Need advice on my exhaust system

[mrdeepseafisher]

I am still going through my "new to me" 1979 Trojan 26' flybridge and was in the process of winterizing the boat yesterday when I noticed the exhaust system. The boat has had the original raw water cooled engine (approx 225 HP) replaced with a new Mercruiser 454 that is fresh water cooled, however the muffler looked like the same one I had on my John Allmand which had twin engines and a muffler this size for each engine. Upon further check out I verified that the muffler has a maximun HP recommendation of 250 HP. It is a fiberglass straight thru with 4" inlet and outlet. The 454 exhaust headers are 4" outlets which have been reduced to 3" and are routed back to the muffler where each header connects to a 4" wye just before the muffler inlet.

What harm (if any) can be done to the engine with this set up? I was thinking of rerouting the exhaust anyway so is it unheard of to route each header seperatly thru the stern each with it's own muffler or should I replace the single muffler with the proper size? Any advice is appreciated.

[JuiceClark]

I have 4" pipes on my 454s as well...I think that's standard. So, I doubt there's a problem with higher than usual back-pressure or something related.

I have an F-36 but want to know about the exhaust as well.

My boat was made for big blocks...by I've redone the whole boat and never thought about the exhaust. When repowering, I just plugged those risers right into the 4" exhaust and moved on.

Also, they're really freakin' loud...thought about putting those tips on the exhaust that points the noise down toward the water.

[dave j]

I have a 1975 F44 which had 454ci 350hp Mercrusers with a 4 inch exhaust system. I repowerd the Boat with 7.4 (454ci) 380hp which came with 3 inch outlets on the exhaust manafolds. I used a boot to adapt up to the 4 inch pipes on the boat and all works well

Good Luck, Dave J