Help! modifying exhaust gas to diesel

Grady Smith · Post by **Grady Smith** » Sat Jun 03, 2006 12:56 pm

I have read in the past that modifying the gas exhaust on the backside of the muffler out will relieve enough back pressure for 250 cummins conversion........any thoughts or insight would be greatly appreciated....... Grady

billb · Post by **billb** » Mon Jun 05, 2006 1:57 am

Grady: you need a 5" exhaust min...so if you have 4" you need to change it, unless your getting the engines real cheap, I would try to get at least 300 hp for a 33, the 250's would be minimal...

TBonedad · Post by **TBonedad** » Mon Jun 05, 2006 4:14 am

I agree with Bill. I installed 300's and feel like they are barely enough to give me the cruise speed I want. Regarding the exhausts, there are many ways to go. Modifying an existing muffler is possible and has been done succesfully many times. Without knowing the exact equipment you plan to modify I would still say you can probably make it work. Part of the "trick" is drilling through the internal baffles in the muffler. Another part is to rob some (a small amount) of the water before it is injected into the elbow so that there is less mass to have to push through the tubes. A great resource for you would be "boatdiesel.com." Tony Athens on that site is a real expert at repowers.

Rick · Post by **Rick** » Mon Jun 05, 2006 4:50 am

I'm sorry but I can't be much help with the muffler's because I just don't know. But I'll 3rd that 300 ! With twin 250's, I think you'd be lucky to make 16 knots cruise and our boats really like the high teen's, low 20's. Rick Ticket 85 SF Falmouth, MA

Brian Davin · Post by **Brian Davin** » Wed Jun 07, 2006 12:13 pm

For my Yanmar 370 conversion this winter, we cut out my old exhausts (cut them in half to get them out of rear bilge). The yard has installed new 6" Centek exhausts that are slightly shorter and are able to make it around the corner from the rear bilge into the side bilge position. On George's 315 Yanmar repower they did the same thing with 5" Centek's. See link below: http://www.centekindustries.com/exhaust ... d-page.htm Additionally, the yard converted both of our exhausts to side exhausts and closed up the rear opening. Brian '90 Bertram 33' FBC Never Say Never Guilford, CT (Homeport) Kinnelon, NJ (Home)