Exhaust size

[vic008 17]

have yet to buy a motor-say 15-20hp.Would like to do exhaust system- hose, new seacock while no engine.My question is what say exhaust line to install?

[wheels 543]

You may need to know which side and where the exhaust exits the engine first.

Will the engine be turbo'd or not?

[Battleship 100]

Come on wheels 15-20 HP turboed?

If it's a new engine you will need to go with the manufacturers specs to keep your warranty but pretty sure a new yanmar or lombardini 20 require a 50 mm exit.

[wheels 543]

Hey, it's early and figures don't register yet because one of my two brain cells is out looking for coffee.

[Battleship 100]

I figured it was probably Friday morning itis.

[wheels 543]

I typed a long reply and it seems I can't have posted it. So here tis again.

 

I expect you would be needing about a 45 to 50mm exhaust. But that is guessing, because there are a few variables. First off, the manufacturer normally requires a back pressure of no more than 1PSI. Less if it is a turbo.

The exhaust outlet on the engine may or may not have water entry for wet exhaust fitted. If it does, then usually that outlet is a good start for exhaust size. If it does not, you need to look at going up 1" in diameter at the point the water is injected. Bends and length add back pressure. Each 90deg bend is equal to about 3M of hose length, I think if I remember rightly.

Also, you are going to need to have a riser to stop water from coming back into the engine, a lift box at the bottom of the riser to blow the water out of the exhaust and you may or may not fit a silencer, depending on how quite you want the exhaust. Finally, you may want a gooseneck at the outlet. This aids in stopping water from being forced back into the exhaust from a wave slapping the back of the boat(if exit is at stern.

DO NOT skimp on hose. Use proper wet exhaust hose. You will wish you did otherwise, the first time you have water lose in the exhaus.

I recommend you look at fibreglass for lift box and silencer and gooseneck. The plastic Vetus ones are good for exhaust flow, but just as expensive as fibreglass and the first time you lose water, you lose the plastic. The polyethylene ones are crap. They melt easily, but the big problem is a 40mm one for instance, has about a 25mm entry/exit which is just oo much restriction. Restriction equals loss of power, excess fuel burn and heat build up.

[wheels 543]

If you did go for 50mm, I just spied this on Trademe.

http://www.trademe.co.nz/motors/boats-m ... 948098.htm

Sometimes I wonder if I spend too much time on trademe nah

[marinheiro 352]

In your power range Beta & Yanmar exhaust outlets are all 50mm, Nanni is 40mm until 29hp when it goes up to 50mm.

This sizing is often more about the manufacturer standardising on parts, eg Yanmar is 50mm from 15hp to 30hp, then 75mm all the way from 38hp to 125hp, than it is about the engine's "breathing" needs.

When I asked Yanmar why my new 54hp (2.2l motor) needed the same size exhaust as their 125hp( or a 6l ford/perkins) they could not come up with an honest technical answer, just bluster.

[wheels 543]

they could not come up with an honest technical answer, just bluster.

That's because the people that represent Yanmar in NZ are not Diesel mechanics, just sales reps.

This sizing is often more about the manufacturer standardising on parts, eg Yanmar is 50mm from 15hp to 30hp, then 75mm all the way from 38hp to 125hp, than it is about the engine's "breathing" needs.

Yes you are correct that engines have some standardisation with the mixer/exhaust. But what is on the end of the engine does not mean that should be the exhaust size. It is essential for the exhaust to be the correct size. Size determins the back pressure. The back pressure is determined by the gas volume being driven through the pipe. Gas volume is determined by (ruffly) engine Volume x RPM. That will change turbo or non turbo and so on.