Is a lot of backpreassure nessasery? (above water exaust)

ferrariman

Petty Officer 3rd Class
Joined
Jan 4, 2006
Messages
82
Being a rowdy 14 year old with no regard for the law and an urge to be antisocial i wanted to put an above water exast on my johnson 25 2 cycle, If i ran a length of pipe from the exast port below the water to well above the surface would there not be enough back pressure for it to run properly? Would there be a way to do this and mantain backpressure? Thnx
 

mikeyzx2

Chief Petty Officer
Joined
May 1, 2005
Messages
633
Re: Is a lot of backpreassure nessasery? (above water exaust)

Kids....they all have loud mufflers on their cars, now they want them on their boats! <br /><br />I don't know if that'll work to be honest. I've seen several hydros with the older Mercs, running the modified exhaust plates and stacks that look like bullhorns. Screamin' loud too. But that's strictly for racing. <br /><br />The only backpressure you might experience will be from the cuffs that police slaps on you for violating noise ordinance. <br /><br />But seriously, I don't know if it'll create the backpressure.
 

Paul Moir

Admiral
Joined
Nov 5, 2002
Messages
6,847
Re: Is a lot of backpreassure nessasery? (above water exaust)

There's a regulation in the Canadian small boat construction regs that limits unsilenced exhaust design to racing with the exception of a "captain's call" type system. I don't know whether that only applies to inboards.<br />They do have such devices for outboards out there. They're essentially pluggable holes drilled in the mid-section. Look at Bob's Machine Shop for an example. They make the engine a lot louder, but don't think anyone has really found a performance benefite (in normal use). <br />But an unrestricted exhaust does not a powerful 2-stroke make. For that you really need to be looking at proper tuned pipes with real expansion chambers. You used to see those on old modified outboard race engines sticking straight out the back. But there's a lot of design that goes into makeing them work right, and they're rather irreversible.<br />
noonan_sun-2_325.jpg
<br /><br />Naturally, when you change one thing you'll need to change some others to get advantage of it. Particularly the fuel mixture will need to be corrected, and likely you'll have to adjust the port timing.<br /><br />Try working on the boat. There's a lot to be gained by proper motor setup and lightening your craft. Definitly more for less.
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: Is a lot of backpreassure nessasery? (above water exaust)

Let's throw this tripple-tuned above-water OMC in too!<br />
OMCtripple.jpg
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: Is a lot of backpreassure nessasery? (above water exaust)

This pic gives a great "window" of what Paul is talking about...Notice the size of the expansion chamber compared to the outlet size...
 

Paul Moir

Admiral
Joined
Nov 5, 2002
Messages
6,847
Re: Is a lot of backpreassure nessasery? (above water exaust)

Ah, now that's more like it! :)
 

ferrariman

Petty Officer 3rd Class
Joined
Jan 4, 2006
Messages
82
Re: Is a lot of backpreassure nessasery? (above water exaust)

I am not trying to gain power as i just want it to be loud. i was just wonderingif i put an exaust peice on if it would require more than tweakng for it to run normaly, and thanks for all the imput you guys.
 

paulie0735

Chief Petty Officer
Joined
Dec 6, 2005
Messages
463
Re: Is a lot of backpreassure nessasery? (above water exaust)

No it won’t run, even if it does run, the answer is still No it won’t run.
 

ferrariman

Petty Officer 3rd Class
Joined
Jan 4, 2006
Messages
82
Re: Is a lot of backpreassure nessasery? (above water exaust)

What would make it not run? Now that a know a little about boats i need to know more. Explain if possible.
 

paulie0735

Chief Petty Officer
Joined
Dec 6, 2005
Messages
463
Re: Is a lot of backpreassure nessasery? (above water exaust)

Ok let me put it this way, If you run the engine long enough with a piece of water pipe or similar adapted to the exast (exhaust) than somebody about my size and age is gunna grab you by the neck and shake it a lot! And I can guarantee you, your devise that’s designed to run quietly and efficiently for years and years will never run again! Time to grow up son, 14 is about the age when you should give it a try!
 

tashasdaddy

Honorary Moderator Emeritus
Joined
Nov 11, 2005
Messages
51,019
Re: Is a lot of backpreassure nessasery? (above water exaust)

what are you running this 25 on. glad to see you are interested in boats instead of drugs. i total encourage you in tour persuit of knowledge. it's a great hobby, i started with a 1950 15 hp evinrude and a 12 ft 3/8" plywood boat.
 

paulie0735

Chief Petty Officer
Joined
Dec 6, 2005
Messages
463
Re: Is a lot of backpreassure nessasery? (above water exaust)

Originally posted by ferrariman:<br />[QB] Being a rowdy 14 year old with no regard for the law and an urge to be antisocial.....<br /><br />Am I the only persom here who can see a problem with this??
 

