Thru hulls vs thru leg back pressure test

[MikDee]

Re: Thru hulls vs thru leg back pressure test

On second thought, I suppose removing the bellows, & putting a rubber exhaust snout (spout) on the outdrive plate instead would prevent any vacuum buildup

 

[havasuboatman]

Re: Thru hulls vs thru leg back pressure test

Chances are all you will accomplish is filling your gauge with water.
We build alot of boats here in Havasu, (there are six manufacturers here) I don't have any numbers for small blocks, (we just don't get many requests for them) But the BB Merc's get between 5 and 7 HP with the installation of thru hull exhaust. IF the exhaust bellows is removed.

If you re using an exhaust diverter to run your test differences, your results are already comprimised since the diverter doesn't completely seal off either passage.

 

[180shabah]

Re: Thru hulls vs thru leg back pressure test

But the BB Merc's get between 5 and 7 HP with the installation of thru hull exhaust. IF the exhaust bellows is removed.

So esentially nothing....

 

[havasuboatman]

Re: Thru hulls vs thru leg back pressure test

Well, 5 extra horsepower can make a big differance in how long it takes to plane, and if you ask anybody who has to plow before reaching a plane, they will say it makes all the difference in the world.

 

[haulnazz15]

Re: Thru hulls vs thru leg back pressure test

Well, 5 extra horsepower can make a big differance in how long it takes to plane, and if you ask anybody who has to plow before reaching a plane, they will say it makes all the difference in the world.

I seriously doubt anyone can tell a difference in 5HP, be it in a boat or a car.

 

[abj87]

Re: Thru hulls vs thru leg back pressure test

Ha, now that's something I can relate to. My wife usually puts the boat in the water and tends to give it a bit too much gas at start up (partially due to carb being out of tune last year). I've seen more than one person on the ramp nearly sprain their neck in an attempt to see what is making that sound.

Dont you just love annoying others with obnoxius exhaust....

that does nothing for performance...

 

[MikDee]

Re: Thru hulls vs thru leg back pressure test

I'm sorry, Ever since I can remember I love the rythym of a V8, not that it has to be that loud either. I remember playing in the street in Brooklyn as a kid when I was less then 9yrs old, hearing, then when I became aware, afterwards waiting for an old Ford rack body delivery truck with a flathead V8 and stock low restriction muffler to drive through on a daily basis, just to savor the sound. Besides, I like being able to check, & see easily if my engine is pumping water, & just how much, at all times.

 

[Zackman]

Re: Thru hulls vs thru leg back pressure test

... My 22' Crownline with a 496 HO (425 HP) has Captain's Call switchable exhaust (side exit). It makes absolutely NO difference on the GPS speedometer or fuel flow gauge whether I am running through the outdrive or through the open exhaust.

I've got the same hull with a 350 Mag and thru transom captains call. I've tested it opened and closed numerous times using a gps and never saw any difference.​

IF you like the sound, fine, but don't expect any gains.​

 

[180shabah]

Re: Thru hulls vs thru leg back pressure test

Well, 5 extra horsepower can make a big differance in how long it takes to plane, and if you ask anybody who has to plow before reaching a plane, they will say it makes all the difference in the world.

...Just trying to justify the expense.

 

[abj87]

Re: Thru hulls vs thru leg back pressure test

I'm sorry, Ever since I can remember I love the rythym of a V8, not that it has to be that loud either. I remember playing in the street in Brooklyn as a kid when I was less then 9yrs old, hearing, then when I became aware, afterwards waiting for an old Ford rack body delivery truck with a flathead V8 and stock low restriction muffler to drive through on a daily basis, just to savor the sound. Besides, I like being able to check, & see easily if my engine is pumping water, & just how much, at all times.

Don't get me wrong i love the sound of a v8, but... engine with out mufflers or any real mufflers sound like crap.....

So why don't the rest of you get mufflers for your engines and be a good citizen. Unless you compensating for something...

 

[havasuboatman]

Re: Thru hulls vs thru leg back pressure test

...Just trying to justify the expense.

Whatever...

 

[wca_tim]

Re: Thru hulls vs thru leg back pressure test

how many of you would buy a nice set of aftermarket heads, have them worked and then put a stock manifold and two barrel carb on it? or a good size cam and then crapy stock old style 1.94 / 1.5 heads?

or leave a set of the old style factory cast exhaust manifolds on and run dual 3 inch pipe with a crossover and low resistance mufflers? of big tube headers with a factory dingle tail ppipe, catalytic converter and muffler behind them?

Even before complicating the picture with the water issue, you have to consider that going with stock or near exhaust manifolds and even running them wide open may not really do anything to reduce backpressure or especially flow rate where it counts at the exhaust port on the head...

If it's all about pushing and pulling more fuel and air mixture through that air pump we call an engine, and when you consider the way a carb is designed, if you get more airflow then you are also very likely going to have to fatten up the fuel mixture to see the potential gains anyhow...

I used to play with bikes a little bit as I'm gathering some folks on here do / have... and if I jetted right for the street with the muffler on, and then slipped it off to run just the bare collector, I'd see little or no gain in performance (also make a heck of a racket and probably burn things up in a hurry as well), but on the other hand if I fattened the mixture up by a couple of jet numbers, then it would run a a bit too rich around town, but when the muffler comes off it's a whole different animal...

so one for the closet (backyard?) enginerring types out there...

