Thru Hull Exhaust

[45Auto]

Re: Thru Hull Exhaust

Everything that I have seen says that thru-hulls won't make any difference (except noise!) on a small block unless you're running something pretty radical at higher than normal RPM. Big blocks seem to be a little different case. According to Mercury, it appears that with an exhaust bellows, at high RPM the big blocks can dump enough exhaust gases through the prop to create a vacuum that will slow you down.

From the Mercruiser Gasoline Installation Manual, 90-863021020, May 2002, page 41 (496 HO, 425 HP):

Quote:

MAGNUM BRAVO MODEL EXHAUST RECOMMENDATION
Important: To get maximum performance from Magnum Bravo Models, through the transom exhaust is required. It is recommended that the exhaust bellows on the transom assembly be removed. This is necessary to avoid creating a vacuum at the exhaust outlet in the propellor at higher boat speeds. This vacuum could degrade propellor performance on some boats.

Note: If noise regulations do not allow the use of through the transom exhaust, an exhaust pipe kit (Quicksilver P/N 44266A6) must be installed for the through the prop exhaust. This kit also contains an exhaust tube that is used in place of the exhaust bellows.

 

[AZSenza]

Re: Thru Hull Exhaust

I'm a hot rodder at heart. EVERYTHING I OWN makes noise and everything I have ever owned has bebnifitted from exhaust mods. I HAVE to hear my engine to "Feel" it. I'm saving up now to put a quick and quiet system on my brand new 186 Larson. I put straight thru hulls on my 85 4Winns. OMG the music was wonderful but, I do believe my hole shot suffered just a wee bit, luckilly there was enough power from the 260HP Merc to get past it in a hurry and my trim tabs kept the bow down when I needed to. The Music thru hulls make as you sit in choppy water, comming in and out of the water is fantastic but conversations were impossible at WOT. PLUS one of my favorite lakes placed noise restrictions on boats 86 Dbls. I think I made more than that...
: )

 

[QC]

Re: Thru Hull Exhaust

The Music thru hulls make as you sit in choppy water, comming in and out of the water is fantastic but conversations were impossible at WOT. PLUS one of my favorite lakes placed noise restrictions on boats 86 Dbls.

We can actually talk at WOT with my 6.2 MX 320. Wind noise is more of an issue . . . I passed the 86 dB thing by 2 dB, no baffles, nada, straight out, stock Merc manifolds.

Good info there 45 Auto! I'm getting my head around the BB thing. It seems to me that the Y pipe must be the choke point with BB flow . . . Not an issue with SB flow . . . In theory though a 310 BB should flow less than a 320 SB, but I am guessing we are really talking about 350+ bhp being the upper limit.

 

[45Auto]

Re: Thru Hull Exhaust

Hi QC,

I would guess that it is an issue with the exhaust passage cross-sectional area available through the outdrive. Maybe above 350 HP or so (big block or small block - irrelevent) the area is insufficient to vent the exhaust gases without causing significant back pressure. Replacing the exhaust bellows with an exhaust tube helps by allowing some of the exhaust gases to go around the outdrive instead of through it if the back pressure in the outdrive exceeds the pressure in the exhaust pipe.

But as I think about it I think you're right. As you say, you would still be restricted by the area below the y-pipe and the size of the opening in the transom fitting that the exhaust bellows (or tube) clamps to - hence the recommendation from Mercury for thru-hulls.

 

[wca_tim]

Re: Thru Hull Exhaust

Let's make it even more complicated, OK?

What about exhaust gas temperature? The density, and hence the volume of a given amount of exhaust gas is strongly related to it's temperature. Exhaust gas temperature increases as we get better at burning fuel and thus higher compression, more effective combustion, more fuel burned, more horsepower, means more heat generated and hotter exhaust temps as well as a greater volume of exhaust gas generated (tell me if I'm missing the boat on this one...), IF we increase the cooling capacity (ie delta temperature) and internal surface area of the jacketed portion of the exhaust system, we might help backpressure, further if we more efficiently atomize /mix the water and exhaust gas at the water injection point, and / or increase the amount of water injected, we further drop the temperature. The gass "shrinks" as temperature decreases and occupies less space. The flip side of the coin is that as it cools and includes a substantial amount of water, it weights a lot more per unit volume and momentum becomes more important. In either instance, a straight shot "out" with a large and consistent cross sectional area is going to be the key to getting max flow through an exhaust system.

