Exhaust back pressure

[Carl]

So I'm moving along finishing up the Port motors exhaust...then I can run raw water and fuel lines.

At that point I was planning to jump to the other side, duplicate what I have and start putting things back together so I can go in....but

but now I'm wondering if I can check the exhaust backpressure with the boat out of the water. I'd like to check before glassing up a 2nd custom surge/transition tube. The 3-1/2" dump and short hose till I transition to 5" has me somewhat concerned. And to check...can I just rig up a line from that little fitting between Turbo and Dump/Shower head to a regular vacuum, low pressure gauge? Is that good enough?

[bob lico]

are you talking about the square head little bolt on the output of the turbo? if so than the answer is yes thats how you check the exhaust back pressure.

[bob lico]

should be 1/8 NPT. cannot remember anything lately

[Bruce]

Carl,
You will not be able to feed enough water to the water pump to keep it from starving itself out of the water at anything above idle. A garden hose won't do it, it will suck a 5 gallon bucket dry before you look at the gauge.

[Carl]

Really, I figured 1-1/4" lines would pump the same as the 1-1/4" lines I had on the 440's. I figured warm it up and then give it a good bump. If not I have a 30 gallon tub I could use.

As far as the gauge, a regular vacuum gauge like I used to adjust carb would work or do I need to get something special?

[Mark]

Carl,
You will not be able to feed enough water to the water pump to keep it from starving itself out of the water at anything above idle. A garden hose won't do it, it will suck a 5 gallon bucket dry before you look at the gauge.

Bruce,

Would a large recirculator work? Something like a 20 gallon portable bait well or a 30 gallon drum with an open top and the appropriate size hose connection on the side at the bottom with the drum placed under the exhaust water and a hose running from the bottom of the drum to the strainer or intake hose. Would that be too much head height for the raw water pump to pick up? Don't know if it is worth the trouble but....

Mark

[Tony Meola]

Carl

When I flush my engines, I use a 20 gallon garbage can. Fill it with water. I have 3/4 inch heater hose running to the flush out plug on the seacock. Can not use a garden hose. it will collapse.

I leave the garden hose running in the garbage can so it keeps on filling it while I run the engine. I can run it like that for a good 5 minutes before it runs out of water. If you use a 30 gallon can you should be able to keep them running close to 10 minutes. that is all at idle. Crank them up and you will pump out that 30 gallons in no time.

If you are not alone, get a couple of 5 gallon buckets and fill them. Keep the hose running and have someone dump the 5 gallon bucket into the can to refill it faster.

By the way, I started doing this when I had the gas engines. Then I could use a garden hose. One year when I was starting the engines on land to make sure they would start before launching, unknown to me, in the middle of my running the engines the water line at the marina broke and I cooked an impeller. This way I can make sure I have water.

[Carl]

Thanks Tony-
I wasn't too worried about running them on land, but now that I know you were able to satisfy your motors that require 1-1/2" hose I know my lil' 210hp with 1-1/4 will be fine. I always ran the 440's that way, make sure they ran before putting in and to winterize I'd fill up a 5 gallon bucket with antifreeze, toss crash pump hose in and start while dumping in another gallon or two to get the pink out the exhaust. Before going in I'd idle'em till they came up to temp, them bring them up to 2k or so...bucket with hose blasting would go down quick, but back to idle let it catch up.
I know I can run them, I now know where to check the pressure...but

What I really want to know, is this a suitable gauge to check back pressure? I know it will, but is this kind accurate enough?

Am I concerned about idle pressure?

Wondering if the exhaust is underwater does it make a lot of difference, that I can't check on land apparently. But I'd guess soon as the initial water is out pressure drops and should be fine. Thinking only as I have seen some boats with exhaust completely under and they seemed ok.

Higher RPM I don't see as an issue as boats going and no water back there to offer pressure. But maybe exhaust needs to be checked with the motor under a load? Or does pressure take time to build?
I don't think so...but not willing to lose a pair of motors because I didn't want to ask a seemingly stupid question.

Once above idle, motor is warmed and I'd like to bring it up...I don't need to keep it there long to check or do I?
Long as in anything beyond a second.

I just want to see if I'm good, should be ok or way off/just not good and need to change the design before hitting the other side.

Thanks

[Tony Meola]

Carl

Not sure if you will have an issue with the exhaust going underwater. With gassers I would say that my exhaust parts were only 25% under water. Now with diesels with all the glass we put back their beefing up the strut area and the additional weight of diesel, they are now a little more than half submerged.

Now mine is a 75 so the exhaust might be a little higher than yours not sure.

