Exhaust diameter and back pressure - is it critical

skyflyer

skyflyer

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Long story short:- I have to custom build a new exhaust elbow for my engine this winter.

The exhaust hose is 1- 5/8" (42mm) Inside Diameter which happens to be the same as the outside diameter of the stainless pipe i intend to use. The ID of this pipe is about 37mm or 1-1/4"

The water injection occurs on the downstream side of an inverted loop.


However the 'normal' output OD at the engine exhaust is 2" (50mm). If I use the 42mm pipe throughout will that cause a problem, or should i used larger paipe and then put a reducer in after the water injection?
 
Are you saying that the engine has been working OK with 42mm exhaust hose? If so, your proposed new elbow should be fine.
 
skyflyer said:
The exhaust hose is 1- 5/8" (42mm) Inside Diameter which happens to be the same as the outside diameter of the stainless pipe i intend to use. The ID of this pipe is about 37mm or 1-1/4"
Click to expand...

I would consider using a pipe with a greater wall thickness than 2.5 mm.
This winter I will replace similar high riser bend that is 10 years old and has done 500 hrs.
There are corrosion crevices at least 2 mm deep by the water injection point. Wall thickness is 3 mm, stainless steel.
 
Last edited:
I was going to use Schedule 10 316 stainless which had 2.77mm wall thickness. I could use Sched 40 I suppose!

Yes it's been 42mm since new. 20 years old and the original elbow.

I think corrosion is minimised because water injection happens only about 50mm before end of tube maybe
 
I can't see that the exhaust hose will be that critical as marine engines are low revving and, unlike a tuned auto engine, nearly all the back pressure is created by the water trap and even that is intermittent. We often read about engines on the forum which have badly coked-up elbows but which are still running reasonably well and you can be sure that the back pressure in those will be significantly greater than your clean, if slightly smaller diameter, piping.

Richard
 
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