The largest problem or first one once we increase performance would be the manifolds / risers, right? on even lightly modified small block engines, there are at least minimal gains to be had from good, free flowing aftermarket manifolds. Headers more, but not enough so at the rpm ranges most of us are going to deal with and the cost combined with the cracking issue is a problem also. If we improve even a little on the mafniolds, then what becomes the bottleneck?

Am I incorrect in my understanding that there is a small (ie 10) horsepower gain to be had moving from bellows to exhaust tube in the lower unit? That tells us that we're definately skirting the limit of flow that the stock, through prop exhaust can accomodate, right?

That being said, doesn't it stand to reason that when making any significant engine modifications - especially ones that will move the rpm up a little and increase the exhaust gas temp a little, that the stock y-pipe assembly will quickly become a limiting factor in our ability to convert fuel to power?

Consider a 350 turing 4800 rpm's. Lets say we want to be a little risky with our outdrive and spin it to max 5500 rpm. Even without considering increased heat, just the volme change of the engine is going to get us halfway to the gas generated by a 454 at 4800 rpm's (and yea, I know... increased rpm's, decreased reliability and eventualy if not sooner, something turns into little shiny bits of mettal everywhere...)

now say we some stroke, we're at 385 ci and running 5500 rpm, even in the simplest model where we take nothing into account except for the volumetric pumping capacity of the engine per unit time, we're pushing almost as much exhaust gas out the tail as a 454 at 4800 rpms. (Y x 5500 rpm/4800 rpm x 383 ci/350 ci = ca. 440 ci equivelent per unit time).

bottom line on a stock 350 seems to be like everyone says... not going to make a difference other than the noise. BUT if you're setting up to make even a little more power overall and are considering everything, the exhaust is going to have to be modified for optimal performance and at some point, the y-pipe flow is going to be the flow limiting factor.

For a stock 350 = noise or even lose a little low end torq without rejetting or otherwise accounting for the chainge in configuration (only plugs will tell the real story)

much above a stock 350 - especially once moving to a 383 or larger, the factory exhaust is worth looking at changing.

SOOOO, I decided I want to go with diverters, keep the y-pipe (ventilated 28 pitch prop) and go through hull, above the waterline, with mufflers... Can anyone tell me how in the world to deal with the powersteering assembly for my alpha drive being in the way?!?!!?

 

[AZSenza]

Re: Thru Hull Exhaust

THAT was my limiting factor. I had just enough clearance to stick the 3" pipes under the steering and above the deck. Very tight fit. Go with side exhaust, thats what I'm doing with my new boat.

 

[QC]

Re: Thru Hull Exhaust

(tell me if I'm missing the boat on this one...)

I think you kinda are, but we should let 45 Auto chime in. My expertise is more diesel and lean burn strategies than Stoichiometric, but my first reaction is that exhaust temp is pretty fixed at full load with a Stoic burn engine (what you have) . . .

 

[NeverEnough02]

Re: Thru Hull Exhaust

Let's make it even more complicated, OK?

What about exhaust gas temperature? The density, and hence the volume of a given amount of exhaust gas is strongly related to it's temperature. Exhaust gas temperature increases as we get better at burning fuel and thus higher compression, more effective combustion, more fuel burned, more horsepower, means more heat generated and hotter exhaust temps as well as a greater volume of exhaust gas generated (tell me if I'm missing the boat on this one...), IF we increase the cooling capacity (ie delta temperature) and internal surface area of the jacketed portion of the exhaust system, we might help backpressure, further if we more efficiently atomize /mix the water and exhaust gas at the water injection point, and / or increase the amount of water injected, we further drop the temperature. The gass "shrinks" as temperature decreases and occupies less space. The flip side of the coin is that as it cools and includes a substantial amount of water, it weights a lot more per unit volume and momentum becomes more important. In either instance, a straight shot "out" with a large and consistent cross sectional area is going to be the key to getting max flow through an exhaust system.

