Is there a formula to figure out what size of exhaust is needed for
a diesel engine?

200 HP , 3.7L inline 6 , governed to 3,800 RPM , turbo charged after cooled .
I am not after max HP, just wondering what the minimum should be.

Thanks

 

How long and how many bends of what degree are you planning?
It's not just the cfm at the turbo outlet you have to figure on. Each trun adds back pressure and there is a static resistance for each inch of pipe.
It's all pretty complicated to figure out but rule of thumb, anything over 1.5" will work and bigger is better.
Look at small delivery trucks, a lot of them used the 4bt and they have full exhausts.

 

Found this which may help you figure out what you need.

 

We used 4" pipe for venting dryers and fart fans....a 90 elbow is worth 5' of straight pipe as far as flow resistance is concerned. Usually you get about 40 lineal feet max length without a pusher fan...I'm sure diesels push harder, Cummins suggested a 4" min to Dodge. Dodge went with 3 and added a 4 " tip.....genius.