Exhaust pipe min diameter for 420 HP

[Joe Monaco]

Had new exhaust riser built. Sent them old one. New one came back with 3 inch pipe from flange to bolt on turbo. Old one was 3 1/2 inches, they want me to do a back pressure test to see if it’s ok as is. I looked at a chart that says 3 inch too small for 420 hp. I want a new part. What do you guys think?

Topic

[Joe Monaco]

Spoke with the owner today. He said they will fix it. So I’m gonna hope for the best and see what happens next. Hard to understand how this got missed but it did.

[Joe Monaco]

This is the company who built the part.

[Joe Monaco]

Hi Tony,

The system was designed by CABO and installed in 2002 when the boat was new. All I needed was a replacement part. The original exhaust company is still in business so I contacted them and sent them the part asked them to just build it again. Somehow they got the diameter wrong. Now they want me to live with it. I want the part built correctly as per original design.

I want to be able to make the case as to why it needs to be built as originally designed. Im suppose to speak to the owner today. I was told he wants me to do back pressure tests to see if it’s acceptable as wrongly built. I think that’s ridiculous, who’s going to pay for that and who decides what’s acceptable? I was hoping someone here knows what diameter pipe is minimum for 420HP to be exhausted into to confirm the chart info I quickly found on the Internet.

This system worked fine for 20 years. If I can get the same part built correctly it should work fine again.

[Tony Athens]

Up to where you are now, IMO, you are dealing with “people” that have zero clue about exhaust design or how to make a proper exhaust connection & possible system.. ..

1, The size ID size of the “pipe” you use for the system must not only be based on the max HP, but also based the the ENTIRE design of the exhaust system from the turbo outlet where max pressure will be seen,/measured and all the way until the exhaust exit to the atmosphere.

2) Then, we need to know is the system Dry and wet, or all dry?

3) Then all the lengths , bends, etc etc etc need to compensated for

4) Then, we need to talk about mufflers, if any..

Nothing here as to addressing any of this other that a comment about “black-smith-ing” a surface to try and make it fit—–Micky mouse would be the only “fit” here ..

How right do you want to make this job?? That is the real question?

Tony

[Joe Monaco]

Picture

Motors are 420 HP according to a quickly searched chart it needs 3 1/2 inch diameter for that much HP not 3. The new risers smaller diameter wasn’t noticed by me until after installed. Also mech had trouble using the pictured flange bolt brackets. They are we’re not getting the pipe flange tight enough to the turbo. Any ideas why this is happening? Mech wants to grind them down so they can draw flange tighter. Pipe manufacturer wants to send me a 1/8 vs currently used 1/16 gasket. Should I have them make the flange thicker if they need to remake the pipe the correct diameter anyway? Right now mech flipped them over just to make it work which obviously isn’t right. I will need another riser built but I was going to wait for winter. This one cracked. Manufacturer said they last 12 to 15 years boats almost 20. I had no idea they needed to be replaced thought SS lasted forever. Anyway So much for a simple R&R…

Thanks for any insights on what’s best to do next.

[Joe Monaco]

Picture of the new part.

I didn’t notice the size difference until after it was installed of course.

[Author]

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