The largest problem or first one once we increase performance would be the manifolds / risers, right? on even lightly modified small block engines, there are at least minimal gains to be had from good, free flowing aftermarket manifolds. Headers more, but not enough so at the rpm ranges most of us are going to deal with and the cost combined with the cracking issue is a problem also. If we improve even a little on the mafniolds, then what becomes the bottleneck?

Am I incorrect in my understanding that there is a small (ie 10) horsepower gain to be had moving from bellows to exhaust tube in the lower unit? That tells us that we're definately skirting the limit of flow that the stock, through prop exhaust can accomodate, right?

That being said, doesn't it stand to reason that when making any significant engine modifications - especially ones that will move the rpm up a little and increase the exhaust gas temp a little, that the stock y-pipe assembly will quickly become a limiting factor in our ability to convert fuel to power?

Consider a 350 turing 4800 rpm's. Lets say we want to be a little risky with our outdrive and spin it to max 5500 rpm. Even without considering increased heat, just the volme change of the engine is going to get us halfway to the gas generated by a 454 at 4800 rpm's (and yea, I know... increased rpm's, decreased reliability and eventualy if not sooner, something turns into little shiny bits of mettal everywhere...)

now say we some stroke, we're at 385 ci and running 5500 rpm, even in the simplest model where we take nothing into account except for the volumetric pumping capacity of the engine per unit time, we're pushing almost as much exhaust gas out the tail as a 454 at 4800 rpms. (Y x 5500 rpm/4800 rpm x 383 ci/350 ci = ca. 440 ci equivelent per unit time).

bottom line on a stock 350 seems to be like everyone says... not going to make a difference other than the noise. BUT if you're setting up to make even a little more power overall and are considering everything, the exhaust is going to have to be modified for optimal performance and at some point, the y-pipe flow is going to be the flow limiting factor.

For a stock 350 = noise or even lose a little low end torq without rejetting or otherwise accounting for the chainge in configuration (only plugs will tell the real story)

much above a stock 350 - especially once moving to a 383 or larger, the factory exhaust is worth looking at changing.

SOOOO, I decided I want to go with diverters, keep the y-pipe (ventilated 28 pitch prop) and go through hull, above the waterline, with mufflers... Can anyone tell me how in the world to deal with the powersteering assembly for my alpha drive being in the way?!?!!?

Very drawn out and very true!

 

[krisnowicki]

Re: Thru Hull Exhaust

I have thru hull and sometimes it is embarrassing. To get out on the open water I have to go through a channel. And we go out alot but, coming back in at night I feel like a scum bag waking people up. But, my 305 does sound pretty healthy with the 2 4' pipes coming out. But my question is about back pressure. I know when I see people driving around with hondas with dual exhaust I laugh cuz it is completely pointless. I have 4' risers and pipes going out the back is this to much do think as far as back pressure...?

 

[Bondo]

Re: Thru Hull Exhaust

If your older and have grown out of that

Do you mean to tell Me that,.. That Might Happen someday,...??.....

I have thru hull and sometimes it is embarrassing.

Ayuh,........ I don't know if I'll go quite that far,...... But thru-hulls Are Noisey....

Back nearly 10 years ago, when I refitted my current Barge,....
I dropped in the 4.3LX,+ even though I had All the parts to do the stock exhaust,.....
I Didn't......
I just happened to have a nice stick of 4, 1/2" SSteel laying around,+ Thru-Hulls are Cool Right,...?..
I just Eyeballed a straight shot out the back of the transom off the risers,+ drilled a couple of Big Holes,+ clamped the big SSteel pipes onto the riser collars,+ cut the Tips off at 45?,.....
Ayuh, She Looked Cool,...

After the 1st day of Trolling for 10/ 12hrs,...... No,.. They Ain't Cool at All.....
Especially behind a 4.3l,..
On Plane,.. It sounds like an Airplane in the distance, only Closer,+ Louder..... A Cessna 152 or something,...
Sooo,.... Plan 1 to Quiet the Racket Down, was to weld in some baffles,.....
This is looking down the pipes from the riser end,.....

That Didn't Work,............ At All....

