Exhaust design question...

[rwthomas1]

Hello All,
I'm in the process of R-n-R'ing the 5432 in my E38 with a used engine. In the process of removing the engine several other things are receiving attention including the exhaust system.

Currently the exhaust leaves the engine from a 2" OD water mix elbow, travels about 10" of hose, and is necked down to a combination NPT street 90* fitting/nipple setup, then to another 10" section of hose to the waterlift muffler. The NPT street 90* setup is at best 1 1/8" internally and the waterlift muffler inlet is 1 1/2" as is the rest of the system. This system looks pretty restrictive, especially the 2" to 1 1/8" combination reduction/90* corner.

I am considering replacing the waterlift muffler and ALL piping with 2" throughout. There are numerous combinations of parts that will allow this. The result should be a happier engine with a lot less backpressure. Less backpressure should mean lower temps and slightly more power. IIRC, Universal's M40 engine which is basically the same as the 5432 has the larger 2" system from the start. The other justification for this is simply the hoses need replacing anyway.

What say the crowd? Seems reasonable and easy to me.

RT

 

[Glyn Judson]

Letting your exhaust breathe.

Rob, What you want to do makes perfect sense to me and is exactly what I did with my set up. My boat was re-engined from a Yanmar 2QM15 to a Yanmar 3GMF by a previous owner. But they stopped the newer 2" exhaust at the factory steel muffler which was 1 1/2" the rest of the way to the exhaust flange. All that went on my watch when the new 2" in-out Vernalift muffler replaced the original along with new hose down stream and of course, a 2" bronze exhaust flange. As an aside, I recently exchanges emails with Loren about how it turned out we both used the same method of fitting that flange by ourselves. We both ended up running a lint through the flange and looped it from the inside of the boat, out and tied it into a loop so as to prevent it from going plop into the briny deep. I see no reason for you not to do the same. Glyn Judson, E31 hull #55, Marina del Rey, Ca

 

[rwthomas1]

Thanks for the head check Glyn. If you "upgraded" to a 2" for a 24hp engine then it would certainly seem mine is woefully undersized as is. 2" will provide a large increase in flow and should make the engine happier. Easy fix too. Thanks, RT

 

[Emerald]

Hi Rob,

I agree with everything you say in theory, and in fact made the same exhaust pipe diameter upgrade when I did the same basic 2QM15 -> 3GM30F switch Glyn did. However, I've been having fun rolling this around in my gearhead, and my gut just tells me that the power and RPM these engines produce/operate at is more than adequately covered by the 1.25" diameter pipe. So, with the knowledge that these are oriented towards gas engines, and yes, we all know it will be a little different for a diesel, but it ain't gonna be that different, IMO, so, have fun with these two links:

http://victorylibrary.com/mopar/header-tech-c.htm

and then just a simple calculator that says you're restricted to 64hp with a single 1.25" pipe:

http://www.mk5cortinaestate.co.uk/calculator5.php

and I'll throw out one more crazy thought. If the diameter is OK, do you put more restriction in the system by having a larger water lift muffler and hence more water wieght/pressure to pump through?

OK, that all said, I'd try to get rid of the 90 bend and I would be likely to do the size upgrade if I had to replace the hose anyway....

Time for more coffee

 

[rwthomas1]

David,
You may well be correct. I know that diesels require far less backpressure than gas engines and any additional backpressure should consume some horsepower just pushing exhaust gas/raw water out. Excluding diameter, the next thing to consider is restrictions.

The 90* street plumbing elbow is less than flow-friendly, you could see that by looking at it. The issue then becomes how to smoothly transition for 2" at the exhaust elbow to 1.5 at the waterlift? The waterlift I'm looking at with 2" inlet/outlet, IIRC, is the same size as my existing unit so there should be no extra restriction?

Lastly, these engines are, or should be, operated at 80%-90% of rated output most of the time. If I'm cruising, its turning 2400-2500rpm, with a max of 2800. Less restriction can only help?

