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Discussion Starter · #1 ·
I'm in the process of swapping out and old 2QM20H (still running strong--just had the chance to acquire an GM) with a 3GM30F. The 3GM has 2" exhaust which I assume is OEM specs (none of the manuals specify exactly). But I was thinking I could adapt to my existing 1.5" exhaust system and avoid the hassle of replacing wet muffler and 10' of hose..

I think I found a post or two where this was done.. does anyone have experience with this or strong caution against?
Thanks

ps. will try and post some specs on fit as I build the mock engine and figure out dimensions, etc;
 

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Our boat has a 3GM30f in it.
The o/o replaced the original Atomic 4 with a 3GM but did not change the exhaust outlet hose or through hull. Yes there is a 1/2" difference in the exhaust diameter. It doesn't seem to effect anything but it is something I plan to fix one day.
I would not recommend this reduction in the exhaust, but like I said it doesn't seem to effect anything. Our motor has over 600 hours on it with this set up. On our boat it will be a pita to change as the engine takes up the whole engine well and accessing the steerage compartment will take a grandchild, yet to be born.
If your engine is out of the boat there is no better time to fix this than right now. Along with the muffler you will need to change the hose and enlarge the through hull as well.
 

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While you would be better off with a 2 inch exhaust system, I think you could get away with adapting to the existing 1 1/2 inch. Might limit the max power a bit due to the increased back pressure, but I don't think you will have any operational or reliability problems.
 

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Thx for reply. So you basically have (assume) 2" exhaust to a 2" wet muffler then coupled down to 1.5" out the wet muffler to the thru hull hose?
Yes, I have a 2" outlet off of the wet muffler with a 2"-1 1/2" reducer and then the 8'-9' long 1 1/2" exhaust hose to a 1 1/2" through hull fitting.
I have run the motor many times at 2700 rpm all day long with no problems. I'm not saying that this is the way to do it, I am only saying that I have had no problems with this set up.
 

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Discussion Starter · #6 ·
ok, thanks again. helpful info. Probably leaning this way since my C&C 34 is going to be slightly overpowered with the 3GM30 anyway.. and I'd need a robot to get to the exhaust thru-hull...
 
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