Exhaust outlet diameter for a volvo d2-40

[stuartwineberg]

Interesting discussion on the Moody owners site - a user has swapped from a volvo to a Yanmar on a Moody 36 and the engineer spotted that the 2.5 inch exhaust pipe on the old engine was reduced down and terminated at a one inch exhaust outlet port - he reckoned this would cause back pressure and potential damage. As a result of that the installers fitted three inch diameter pipe all the way through to a new 3 inch outlet. I have a D2-40 in the same type of boat and was wondering if this concern about the diameter is justified. I can't find any recommendations as to the correct size on the web

Its on saildrive btw

 

[pvb]

The D2-40 requires exhaust hose with 45mm internal diameter.

 

[stuartwineberg]

The D2-40 requires exhaust hose with 45mm internal diameter.

Thanks - does that mean that the exhaust port in the hull needs to be at least the same diameter and its a problem if it isn't?

 

[pvb]

Thanks - does that mean that the exhaust port in the hull needs to be at least the same diameter and its a problem if it isn't?

Well, logically it should be, shouldn't it? Any restriction in the exhaust is going to cause unwanted back pressure, especially at high revs. VP say the maximum exhaust back pressure for a D2-40 is 20psi, but I have no idea how you'd measure back pressure accurately.

 

[BabaYaga]

Well, logically it should be, shouldn't it? Any restriction in the exhaust is going to cause unwanted back pressure, especially at high revs. VP say the maximum exhaust back pressure for a D2-40 is 20psi, but I have no idea how you'd measure back pressure accurately.

Isn't it more a question whether the given exhaust diameter is marginal for the engine size or not?
In this case perhaps it is, as the next size up, D2-55, has an exhaust dia of 57mm.

I have a 20 hp Beta, exhaust dia 51mm. Nanni uses exactly the same block, exhaust dia 40mm.

 

[pvb]

Isn't it more a question whether the given exhaust diameter is marginal for the engine size or not?
In this case perhaps it is, as the next size up, D2-55, has an exhaust dia of 57mm.

I have a 20 hp Beta, exhaust dia 51mm. Nanni uses exactly the same block, exhaust dia 40mm.

I've always thought that you can't go far wrong by following manufacturers' recommendations.

 

[Tranona]

Isn't it more a question whether the given exhaust diameter is marginal for the engine size or not?
In this case perhaps it is, as the next size up, D2-55, has an exhaust dia of 57mm.

I have a 20 hp Beta, exhaust dia 51mm. Nanni uses exactly the same block, exhaust dia 40mm.

Same with the Nanni 16 which I fitted to replace a Yanmar 1GM which is 45mm. Pretty sure size is not critical - I just used an adaptor to keep the original larger exhaust from the water lock on.

Don't think back pressure can be too critical as there are no specific recommendations for waterlock or silencer sizes but it makes sense to not reduce the pipe size lower than the manufacturers recommendations.

 

[stuartwineberg]

All interesting comments and unusually for the forum no absolutely definitive answer. Other sources tell me that If the diameter is insufficient the engine will tick over happily but as revs increase will show signs of oxygen starvation as it won't be clearing the exhaust gases effectively and it will start to smoke due to unburnt fuel. We shall see.

 

[pvb]

All interesting comments and unusually for the forum no absolutely definitive answer. Other sources tell me that If the diameter is insufficient the engine will tick over happily but as revs increase will show signs of oxygen starvation as it won't be clearing the exhaust gases effectively and it will start to smoke due to unburnt fuel. We shall see.

So just how small is your exhaust hose/outlet?

 

[macd]

I've always thought that you can't go far wrong by following manufacturers' recommendations.

+1
Also worth noting that if the exhaust system is particularly long, many manufacturers suggest going up a size on the exhaust. Since the supplied exhaust/mixer is usually only available in one size, this can be a problem. Luckily the Vetus drum-type water traps can be supplied with different sized hose connectors: mine is 50mm in, 60mm out. The short run of 50mm to the trap is of no great consequence.

 

[scottie]

So does the engine power and gas produced or the exhaust outlet determine the hose size or is it the manufactures convenience in having one fitting the is used on a range of engines from 12-50 HP ?
If the latter then one would assume that a smaller hose and hull fitting would cope assuming again that the water trap allowed for the water flow when the common water pump is used