Ilikesalmon

Seaman Apprentice
Joined
Jun 10, 2004
Messages
34
Re: Is a lot of backpreassure nessasery? (above water exaust)

Paulie, you must be the only person that is willing to say anything about this! :p
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Is a lot of backpreassure nessasery? (above water exaust)

Having gone thru this same thing at about that same time in my life I think I understand what he's trying to say. I don't really think he wants to become the next Jessie James but is rather looking to experiment with mechanical things. I’m sure he does indeed have regard for the law or he wouldn’t be asking this on a public forum.<br /><br />To answer your question Ferrariman, yes – it can be done and the motor will run. Perhaps (most likely) not as good as it did but it will run. We see this all the time. But it has to be done correctly. If not, your next project will be learning how to replace pistons. Lets not go there yet.<br /><br />Someone sees a performance or race boat and its loud. They go home and drill holes and now their motor is loud. 99.9% of the time the boat actually slows down. Changing exhaust restriction is only part of the change. We mess with jetting, timing and the way the water exits the motor among other things.<br /><br />I'm glad to see some young blood interested in boating. We need more. Ferrariman, I can tell you for a fact that messing with boats, then race boats and then stock cars kept me out of a lot of trouble that all my buddies got into.<br /><br />Theres a lot of folks here that will help you. But lets start with the basics. Tell us more about what you have and we'll go from there. What year motor do you have? What boat? Whats the baseline on performance now?<br /><br />One more thing -- Look at the pic Paul posted. Then go look at race boats. Whats the one thing they all have in common? Helmets. Or more precise -- ear protection. 2 strokes may sound cool from a distance but let me tell you they are annoying as hell without ear protection when youre right in front of one. Instant headache.<br /><br />Anyway, tell us more about what you have.
 

mikeyzx2

Chief Petty Officer
Joined
May 1, 2005
Messages
633
Re: Is a lot of backpreassure nessasery? (above water exaust)

f-man, I agree with what everyone's said. Those hydros have motors with a tuned exhaust, otherwise they'd run about as good as a two legged dog. you have a lot more to learn about outboards, I'm still learning and have a long way to go myself. It is very costly to rebuild an outboard, and I really hope you put a lot of thought into that. Did you know that simply having the wrong heat sparkplug can cause the motor to fry a piston? That's sums up how sensitive a two stroke outboard is.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Is a lot of backpreassure nessasery? (above water exaust)

On the old Mercs you used to be able to simply remove the exhaust cover on the side of the powerhead and run it after adjusting the mixture.(very important)It sure was loud!!!This was over 45 years ago when I was about 18.You couldn't do it on an Evinrude because water circulated through the cover.Probably can't do it on your motor either.Definately don't run it without adjusting the mixture richer or you may end up with toast.
 

tashasdaddy

Honorary Moderator Emeritus
Joined
Nov 11, 2005
Messages
51,019
Re: Is a lot of backpreassure nessasery? (above water exaust)

apparently he ain't that rude or antisocial, probably smarter than the average 14 Y/O, he has enough sense to ask the help of us old farts.
 

ferrariman

Petty Officer 3rd Class
Joined
Jan 4, 2006
Messages
82
Re: Is a lot of backpreassure nessasery? (above water exaust)

I used to work on dirtbikes alot when i was younger tuning the hell out of them, i managed to make a susuki ds80 beat a kawasaki 125 in a race across the fields at school. So working on my recently aquaired outboard is just a way to be around two cycles. And about haveing no regard for the law and wanting to be antisocial, that was supoosed to be a joke paulie, u know ha ha, he he. And to whomever asked i am running it on a 13 foot alminim v-hull.
 

lark2004

Lieutenant Junior Grade
Joined
Jul 12, 2004
Messages
1,080
Re: Is a lot of backpreassure nessasery? (above water exaust)

What year is the motor? Can you give us the model number? I don't really know why you want a loud outboard, most of us are trying to make them quieter. (is that a real word? if not it should be..)<br /><br />Apparently, if we can't hear the motor, neither can the fish????? :p
 

MCM

Lieutenant Junior Grade
Joined
Jun 1, 2005
Messages
1,201
Re: Is a lot of backpreassure nessasery? (above water exaust)

Maybe someone could come up with a way to put some Glasspacks on it, now that would be coool :cool:
 
Top