The only negative I can see is the increase in pipe size would reduce the velocity of the gas/water stream and somehow actually increase backpressure?

But I also have to ask myself why did Universal upgrade the M40 to 2" when they sold it?

Pros and cons to be sure however if its working for Glyn, the hoses are starting to dryrot, I might as well do it anyway. The only thing I'm paying extra for is the muffler and transom fitting. The hose will likely cost more than both....

RT

 

[Glyn Judson]

Exhaust hose replacement.

Rob and all, Let me jump in her on a somewhat related topic having to do with replacing exhaust type hose(s). All of a sudden a couple of years ago my engine exhaust developed that sickening "hollow" sound emanating from the exhaust flange, no water coming out, Ugh!! Fortunately I was able to limp back into our slip less than 200 yards away. To make a long story short by skipping all the what the?? and head scratching, it turned out to be the raw water hose from the seacock to the Groco strainer that was at fault. The vulcanized inner lining of the hose had separated and was acting as a flapper valve when water was being pulled by it, effectively sealing off the hose and all water flow. Needless to say, I replaced all of that hose including that which went from the strainer to the engine. With the job done, I directed my attention to the offending length and proved to myself that by blowing into it, I could replicate the hose failure. I only mention this because replacing old dried out hoses with new can only give one added peace of mind. Go for it, Glyn

 

[Emerald]

Hi Rob,

I think as long as it's not a budget issue, I'd go for the 2" upgrade. I'm honestly wracking my brain trying to remember if I did my exhaust upgrade as part of the engine swap or a year later after the old cast iron water lift started to clog up. I think I ran it a year with the restricted flow, and did feel like more water came through with the upgrade, but that would be tied to increase water lift volume. I thought about the reduction in velocity, but I can't see it being an issue. I think that you're also on the right track to straighten it as much as possible, and avoid size reduction in a pipe mid stream. If you neck it down in a straight section, I would have a little concern on turbulance when it hits the reduction. So, this all just says go 2" all the way and make it easy and know you have more than enough capacity for WOT, but I guess I probably shouldn't call it WOT with a diesel

(WOT = wide open throttle, which to me implies a throttle body regulating air flow, which diesels don't have...)

 

[Loren Beach]

"Mine's smaller 'n your's"

Interesting discussion to eaves drop on....
Having just worked on the exhaust transom fitting on our boat I can confirm that it uses 1 5/8" ID hose all the way thru including the water lift muffler.
That's on our 23 hp Universal three cylinder diesel.
Loren

 

[rwthomas1]

I was just reading the Centek/Vernalift homepage and they have general recommendations for exhaust sizing based on engine horsepower..... They consider 1 1/2" good up to TEN horsepower with a diesel! A 2" is good up to 25hp and 2.5 good to 40hp. http://www.centekindustries.com/pdfs/CentekNEWRev.pdf They state "general" size recommendations however it would seem that most engine installations are woefully undersized based on their charts. Of course, they have something to sell us too..... Still, its going bigger for sure.

RT

 

[Glyn Judson]

Proper exhaust size.

Rob and all, Once again luck surpassed innate ignorance and I chose the right sized exhaust hose. Thanks for the link, it was nice to see the chart. Glyn

 

[Emerald]

and looking at page 14 of the .pdf you linked above, I am quite surprised at how much more they require for diesel over gas. I'll have to eat my hat on this one. :eek:

 

[Glyn Judson]

Read the chart.

David, If you do eat your hat on this one, make it one of those corn-ball Greek fisherman's wool hats will you? Send photos. Glyn

 

[Emerald]

That's fried, right?

 

[footrope]

When I had the exhaust system redone in 2004, the engine guy said the M-40s used 2" and recommended the upgrade. We did and I think it was the right way to go. It's 2" all the way from the flange to the transom, with a new muffler, valve and thru-hull. We get more water through and combined with the new heat exchanger it runs a lot cooler.