I used to know a guy that carried 300 gallons of fuel when running offshore. Between fuel and ice and guys his exhaust was totally submerged. Those gassers always started and ran fine.

[Carl]

Thank Tony. My 440's had the exhaust about 1/2 way up when loaded with fuel and gear, empty belly the flanges were out of the water by a good amount. It is the way I got the boat, the way it ran...I never gave back pressure a second thought.

Some setups I know have the exhaust pretty much completely submerged when loaded...a buddy with an Albamarle and 240 Yanmars runs a little iddy biddy exhaust system right from the factory, a good amount smaller than the 4" 440's exhaust system.
I don't think the Yanmars are so touchy about backpressure.
From what I see the Cummins 6BT motors do not like backpressure with 3psi being the MAX. Manual says my motors need 4" minimum, but my motors came with 3-1/2" dumps/sprinkler head !?!? I was planning to transition to 5" right after the 45 bend came through the bulkhead...but with so little room in that spot (exhaust, raw water seacock, dripless packing, shaft and coupling plus the surge tube) I left it at 3-1/2" for another 24" and glassed the 3-1/2" - 5" transition directly to the surge tube.

What I'd like to do is test the backpressure at this point. I have open access to everything and have not committed myself to the Starboard motors exhaust system. The individual parts are made...just not fit and glassed together.

[1962 31]

I would say it will be ok those were running take outs correct ?
the po probably didn't go up to 5 inch
build the other side the same way and test for back pressure in the water under load
if its too high you will need the 4 inch shower heads and can easily make a new transition at the surge tube

[Carl]

I would say it will be ok those were running take outs correct ?
the po probably didn't go up to 5 inch
build the other side the same way and test for back pressure in the water under load
if its too high you will need the 4 inch shower heads and can easily make a new transition at the surge tube

Tom,
Running take outs is correct...my understanding from a Coast Guard boat. I'd guess the dumps are stock and kinda sure they didn't bump it up to 5", maybe not even 4".
But I have no clue as to the length they ran the exhaust or the configuration to achieve the rated backpressure.

That easily make isn't all that easy as I'd have to pull the boat, pull floor, side panels, cut, and reconfigure.

Ok, it's easy and not pricey but I really don't want to pull the boat to redo the exhaust over after spending so long working on it. Be nice if I could prove before moving on....such is life.

I'm really assuming they originally needed to go at least 2-3' with smaller exhaust and my short run going into 5" will more than make up for it.

Not sure if I'm assuming on my hopes over reality...


I'll move forward and check on land...if it's not good there without a load, it won't be better in the water.

[Tony Meola]

Carl

Unfortunately I can't help you there. Mine were 5 inch exhaust from the factory. The Mercruisers had 4 inch that transitioned to 5 inch. I have no back pressure problems, but not sure if you remember John Jackson (JJ). He has the 270's also, but he had to go up to 6 inch exhaust. He ran the exhaust down the outer alley then he used two 45's to transition to the exhaust ports that he left in their original position. It seemed that bend was enough to cause back pressure that the engines did not like.

Now since you are going from 3.5 inch to 5, I would say if you wanted, get a low profile muffler and run the exhaust down the outer alley and do what JJ did. When you get near the transom, use two 45's and then you will clean shaft alley's.

[Mark]

Carl,

I was surprised to see that you didn't go down the outside. But then again, I'm sure you would like to use the boat instead of just working on it. Maybe next winter. Would require you to go over the top to get the full shaft alley on the port, right? Which would mean new elbows and pipe, insulation, etc.

Mark

[Carl]

Tony, your running 5" exhaust with mufflers on your 270's and back-pressure is fine? If you are that makes me feel a lot better. Your motors call for 6" minimum. That means there is some leeway and good news for me.

My 210's call for 4" minimum exhaust and recommend 5" if running mufflers. I looked at the slime line mufflers which would have been nice as the ones I bought are BIG. big as in decks gonna have to go up a bit to clear.

My reasoning was-

After hearing Toms boat at idle I knew my wife would hate being on the boat when trolling. I trol often.

The larger mufflers flowed better and I knew the 3-1/2" sprinkler/dump was already restricting flow.

The price of these Big Mufflers is much less then the slim line and in stock.

I am a marine dealer and got a nice discount on their stocked exhaust, special order not so special price.


So cost, flow and sound kept me down the middle...I should say cost again as it is a big factor.


