5" or 3" individual outlets for each one? (Just trying to figure out the best options and plan of attack using yours and Rick's advice). Not to mention, quite a cost difference from 3" to 4" exhaust systems hehe.
How would you plan to go from 95mm to 3" or 4" thru transom exhaust tips?
Click to expand...
 
The way I plan to go through 3" or 4" exhaust tips, is the following:

There are quad wall Turbo couplers (up to 500° degrees wet exhaust couplers) that go from 3.75" to 4", or I can get one that goes from 3.75 to 3".

So, off of the elbow I'll run a silicone plyed coupler that goes from 95mm/3.75" to 3" or 4" pipe. Then, the 3" or 4" pipe will be sleeved to the 3" or 4" exhaust tips.

So, basically, each side's run will be:

- 3.75"/95mm turbo coupler sleeved from elbow to straight pipe
- Then straight pipe
- Then sleeve from the straight pipe to the matching size exhaust tip. (All connected with T-Type turbo clamps that is.

Turbo Coupler:

https://www.hpsperformanceproducts....g7qxy8Qnn5jbDvQf2N4yPtpqj08_sSTxoCNS8QAvD_BwE
 
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Thanks Chris, I didn't know things like that even existed lol.

How do you clamp them/connect them to fit air tight? We they really firm to where you van clamp a clamp ring super firm onto them?

Yeah, the turbo reducers are super nice, because they are 4 ply and heat-worthy up to around 450-500 degrees.

I'm just debating on whether I should upscale to 4" or downscale to 3". I'm afraid if i go straight 4" I'll lose low end torque/power due to virtually zero back pressure on the engine. Whereas currently it's two 3.75's mating into one exit hole to the bellow at 3"... So, would I want to run 2.5" or 3" dual exits to try and compensate for the loss of back pressure by running duals? Or, would the loss of power be negligible anyways allowing virtually no backpressure?

(Exhaust CFM metrics are very confusing to me).
 
You slide them over the elbow. In order to slide the 3.5" ID ring over the 3.75 OD elbow, I just used dawn dish soap on the elbow and the inside of the ring. This flexed the outer diameter of the ring to grow from 4 to 4.25" so I ran my dremel tool around them a few times with a deburring type tool and got them back down to 4". Really just needed to take off 1/8" all the way around. Once those are on, the 4"ID hose was slid over the rings (more dawn dish soap). And then double clamped. I've run it this way for 2 seasons now with zero issues. I went with 2 4" and really didn't lose anything on the bottom end at all in my opinion. I do have mufflers in which provide a little back pressure I'm sure. However, you are at 305ci and I'm a 351. If I were 305, I'd consider going with dual 3" pipes if possible. I had a AQ225E on a 86 Bayliner before, and it didn't make a ton of power, so I definitely would not want to lose any. I think our 21ft Bayliner topped out at 19 or 20 on the speedo.

Keep us posted. : )

Here's a quick video of mine just after install. First time running them.
https://www.youtube.com/watch?v=V1nKHkiUUlU
 
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Thanks! I'm definitely going to go with 3's now. Yeah, the 305 is an overall good engine for leisure and etc, but doesn't really do anything for power overall. With my big 25' boost, I'm often wondering why they didn't just go with the 350 like most others that size. But, I guess we'll see how she turns out.
 
The MERC install manual for 305/350 SBCs stipulates either a minimum single 4" exhaust outlet or dual 3"... and a SBC is an SBC regardless of whose decal is on the engine for most things.
 
Thank you Sand, for the info. I figured it would be somewhere around the 3" mark for duals give or take some. I'm just gonna run with 3's then I guess. When I drill out the holes for the exhaust tips, do I just use epoxy on the transom wood and exposed core, to help seal and protect it, prior to final installation of the exaust tips?
 
FYI... I measured well over 6 psi back pressure with my 5.7 thru the 280 Y exhaust. When I went to thru transom w/ mufflers ( NJ laws!) , I instantly got another 450 RPM... and it was quiter @ cruise at the helm than the original 280 exhaust port. Sounds neat @ idle :)

The exposed core of the transom should be well protected before you install exhaust tips.
 
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