Exhaust Sizing – Part 2 – Wet Exhaust


Below are some notes on 3 most common WET exhaust sizes that have shown to meet all restriction requirements for Cummins engines over the last 20 years and are based upon well over 300 installation reviews and exhaust tests.

Keep in mind that the dry section and wet section are different and must be treated so. Also take into consideration the type of muffler, the overall length of the system, the amount of both dry and wet bends, the amount of water that you inject into the system (some of it, if not all of it), the angle of the discharge AFTER you inject water and how the system exits the vessel. All of these affect the total system back pressure. I am using Cummins engines as that is what most of my experience is with and I am using 3″ Hg or 41″ of water column as the maximum back pressure limit:


Learn to understand the nomenclature difference between “pipe,” “tube” and “hose” dimensions.

For this discussion, pipe size is always shown as a nominal size in SCH 10 wall ( .120″-ish) – I.E.
2.5″ = 2 7/8″ OD,
3″ = 3.5″ OD,
4″ = 4.5″ OD,
6″ = 6 5/8″ OD, etc.

“Wet” size is always the actual ID of the exhaust hose. Tube is always the actual OD of the tube (pipe) and is usually about 1/8″ wall meaning a 5″ WET tube or FRP (fiberglass) exhaust tube or pipe has approx a 4 ¾” ID.

Exhaust hose (any hose for that matter) is always sized as an ID measurement and always is sized to FIT OVER a tube OD dimension or pipe outer diameter measurement.

4″ Tube – 4″ pipe size ( 4.5″ OD) & 5″ WET SYSTEMS – 6BT 210, early 6BTA’s – ( 200-250 HP) Factory recommendation is 5″ WET (Yanmar 6YLA too), or most any diesel in between 180 and 250 HP.

Experience has shown that 2.5″ pipe (2.875 OD) for a short length or one “90,” OR 3″ pipe size ( 2-3 90’s) for the dry section, and 4″ tube to 4″ pipe for the wet section and wet muffler is usually fine. This would also apply to a 4LHA 240 Yanmar and 6LPA 315.

6″ WET SYSTEMS – 6BTA’s – 300 ~ 370 Diamonds and QSB’s thru 480’s, QSC’s , QSL’s ( 300-500 HP) – Factory recommendation is 6″ wet.

Well, this is where we have some “fudge factor” if you want to design it right and make it “fit” the boat. Going from a 5″ wet to a 6″ wet ( typically in 300-450+ HP engines ) is HUGE and opens many install capabilities. We like 6″ mufflers and piping from the typical engine room bulkhead aft to the exit near or at the stern. But we like to do our engine risers/custom elbows and wet mixers in 4″ pipe for the dry section and 5″ WET outlets where space is an issue. Using a properly designed dry riser w/ a 5″ mixing elbow, and then transitioning to 6″ at the engine room bulkhead, saves valuable space in smaller engine rooms, saves money for the customer, and is just about always easier and a better overall and “politically cleaner” install. The transition from 5″ to 6″ wet is always easy as 5″ to 6″ FRP tube is an easy make for a custom elbow of any degree at 45 or less. If a highly restrictive muffler is used, or the available space allows the use of 6″ close to the engine, then a 6″ mixer might be the best choice.

8″ WET SYSTEMS – Some “C” & QSC installs, QSM’s-400++ to 800-ish HP-Cummins has “recommended” both 6″ and 8″ and seems wishy-washy on this.

Well, I have yet to see a C, QSC, QSM, or any Cummins (or other make) engine UNDER 500 HP need an 8″ wet exhaust system to meet specs. But, I continue to see this as a RECOMMENDATION for the exhaust size in many cases. I guess if you have the room, the budget, and have an extraordinary amount of bends and/or an overly restrictive muffler, then maybe you should go for it. But from a vast amount of experience, we always stay with 6″ max if under 500 HP.

When we get up into the 500 – 800 HP-ish HP range, this is when we need to think about some, or all, of our WET side of the system being 8″. Since most of my work and the discussions on this site center around repowers of sportfishing boats like Bertrams, Vikings, Hatteras, etc, let’s confine the 8″ to this general style of boat in the 40-60 ft length range. And, with so many older Detroit powered boats/owners still out there that are now seriously looking at repowers, and most of these having 8-71’s , 6-92’s, 8-92’s, etc., this is now even more important to understand.

Usually, these boats have 8″ or even 10″ WET systems aft of the engine room. This is typically where the mufflers are located too. If all is good aft of the engine room – all piping, mufflers, outlets and hoses – (it is usually not), then this is a good size to start with for adapting to the newer 4-stroke engine. If some or most of this piping needs to be replaced because of age, it’s usually easier just to stay with the same size for the section aft of the engine room. If you’d like to size down for space or cost reasons, then a good look at the overall system and engine requirements will be needed.

Let’s look at a new QSM at 670 HP as to what will easily meet the spec. The factory supplies an 8″ fabricated wet elbow or mixer and IMO is of questionable use or quality. Yes, they do work in many applications when used correctly, but also seem to be subject to both internal and external leaks before their time because of the many welds and thin wall construction. Even worse, though, are the “installers” who re-weld or bolt these units in a dangerous “up” orientation in order to build a cheap “Mickey Mouse” type riser. I not only see this with the QSM’s but also in many other installs using the factory 5″ and 6″ “factory” wet elbows.

For those who cannot grasp what the issue is here, a simple explanation – This is a double walled 90 degree wet elbow that was never designed to “hold water” in this orientation but, rather, be mounted horizontally so as to exit downhill and self drain. When installed like this, the sea water that will eventually corrode thru the elbow will now go down the pipe into the turbo or worse. I hope you do not recognize this type of system on your boat!

Now, back to 8″ – This is what we have found with engines in the 600-700 HP range as to designing a wet exhaust system and working around the existing 8″ – 10″ piping or “book” recommendations. We typically use 4″ dry piping from the engine up to a well designed 6″ wet mixer. This usually includes 3 to 4 pieces of 4″ pipe size “weld” 90 degree dry elbows and then a double walled wet mixer of our design orientated on the downhill side of the system. The mixer is always pointed to a spot on the vessel so we DO NOT have to incorporate any WET bends / “90’s”, etc., BEFORE the transition to 8″ (or 10″) from the engine room aft.

With good design and thought, along with some first class fabrication, meeting exhaust restriction requirements can easily be done with a mix of both 4″ dry, 6″ and 8″ wet which will make for a much cleaner install while making the engine room more user friendly as to maintenance, etc.

To recap, exhaust size is something that can be adjusted to not only meet requirements for the engine, but also fit the vessel’s constraints. When sizing the system to meet acceptable restriction requirements in order to protect the engine from excessive back pressure, keep bends to a minimum, especially “wet” 90 degree bends. With some planning, most exhaust routing designs can eliminate a one or two 90 degree bends by building a custom riser/mixer and make for a less restrictive, less cumbersome and safer exhaust system.

DeAngelo Marine Exhaust Systems Contact DeAngelo Marine if you require an inspection / survey of your engine room, main engines or generator exhaust systems. We invite your inquiry for exhaust parts & accessories and exhaust repairs and service with your existing vessel. If you are in the design phase of new vessel construction or contemplating an engine overhaul, contact DeAngelo Marine Exhaust by email or by telephone: +1 954.763.3005.

2017-04-24T20:57:20+00:00 Design & Engineering|