Then like Mark mentions was the idea of making a custom exhaust riser going up and over, blanket the high heat and come down the other side all larger diameter. Nice neat, no worries about water going back to motor...maybe run outside alley. I can do that for slim money aside from the blanket to insulate.
But that comes at another price...I'd have to complete everything, put floor back in so I can put the motor boxs back in to take measurements and fit...that adds way too much time to the project...then get blanket made...more time and money.

Big issue for me aside from what I mentioned is the motors are running take outs at 210hp...Yes I had them checked out but it was done on a pallet, ran easy. While sure they are good and performance will be good! worth all the time and effort...I'm also sure it's still a crap shoot.

I need to take boat for a run with these motors to know...

[John F.]

Carl-

Crows Nest has a 5 inch exhaust with a big muffler on each side and something that I believe is a surge protecter on each side. The exhaust runs in the OEM position. Crows Nest has twin 300 hp Detroit 8.2s, and has been running with that exhaust for about 30 years from what I can tell (at least 20). You can see all the stuff in the exhaust in the last picture in the thread below. I remember Walter's posts and talking to Walter about his repower on Bluebeard. He went with the 210s because it was easy--I believe he stayed with the 4" exhaust from his gassers and 1 3/8" shafts. I'm sure he posted about it so you can check. I'm a big fan of easy.

http://bertram31.com/newbb/viewtopic.ph ... ilit=+deck

[Carl]

Thanks John.

Not too sure I've ever seen a setup with two mufflers...maybe as you said a surge protector. What I did notice is your deck is considerably higher than mine and your rudders are more forward than mine are. My rudder post look to be at least a foot further back...not sure if that was even possible. Your tillers point back to the stern, mine go forward and the back of the tiller is right up to the transom. Interesting...

As to your motors having 5" exhaust and being fine...it could be. From my understanding, Cummins 6Bt's do not tolerate the backpressure other motors are perfectly fine with.

Walter did leave the 4" exhaust from the 440's when converting to the 210hp 6Bt's as I have. Only Walter, like Tom ran straight 4" hose...no mufflers. Without mufflers, the Cummins manual says 4" is ok.

I almost went that way, striaght 4" EZ and done till Tom sent me a sound clip of him trolling...while a nice sound for a few minutes, maybe even an hour or two BUT after many hours of trolling that can get old. I talked dad into running straight exhaust when he put a 327 in his last boat. Awesome for the 1st few hours, starting her up...all afternoon, all night into the next morning trolling before heading in around noon...the sound got old.

...and as I said my wife wasn't going to like it.

[John F.]

Carl-

I get it. I like quiet boats. The quieter (or less loud) the better.

John

[micky]

In my case with the old Detroits the risers were 6" and someone installed 4" mufflers so they came out 6" from the engine, got reduced to 4" for the mufflers and then went back up to 6" to exit the boat. The Yanmars are 4" so they will be 4" from the engines until after the mufflers where they will be 6" when they exit the boat..

[EarleyBird]

Carl,
When we Carver Boat (Chris) and I converted to the 270 Cummins 5” exhaust was the choice. The mufflers are outboard of the outboard stringers. 30 gallon fuel tanks were there. Glassed in the original overboards and they are now 5” with a flare slightly above the water line inline with the mufflers. Fuel fill holes in the gunwales are now verticle swivel rod holders for teaser rods when used. Never had a back pressure problem. Fiberglass surge tubes on each motor after the turbo exhaust elbows that run slightly forward. Most likely you have read the engine hours now on a previous post. If you would like pics coming out of the turbos, not a problem.

EarleyBird

[Carl]

I'm feeling pretty good that my setup will be fine after reading the past few posts. I'll check, but keep moving forward.
Thanks

[Tony Meola]

Carl

When we set mine up, forked river Diesel said the 270's needed 5 inch exhaust. It is 5 inches all the way, with the original mufflers that were on the mercy.

Your hoing up yo 5 inches, it it comes off the engine at 4.5 i nvvhes, then you should be good.

Not sure how they sound without mufflers, b it I was told they are not that loud.

If I ever get her painted, or hit the power ball, then I will move the exhaust out of the shaft alley.

[Carl]

Tony-

Exhaust comes off the motor at 3-1/2"

Yup, 3-1/2" shower head into a 3-1/2" 45° elbow then flows into 3 -1/2" hose for 24".

At that point I made the transition to 5" at the surge tube and its 5" all the way out.


Running I'm sure the motor noise will be louder than the exhaust...mufflers are for idle speed.


only benefit I see for going outboard with the exhaust is to get the fumes out from behind the boat.



Once it's running good and I'm confident in the motors and setup...I have no issue going back later to make upgrades and changes.