Exhaust Back Pressure

[Pete]

I have a new project. Our boat, a 76 48 LRC has a pair of DD 453N's. It has two nice 4 foot long by 9 inch diameter Veratone fiberglass mufflers using 4 inch tubing through the transom, as installed by Hatteras. This muffler was the standard back then, but anything you can see through is going to pass a lot of noise. So, my solution is a pair of Centek Veratone MKII Dual Stage mufflers, presently in my garage. With a flash light and my hand I determined there are two chambers; each a water lift design; but different in the details. So no more straight shot through the muffler.

Thus, my post to understand better the subject of exhaust back pressure. The energy required to lift the cooling water twice in the muffler would appear to guarantee more back pressure. I have tried to discuss this with Centek, but they are unwilling to engage me with technical details and test results. A simple it is OK for your engine is the net of my feedback. I have seen written material that suggest everything from significant increase in back pressure to decreases in proper water lift designs.

So two questions. 1)Do water lift design mufflers increase back pressure or not? 2) If the muffler design does increase back pressure somewhat, is this a problem and if so why? Thanks for the help.

Pete

[jim rosenthal]

You might get better answers and service from Vetus. They make a lot of this sort of equipment and have a section in their catalog on it. They may be more willing to advise you as well. Also, have you talked to any DD guys about this? They can probably give you specs on what sort of back pressure is acceptable.

To look at this another way, it is very common to put waterlift mufflers on small naturally-aspirated diesels. We do it with gensets all the time, and they work fine. The usual caveats about not overcranking the generator and making sure that the vertical drop to the waterlift muffler is far enough all apply here.

What about another, better, inline muffler? Tom Slane is using Centeks in my Hatteras. They would fit where the old ones came out, and you wouldn't have to deal with all the waterlift issues.

[Pete]

Jim, the Centek mufflers that I purchased are of the same general shape as the original Veratones. The setup on my boat has the engine end of the muffler's 4 inch inlet laying in a saddle. The new ones also have a 4" centerline inlet so they will drop in on that end with no modifications. The exit port is lined up on the centerline in the vertical plane but the horizontal plane is 2.75 inches above the horizontal centerline. Bottom line the only exhaust routing modifications are to drop the outlets the 2.75 inches and I have over three feet of space to the rear engine room bulkhead to do this. BTW, the original mufflers are 9 inches in diameter by 48 inches in length. The new ones are 10 inches in diameter by 30.5 in length. It will make a good looking installation and the existing aft muffler saddle can be modified slightly to do the job. I am more concerned about any difference in engine back pressure due to the different technology. Without turbo chargers to break up the exhaust sound pulses, I know of no other technology available today that will significantly dampen DD NA engines.

Pete

[jim rosenthal]

Pete, I looked on the Centek web site. I was confused- I thought you were going to a vertical waterlift muffler like I use on my genset etc. Now I see what you have.

As I understand it, the vertical waterlift muffler uses the exhaust pressure to force the water charge out; the pressure builds up and when it is high enough, a slug of water and exhaust comes out. How much pressure is needed is a function of how much vertical distance the water charge has to be propelled, I imagine.

I think what you have is the same kind of thing, basically, except that the vertical distance is much lower. If I had to guess, I would assume that there is some kind of baffle or perforated plate inside that the water has to get over or through, and that the exhaust pressure accomplishes that task. The rise can't be very great- the distance is limited by the vertical dimensions of the muffler. I suspect you are not going to have a problem with back pressure.

You have a couple of alternatives: you can try and pry an answer out of them, or talk to a DD mechanic who can tell you something about what kind of back pressure the engines will tolerate without trouble. Their mufflers are sold by size, not application. I did call Centek and although they don't release details of the internal construction of their equipment, the lift distance is small enough that I would feel comfortable installing what they specified and that it would work as you expect. From what I can gather, they sell more of this stuff than anyone, and I suspect their first consideration is to supply something that will do the job, and not create problems for the owner- or them.

[SKYCHENEY]

The problem with back pressure is what? I suspect the problem is that too much back pressure causes increased exhaust temps. If this is the case, then you will probably be okay since you have naturally aspirated engines at low hp. I would guess that your exhaust temps are already much lower than the maximum acceptable level for those engines and that you probably can tolerate much more backpressure than most other applications. If you were trying this on high-output 6v71tib's, you might have a problem, but on "n's", I think you would be okay.

[jim rosenthal]

Good point- increased exhaust back pressure causes poor exhaust scavenging which traps heat in the engine. But DD naturals are not running near the edge of the performance envelope on internal pressures or heat generation. Like you said, he should be fine.

[REBrueckner]

The difficulty you face is that even if you got a back pressure figure from Centek, let's say they quoted 1.1 psi...what would your do then? You'd really have to know the figure for the old muffler to be able to compare. And if the new one were, let's say, .25 psi higher than old what would you do then?

In all probability a modern muffler with the same diameter connections should be ok. And as noted, 4-53N's are not at critical HP ratios. On the other hand, you don't really know how much leeway there is in your exhaust design backpressure either.

I'd be inclined to check with Hatteras or Slane and get a quick opinion. In production boats, my understanding is that engine manufacturers, who warranty engines, test installtions making actual readings for such things are exhaust back pressure. Otherwise they could be rebuilding engines under warranty willy nilly.

Bottom line for me is I would not worry, but that's a judgement call, not one based on hard facts nor direct experience.

[Lucky Dog]

I had a pair of "see thru's" and replaced 'em with Vernatones as you described on my 8V-53N's. For a non-related reason, I built a water manometer and measured exhaust back pressure on the starboard engine up to WOT. It was under the max allowed by Detroit thru-out the range.

[dastahl]

I had a pair of "see thru's" and replaced 'em with Vernatones as you described on my 8V-53N's. For a non-related reason, I built a water manometer and measured exhaust back pressure on the starboard engine up to WOT. It was under the max allowed by Detroit thru-out the range.


Luck Dog, Did you notice any noise reduction?
David
Skooch

[Lucky Dog]

Yes - but that's only subjective. I wish I had gotten a sound pressure meter (Radio Shack for under a $100) sooner to quantify it. The noise level reduction is most noticible on the bridge.

[Brian Degulis]

My last boat had DD 671 naturals I wanted to reduce exhaust noise so I added a 6" centec lift style muffler (same as genset only bigger) This acomplished a very big reduction in sound and with the lift muffler and the original thru muffler installed backpresure was easily within normal spec.

Brian