Are you installing a name brand generator (Onan, NorthernLights,etc) or are you putting together components yourself? If name brand the installation instructions should certainly cover this clearly and it depends on the genset size.
For example, the older NL 6/4.8kw (which has a Kubota 10hp engine) calls for 40mm throughout and the exhaust elbow is 40mm (the new 6/5kw model has 38mm). Their 12/10kw with a 20hp engine has a 51mm elbow. In general, though, you don't want to introduce a reducer. If the exhaust elbow is 50mm, then carry that through, unless the manufacturer specifically calls for the reducer (to increase exhaust pressure - unlikely).
---In amelyachtowners@..., wrote :
Hi, I am installing a new genset into a super maramu 2000.
The engine is a kubota with a 50mm hose going to the vetas water lock. On the outlet of the water lock the hose reduces to 40mm. This is how the original is installed. It has a reducer 50 to 40mm glued into the water lock.
Can anyone help with the reasons why the hose is reduced in diameter?
The generator supplier advised to have a 50mm hose going on the water lock outlet.