Thru Hull Exhaust

[MikDee]

Re: Thru Hull Exhaust

I've been in lots of boats with switchable exhaust... there's no speed advantage with thru-hulls over thru-hub... it's all for the sound. We've done extensive 'testing' on the topic
Kick this one around...
The thru-hull is open exhaust... and therefor less back pressure of course. At speed though the drive pushes a hole in the water creating a vacuum to 'suck' the exhaust out thru-hub.

Yes, This is a fact, but check this, I had a mechanic at the boatyard yrs ago tell me that a boat he worked on lost speed with thru-hull exhaust because they blocked the Y-pipe hole, and it created a vacuum, slowing the boat down some? Go figure?

But it sure is easy to see if you're pumping water.

 

[wca_tim]

Re: Thru Hull Exhaust

I've been reading this thread with a lot of interest considering I'm going to decide in the next few weeks whether to go with through prop, through hull or both with diverters... (definately some muffler on any through hull)

For the notes on performance saying no difference, I understand that stock or similar manifolds are the bottleneck if they're used, that without good aftermarket performance manifolds or tuned headers there is absolutely no performance reason to open up anything past that point. How many guys that see no gain are running factory manifolds, and for those running performance manifolds or headers, did you tweak the jetting on the carb or injection system to make up for any increase in airflow? Note that I'm not necessarily arguing that there will be a gain for a small block even with big tube headers and meticulous tuning, I'm curious at what point the exhaust path through the y-pipe and outdrive becomes the limiting factor? As an aside, I did notice a slight difference in plug readings after switching from exhaust bellows to an exhaust tube that didn't seem to be attributable to anything else.

My other question / issue is relative to the alpha drive. A local performance marine engine builder told me that heat transfered to the gear case by the hot exhaust passing through the drive was an important consideration. He told me that the heat can have a significant effect on the life of an alpha drive behind a relatively hot engine, especially when making long runs at open throttle, and that it had more of an effect on drive temperature than most people realized.

Note that where I boat the vast majority of the time, it's common for the crew from Fountain to be out test running boats... or one of the locals to be running theirs... NO amount of noise that I can make with my little toy is going to stand out...

 

[wire2]

Re: Thru Hull Exhaust

I'll add my 2?,
I added a switchable Y to my double wall stainless headers;

As already stated, the performance difference between open thru hull and the vacuum of a thru prop is negligible. It's strictly the "hot rod" aspect. It does start easier with the butterflies open, even partially, with little back pressure.
I do get a "novelty" thrill from the echo while passing under a concrete bridge, either at idle or at speed. I built the headers to mix the cooling water with exhaust halfway down the 3.5" tubes, which takes away the harsh "bark" of the exhaust. I frequently have locals urge me to open them and give the throttle a jab.
Bottom line; it's a toy.

 

[QC]

Re: Thru Hull Exhaust

I agree with you tim, the manifolds matter. FWIW, I have no idea why small blocks would not respond the same as a big block or even a micro block with the understanding that a 10% gain from 260 bhp is 26 and a 10% gain from a 385 is 38 bhp, so it is obvious to me that you will get more net horsepower from any percentage gain from the biggest starting point . . . Either way, performance gains are expensive, but not completely limited by displacement. Remember, our Nascar boys are squeezing 800 out of a carbed NA small block . . .

 

[John_S]

Re: Thru Hull Exhaust

wca,

Search the net for modest SBC build-ups. There are a number where they make a change and dyno after each. What you will find is that the scavenging effects of headers is just starting in the 4-5K rpm range and show the most gain in 5-6K. Large tube will push that band higher.

The stock cam in our marine engines have been designed to help compensate for the restrictive exhaust. The exhaust valve has a longer duration and higher lift than intake.

Headers plus open exhaust will provide only a little improvement over stock if you are staying below 5K. To get the most effect, engine needs to be built as overall pkg to perform best in a certain rpm range.

SS water cooled headers are expensive, and I have seen allot of reports of cracking over time.

 

[Bondo]

Re: Thru Hull Exhaust

I had a mechanic at the boatyard yrs ago tell me that a boat he worked on lost speed with thru-hull exhaust because they blocked the Y-pipe hole, and it created a vacuum, slowing the boat down some?

Ayuh,........ And You Believe That,..??..??.....

I Don't,........ Pure Bull$h!t in My book..............

Remember, our Nascar boys are squeezing 800 out of a carbed NA small block . . .

Ayuh,..... At What RPMs,..... 8000,..??.. 10,000rpms,..??..

Remember,...... Horsepower is a Mathamatical # derived from Torque,+ RPMs.....
At the 5000rpms that We're limited to,... I doubt they'd pull much more than 300/400hp........
And,......
That's probably a $50,000. motor......... Or More.......

 

[magster65]

Re: Thru Hull Exhaust

Ya' gotta' love thru-hulls! The video of this guy warming up his u-joints sounds great.
video

 

[QC]

Re: Thru Hull Exhaust

Ayuh,..... At What RPMs,..... 8000,..??.. 10,000rpms,..??..

Remember,...... Horsepower is a Mathamatical # derived from Torque,+ RPMs.....
At the 5000rpms that We're limited to,... I doubt they'd pull much more than 300/400hp........
And,......
That's probably a $50,000. motor......... Or More.......

Totally agree, just pointing out that a small block doesn't have some sort of physics wall that limits it as opposed to others. I am NOT talking about displacement. I just don't understand the mind set that says "it can make a difference with a big block, but forget the small block" . . .

Yup, hp = torque x RPM / 5252 and leads to why I constantly harp on the "torque is all that matters" discussion. Uh, no . . . BTW, we are not limited to 5000 RPM, our cams and valve train are . . .

