Exhaust Back Pressure

[Mac G]

Have Deere 6404 diesel with no turbo.

Existing exhaust exits engine manifold with 2.25-inch Inside Diameter pipe into riser and mixing elbow (also 2.25 inch ID) where it eventually attaches to standard 3-inch ID exhaust hose to muffler and out the boat.

If I were to decrease the ID of the riser and mixing elbow to 2-inch ID would that 1/4 inch reduction in ID create any adverse back-pressure or any other adverse consequences?

Will still connect to the 3-inch ID hose to the muffler, etc., but would have 1/4-inch reduction in ID between manifold and muffler hose.

Any issues or problems with that?

Please advise

Thank you

 

[mvweebles]

I have no idea of the answer to your question (beyond the obvious that Deere likely engineered the spec for a reason), but wondering why you ask? What's driving desire to change?

Peter

 

[DDW]

What does Deere require for the engine? Manufacturers often give a back pressure limit in the specs, you can measure it.

Gas flow in a tube is proportional to the diameter to the 4th power, So even though a 1/4 inch doesn't sound like much, it might be significant. For constant flow, the 2" will have 62% of the flow compared to 2.25, with the same pressure drop.

 

[Turtle Blues]

an 8% size reduction right at the outlet of a 6L. yes that's significant don't know how to calculate other than actually measuring pressure and exhaust gas particulate before and after.

Second issue is heat increase will be significant. Looking at the photos in your other posting that pipe has been extremely hot for a long time and there is further restriction caused by the water inlet.

Guessing you are going with a custom fabrication? Why the size reduction? Bigger would run cooler.
DDW got it while I was typing. 62% is certainly severe

 

[Mac G]

Okay. Thank you for that insight and explanation.

So conversly then going from 2.25 ID (existing) to 2.5" ID custom would not be a problem??

Am trying to build something myself with either 2" or 2.5" ID fittings.
Sounds like smaller is not good. What about slightly larger?
Would that cause any issues with velocity or anything else?

Eventually it all ends up in 3" ID hoses to and out the muffler.

Please advise

Thank you

 

[Turtle Blues]

Not a problem. Bigger is actually better. Just make sure that your design moves the cooling water out. The dry stack version of that engine exhaust goes from 2-1/2" horizontal to a 3" vertical with an even larger muffler.

 

[Nick F]

What pipe are you looking at using? I see no pipe size that has 2-1/4" ID.

OD wt ID
2" sched 40 2.375 0.154 2.07
2-1/2" sched 40 2.875 0.203 2.47
2-1/2" sched 80 2.875 0.276 2.32
3" sched 40 3.5 0.216 3.07

 

[O C Diver]

2 parts to this discussion. As mentioned above, reducing the diameter by a quarter of an inch will be significant. Secondly incorporating the reduced size in an elbow will make an even bigger difference. There is increased back pressure from frictional line loss with each elbow. Having the elbow with a reduced diameter significantly compounds the problem.

Most new engines include a small fitting in the exhaust elbow to measure back pressure. Might be worth adding the fitting and finding out what the maximum back pressure is for your engine.

Ted

 

[Keysdisease]

If you knew what your backpressure is now, and what the factory spec max was, you would have an idea how much "room" you might have to increase backpressure by downsizing your piping.

The biggest contributors to BP are, any turns in the hot piping, the water mixer, the muffler, and any rise from a lift type muffler.

A good exhaust shop can fabricate a mixing elbow that will provide good cooling while minimizing BP.

 

[Mac G]

Thank you all for that insight and education.
Will definitely not go smaller!