Plan 2 was to get the Tips of the pipes, Just underwater,.... Maybe venting'em slightly on the underside of the Bend....
The drawback to plan 2 is that the only SSteel pipe I had left was the drop pieces of the 45? cuts,...
2 45? cuts welded back together are straight or 90?,.... Which is Really Ugly,.... But it's all I had to work with.....
Soo,... Plan 2 worked Much Better than Plan 1,... Although it was but Fugin Ugly..... And the vent holes were totally unneeded,+ sprayed water All over the transom area....They got filled... The Tips were about 3" underwater at idle, at the dock,... Dead QUIET..... Til you started to move,.... Trolling it would get Quiet,+ Then Noisey,+ then Quite,+ Then Noisey... For Hours on end....
Cooler, but still not Cool....

At Plan 15somethin',.... It's still atad Ugly,..... But Nice,+ Quite now, up to about 8 or 10mph,.... Then she'll start to thunder,.... Conversations in the cockpit at cruise,+ above are in Loud Voices,....
But Trolling,..... She's Dead Quiet......... Unless of course the wind is Howling, then there's the occasional Bellow from the pipes.........
I got some more SSteel,+ recut the angles on them, leaving them Long to experiment with the lenghts.... That was a few years ago,... Never got around to shortening 'em up,....

On the next Barge Refit,.... I'm moving the motor forward,+ bringing the pipes out Low at near the bottom of the hull I think....
Still open pipe Right off the risers though.....

 

[wca_tim]

Re: Thru Hull Exhaust

Bond-O... thanks for sharing that...

Aside from being "fun", sometimes I wonder if having so many things in a state of constant reengineering is some kind of an addiction or disorder...

 

[Bondo]

Re: Thru Hull Exhaust

sometimes I wonder if having so many things in a state of constant reengineering is some kind of an addiction or disorder...

Ayuh,.... Whatever you call it,....... I've Got it.......

 

[AZSenza]

Re: Thru Hull Exhaust

Ayuh,.... Whatever you call it,....... I've Got it.......

LOL MEEE TOOO

on my old boat (85 4Winns with the 350) I was able to get them low enough to exit below the waterline (Quiet) when trolling in and out of no wake zones but once up on plane they were making their music. LOTS of enviable stares and "WHAT HAVE YOU GOT IN THAT THING" She was fast and handled like a gace car with the trim tabs. I cant wait to get going on our new boat...

 

[45Auto]

Re: Thru Hull Exhaust

If your motor is running right, your EGT (exhaust gas temperature) as it exits the cylinder is already at about the max temp. As QC said, you get max temp from a stoichiometric (exactly enough molecules of air and gasoline to react with each other with nothing left over) mixture, which is an air/fuel ratio of about 14.7 to 1 in a gasoline engine.

Skip the rest of this paragraph unless you really care about mixture ratios and how they affect EGT . Lean mixtures produce cooler combustion gases than does a stoichiometric mixture, primarily due to the excessive dilution by unconsumed oxygen and its associated nitrogen. Rich mixtures also produce cooler combustion gases than does a stoichiometric mixture, primarily due to the excessive amount of carbon which oxidises to form carbon monoxide, rather than carbon dioxide. The chemical reaction oxidizing carbon to form carbon monoxide releases significantly less heat than the similar reaction to form carbon dioxide. (Carbon monoxide retains significant potential chemical energy. It is itself a fuel whereas carbon dioxide is not.) Lean mixtures, when consumed in an internal combustion engine, produce less power than does the stoichiometric mixture. Similarly, rich mixtures return poorer fuel efficiency than the stoichiometric mixture. (The mixture for the best fuel efficiency is slightly different than the stoichiometric mixture.)

If you had an EGT guage and a CHT (cylinder head temp) guage (like piston engined airplanes and helicopters do, cause they are VERY concerned if their motor quits!) you can tell when your engine is running rich or lean because the EGT would be low. Best we have on most boats is a coolant temp guage. A cylinder running lean and hot will usually not show up on a coolant temp guage until AFTER it trashes the cylinder, since the heat is dispersed into the cooling water.

"Wait a minute," you say. "I know that when I lean the engine out it just keeps getting hotter until it welds itself together!" If all you have to go by is CHT or coolant temp you're absolutely right. When your engine gets too lean, the skyrocketing temperature you would see on the CHT (but won't see till too late on a coolant temp guage) is probably not really an indication of hotter combustion. Most likely it's a warning sign of DETONATION. Detonation is the collision of two flame fronts inside the combustion chamber, where there should be just one, and it's the single biggest cause of heat related engine failures. If you see CHT going UP and EGT going DOWN, it's a sure sign of detonation and a mixture too lean for conditions.