I've mentioned the drive heat thing a couple of ties and no one seems to say a word about it... I know that heat is a big enemy on overpowerd drives, and that people claim a big difference with drive showers, but why then don't we worry aobut running ALL of the exhaust gasses throughthe outdrive... Surely when runing flat out for a good bit that contributes as much heat as anything to the drive? (unless the water is cool enough that it drops the temp to below oil degredation temps... but wait, we let the water drain on the bottom part... I have a feeling that the heat capacity of the water as well as the heat of vaporization is the reason that it's not a problem... but then again, maybe is isn't/ i've never had the oil temp gauge in my oitdrive when the exhsuat went through the drive...

 

[TilliamWe]

Re: Thru hulls vs thru leg back pressure test

Could you post a link...this should be intresting...I do have a extreme intrest in this subject.

Here you go:

http://www.trailerboats.com/images/elements/2030761_p38-43_TBM309_Mufflers.pdf

For those of you who don't want to wait on this .pdf link to load, it was a muffler shoot-out article. Baja 247 Islander with 350MAG MPI, with Corsa Captain's Call exhaust was the test boat. With the exhaust going through the prop, the boat's top speed was 48.1mph. Open the Captain;s call and it went 49.1. Using different mufflers, it achieved a top speed of 50.6. So open exhaust made a difference on that boat.

 

[havasuboatman]

Re: Thru hulls vs thru leg back pressure test

Here you go:

http://www.trailerboats.com/images/elements/2030761_p38-43_TBM309_Mufflers.pdf

For those of you who don't want to wait on this .pdf link to load, it was a muffler shoot-out article. Baja 247 Islander with 350MAG MPI, with Corsa Captain's Call exhaust was the test boat. With the exhaust going through the prop, the boat's top speed was 48.1mph. Open the Captain;s call and it went 49.1. Using different mufflers, it achieved a top speed of 50.6. So open exhaust made a difference on that boat.

But nobody noticed...

 

[IVAZ]

Re: Thru hulls vs thru leg back pressure test

If you dont like the sound of a V8 you need to check your pulse or testosterone level.
With that said I like to go to the lake to fish and relax. A V8 at WOT kills the tranquility. For the marginal performance gain its not worth ticking everybody off that has to put it with it.
The proper place to hear a thundering engine is at a race track.

 

[MikDee]

Re: Thru hulls vs thru leg back pressure test

Don't get me wrong i love the sound of a v8, but... engine with out mufflers or any real mufflers sound like crap.....

True, It sounds like a tractor! But, with the right kind of muffling or baffling, it can sound more mellow without destroying performance

I wonder if there would be any performance advantage with running both, thru-hull, & thru-prop exhaust, at the same time? Considering that thru-prop gives the extra advantage of vacuum pulling out part of the wet mixture.

Oops! Just noticed the link above answered my question, the captains call system leaves the thru-prop exhaust open all the time, regardless of the setting on the thru-hull exhaust.

 

[cannonford57]

Re: Thru hulls vs thru leg back pressure test

just an ol rednecks 2 cents but would it not have somethimg to do with the ex. being "helped" out of the stream by the negative pressure behind the prop allowing a bunch more good stuff to be put in the cyls. opposed to being forced out into the atmosphere on its own deforming baby seals and such?

 

[bucky7680]

Re: Thru hulls vs thru leg back pressure test

Since I only run my boat wide open for only a small fraction of the time I?m boating, I would like to know the benefits of running an open exhaust at cruising say 3,400 RPM? I seems to me that I could get a 2.5 mph gain cheaper by investing in a well tuned prop.

 

[TilliamWe]

Re: Thru hulls vs thru leg back pressure test

Considering that thru-prop gives the extra advantage of vacuum pulling out part of the wet mixture.

That's the Fallacy that Mercruiser tried to perpetrate for years. That doesn't happen, sorry. At least not enough to outperform open exhaust.

As far as the muffled, versus unmuffled noise:
Yes, unmuffled Chevy 350s are just loud when revved, but...

To me the water in the exhaust of a boat provides the perfect muffling/sound enhancement.

 

[n2ostroker]

Re: Thru hulls vs thru leg back pressure test

My thru hull exhaust exits below the water line. At idle and low speed you can"t hardly hear it. Getting up on plane it starts to come alive and with the water in the exhaust and the 2 90* turns it isn't that loud. I actually think it sounds really good without being too obnoxious. I've never had it run throught the prop so I couldn't tell if there is a performance difference.

I don't know how much backpressure you'll find on the thru prop exhaust. Anything less than a 350 Mag motor I think is just for looks and/or sound. I wouldn't think there would any performance gain. Even on a factory set up of a 496 ho motor I don't think you'd notice any gain until maybe a reprop til you started doing performance mods. A friend of mine running a 275 Baja gets 1 mph hitting the captains call button and he runs an all stock 496ho motor.

I have a Megasquirt computer I used to run on one of my Mustangs that I could hook up to a pressure line from the exhaust if anyone was interested. Instead of a 1psi increments I would be measuring in KPa with a 4 bar map sensor. I wouldn't be able to do a thru prop comparison. I could show a datalog of back pressure and try to plot it against rpm though.