 

[Bondo]

Re: Thru Hull Exhaust

BTW, we are not limited to 5000 RPM, our cams and valve train are . . .

Ayuh,......

Along with the Outdrives,.... Gear Ratios,.... Prop Pitches,...
And Prop Efficiencies.......

No Arguements,.....
Just trying to keep it within the parameters of the boats Most of Us have.......

 

[mercrewser]

Re: Thru Hull Exhaust

You can not enjoy the advantages of a vented propeller with open exhaust. Faster/easier holeshot.

 

[MikDee]

Re: Thru Hull Exhaust

Ayuh,........ And You Believe That,..??..??.....

I Don't,........ Pure Bull$h!t in My book..............

(Well I'm finally back online, after 6hrs without power, light, water, or heat, from 7am to 1pm, and after a small flood in the downstairs, due to a light snowstorm, followed by a sleet storm. )

Bond-O, I don't know what to believe, but it does sound logical that if vacuum at the thru-prop, can suck out the exhaust, then the law of physics would also create a vacuum if the Y-pipe were blocked, No? just my opinion Not that it even matters to me at this time.

 

[QC]

Re: Thru Hull Exhaust

Just trying to keep it within the parameters of the boats Most of Us have.......

Agree once again. With that said, you need to visit Lake Havasu someday, bring your earplugs

 

[MikDee]

Re: Thru Hull Exhaust

Agree once again. With that said, you need to visit Lake Havasu someday, bring your earplugs

LOL QC, I bet you could say the same for lake Norman in NC, where all our NASCAR boys are located, or so I've heard

 

[John_S]

Re: Thru Hull Exhaust

"(Well I'm finally back online, after 6hrs without power, light, water, or heat, from 7am to 1pm, and after a small flood in the downstairs, due to a light snowstorm, followed by a sleet storm. )"

Sorry to hear that MikDee. I hope the damage is minor and the clean-up goes quick. I wish I could say that it never happened to me, and couldn't relate.

 

[Bondo]

Re: Thru Hull Exhaust

Agree once again. With that said, you need to visit Lake Havasu someday, bring your earplugs

Ayuh,........

Probably Alot like up on the St. Lawrence River during the bi-annual Poker Runs,.......
Only Every Single Day.................

Bond-O, I don't know what to believe, but it does sound logical that if vacuum at the thru-prop, can suck out the exhaust, then the law of physics would also create a vacuum if the Y-pipe were blocked, No?

Ayuh,........

Except the exhaust cavity is still Wide Open from the bellows back,+ the 1s I see don't have a bellows....
So,......
There's No restrictions........Plenty of Air to fill that low pressure area........

 

[Coors]

Re: Thru Hull Exhaust

Ya' gotta' love thru-hulls! The video of this guy warming up his u-joints sounds great.
video

Think he needs to lower the bow, in order to lower the drive?
Or he ain't read the book.

 

[Bondo]

Re: Thru Hull Exhaust

Think he needs to lower the bow, in order to lower the drive?
Or he ain't read the book.

Ayuh,.......

That's 'ell on the U-joints..........

 

[wca_tim]

Re: Thru Hull Exhaust

The vast majority of through drive props are designed to create a low pressure zone / bubble directly behind the prop, whether that costs much of anything or not in terms of drag, I would doubt with the bellows gone (circa Bonda), bye the same token, there might be be gains to be had by running a different style prop all together if you weren't using through hull at all.

mercrewser's point about the vented prop is a really good one - especially if you're running a hotter than stock cam and need the slip to get the engine to the bottom of the power band... I know that I get just as good a hole shot with my vented 26" turbo as I do with a non-vented 23, and a few more mph on the top end with the 26.

imported_John_S: point(s) taken. In many cases, if other modifications aren't made - even simple ones like jetting, and so forth, when you open up the exhaust, you'll lose a little torque - especially on the lower end of the rpm range - isn't it really more a matter of how well matched all of the associated copmponents are and what the whole package is designed to do? I'm pretty much decided that I'm going to go with good manifolds and risers (light, dry joint, and water injection a little further back than factory set-up to minimize chances of inversion) and stick with the y-pipe and through prop for the time being.

The bottom line (for me) is just as several others have mentioned - it's a toy! Personally I get almost as much enjoyment from taking things apart, putting them back together, trying to get the most out of what's there and seeing what I can learn or figure out in the process as I do being on the water. Does it really matter how fast my boat, bike, car, bicycle, etc... will go? (in an appropriate setting of course...) especially within the range of what I can reasonably afford without spending the kiddo's college money? Nope. but it sure is a lot of fun to play with - especially in the winter when I don't have the time to get out on the water as much and the people I enjoy boating with the most would be too cold to enjoy going along....

 

[Captain Neckbeard]

Re: Thru Hull Exhaust

I am planning on putting a "Silent Choice" setup on my 5.7 Merc Outboard. If I want loud I flip the switch. If It's early or I feel the need to be quiet I switch it back to thru-prop.

It is nice to have choices. =D

 

[NeverEnough02]

Re: Thru Hull Exhaust

Well how old are you? If your in your 20s or 30s then go with the loud pipes! If your older and have grown out of that then stay with the stock set up. Its all about where you want extra power. Are you going to be wakeboarding or tubing a lot? Maybe not a good idea to have through hull. Just figure out what YOU are looking for. I dont think there is a correct answer to the question without knowing what the priorites are for this person. For example, if more HP is the priority, you can get a lot more HP from with less backpressure with the appropriotly match mods than you can with mods and a stock set up. I dont think anyone will doubt that. How far are you willing to go if HP is your main concern? Everyone has different priorities and it seems as though everyone is thinking of their own while answering.