The trigger was similar to Glyn's experience, except we had only a half mile to a nice open dock, with friends waiting to catch lines. The delamination occurred in the old 1-5/8" exhaust hose between the elbow and the muffler that blocked the exhaust and then blew the hose off the muffler. We don't use hose with a liner anymore, but I still replaced that piece after 5 years.

 

[rwthomas1]

As I removed the exhaust I realized what a hodge-podge of parts it really was. 2" hose off the elbow sleeved to a smaller hose to reduce diameter to fit to a 1 1/2" nipple screwed into a street 90* elbow that had a 1 1/8" ID, to a short length of 1 1/2" hose to the muffler. Then out of the muffler with 1 3/4" hose cranked down with clamps to fit the 1 1/2" muffler outlet. The 1 3/4" hose ran to the transom where it was hooked to a grey PVC nipple that fed into another street 90* attached to the Marelon thru-hull that only had a 1" ID.

Restrictive to say the least. The new system is 2" wire hose from the elbow to the muffler, (hoping I can make that tight bend happen), then the muffler out to 2" hose all the way back to a 2" bronze nipple, 45*fitting and 2" bronze thru-hull. That should "uncork" it nicely.

RT

 

[footrope]

The other thing the engine guy did was increase the sea water intake hose diameter. That had to help the cooling. I re-plumbed last year with 3/4 from the thru-hull to the strainer. I think the strainer to the Oberdorfer and beyond is 5/8.

 

[bigd14]

Thread back from the dead

Ericson 30+ with M18 2 cyl diesel.

I have the same issue as Rob originally posted: 2 inch hose from the Westerbeke mixing elbow, a reducing elbow down to 1 1/2 inch hose over to the waterlift muffler then same 1 1/2 inch hose back to the stern. The mixing elbow's all seem to be 2 inch outlets now so I am stuck with a reducer if I want to use the 1 1/2 inch hose size. Everything in the system from exhaust manifold back except the transom thru hull needs replacing. Since it all needs replacing, I am wondering if a 2 inch hose run all the way back to the transom would be overkill for this little motor? Or should I stick with the reducing elbow and continue using 1 1/2 inch hose? Is anyone else with a small motor like this using 2 inch exhaust hose?

Thanks.

 

[Loren Beach]

Hose Size

I recall that a ship wright I know mentioned that for modern re-powers using BetaMarina diesels, most owners have to change up from their older ID exhaust hose to a larger size.
Since the whole install normally would involve a new hose anyway, this is does not add any significant extra cost... except that the transom outlet has to be changed out.

I just looked at the specs for the current Beta 16, 20, and 25 hp, and they all show a 2" ID exhaust.

I recall that he said that new engines are designed for less back pressure to achieve a little more hp.

Our original hose size for our 23 hp Universal is smaller than that. "Times change" indeed.

Loren

 

[Kenneth K]

One thing to consider if increasing the length and diameter of the exhaust hose is the capacity of your lift-muffler. From a related article I found online:

"The concern here is that the water in the lift pipe will fall back into the muffler when the engine shuts down. This is a critical issue. The muffler volume must be large enough to accept the water that falls out of the lift pipe. The rule of thumb is that the muffler should have at least 130 percent of the volume of the lift pipe."

reference = Good Old Boatmagazine: Volume 1,Number 2, September/October 1998.

​

In other words, when you shut your engine down, a 2" exhaust line will let 80% more water (per unit length) flow back down the vertical pipe rising from your lift-muffler than a 1.5" line will. If you overflow your lift muffler, water can back up into the engine.

Ken

 

[Loren Beach]

Important detail, indeed

Good point Ken!
OTOH, on our boat the distance from the muffler to the top of the loop is maybe a couple feet, so the actual amount of water in it would seem to be limited. But then I have not done the calculations, either. I have no diagram for the inside of the muffler, so I do not know what volume it can hold.

Loren