Injecting water into the exhaust definitely cools it (try running rubber exhaust hoses on your car) and condenses it. Not real sure of the numbers on how it works out with the lower volume due to the cooling versus the increased density due to the water affecting the required exhaust area.

As WCA_TIM and QC point out, and seems to be backed up by the Mercury factory advice, anytime you exceed around 350 HP (big block or small block) you will probably benefit performance wise from straight exhaust. How much? Maybe 20 or 30 HP max would be my guess (assuming we're not talking 600 CI supercharged stuff here). Since it takes about 10 HP to gain 1 MPH once you're over 50 MPH, you'll have to decide if the increased noise is worth the 2 or 3 MPH speed gain.

My boat has the "captain's choice" switchable exhaust on a Mercruiser 496 HO (425 HP). Call me immature, but I really enjoy having the "rumble" sometimes ! I also enjoy being able to talk in normal tones or hear the stereo when I want to! I just put a Smartcraft GPS tach and speedo on it, which ties into the engine computer and gives real time fuel flow data as well as all the other sensor readings. It'll be interesting to see how much difference the exhaust makes on performance and economy in the next few weeks.

Bond-O, your exhaust is definitely .... uh ..... effective! I've seen a few creative straight exhaust modifications due to noise checks. The Home Depot exhaust department seems very popular!!

 

[NeverEnough02]

Re: Thru Hull Exhaust

Some ricers have super large dual pipes for a reason...Turbos. You want zero back pressure after a turbo. The pressure trys to spool the turbo the wrong way. In most cases there is no turbo and you are dealing with someone who is just into the look. What we have been saying is, if you plan on increasing your back pressure with engine mods, you will need a thru hull to make the most out of your final tuning. Otherwise, they are just there for looks and sound. IMO thats good enough!

 

[Flinging-Arrow]

Re: Thru Hull Exhaust

True, the V6's, & V8's with the late model center riser manifolds are the only ones this can be done with. My OMC V6's had those odd "batwing" style manifolds as orig. equip. I had to replace one, it was a b*tch to find! and not cheap.

I have a cracked y-pipe on a 1993 4.3L vortec. I got the motor pulled and was in process putting thru the hull. It's not about the noise, or hp.... but the cost of a new y-pipe. I'm shopping for parts, but the comments above scared me a bit.

Is 3 in good enough and is it possible. I'm worried about finding a elbow that works?

Regarding the noise... I'm not concerned, its only a V6 and I'll be under the water line. Also how in the world would this decrease performance. Idiot terms please.

Thanks in advance for your feedback!

 

[45Auto]

Re: Thru Hull Exhaust

Also how in the world would this decrease performance.

Quick simple answer:

The propellor creates a low pressure area behind it as it moves through the water. With thru-prop exhaust this low pressure is like a vacuum cleaner sucking the exhaust products out of the cylinders. The lower pressure in the cylinders also sucks in more fresh mixture when the intake valve opens.

This works as long as your engine is not moving enough air volume so that the size of the exhaust passage in the outdrive becomes a restriction. You won't have that problem with your 6 cylinder, only place you'll see that is in modified 350's or big blocks making more than 350 horsepower or so.

 

[Mkos1980]

Re: Thru Hull Exhaust

I finally got all my stuff I need. Should be installed within a month. I'll post before and after performance. Prob none but it'll sound a little meaner our on Lake Erie Sure looks purdy though!!

 

[Don S]

Re: Thru Hull Exhaust

Flinging Arrow and Mkos1980.
Best to start your own threads with questions or status of your own projects.
This isn't a thread to cover all exhaust problems, only samm's question.

 

[fishingman220]

Re: Thru Hull Exhaust

As already posted, the main increase is noise. Many places have restrictions on that, so check your state and boating area. As far as hp gain, it will only be minor in a SB V8 (10hp?) and not much more in a BB V8.

As for how hard it will be to do it, it is going to depend on your engine compartment access and transom. Many people have done this DIY.

If you would prefer some additional noise, only on plane, consider changing to an exhaust tube bellows.

what is a bellow and how much louder is it? is it hard to put on?