Merc exhaust

[Moonshot180]

I was just down by the river looking at the boat traffic and I noticed a boat with what looked like a Merc' and that baby was roaring!

What exactly makes the 2.5s sound so good? My promax is quiet and after hearing that guys mota, I want mine to scream too. What do I need to buy to get that sound??

thanks in advance:)

[SnakeBit]

by One - It's The Best:)

[Moonshot180]

I've already done that.:) A 225X promax.:)

I'm just unsure about what aftermarket parts I need to hunt down and buy to get that screamin' sound...:confused:

[Jay Smith]

Most all 2.5 260/280/Drag type engines have some above the water exhaust relief. The "Bass" model 2.5's have 10 1/2 " holes drilled in the down housing letting it "wail" and the OS models relieve exhauxt out the rear snout allowing it to wail and have that OH SO GOOD 2.5 ( small block chevy high pitch schreel )sound.....

good luck,

[John Richied]

Your ProMax is probably a 20” shaft and the exhaust is pretty much closed off and pushes the exhaust out through the gear case. A short shaft (15”mid) or offshore midsection has more of an open exhaust.
You can relieve the exhaust on a 20” midsection, but you want to be careful how you do it… done wrong you can weaken the midsection and this can lead to cracks.
Basically open exhaust and MORE Rpm’s is that SWEET sound you are hearing. :D

[Moonshot180]

Hey thanks for the info Jay. So I need to hunt the buy and sell forums for an OS exhaust? Or drill some holes in the stock midsection? What is the preferred method?

Is putting on an OS exhaust a 'DIY' job? I have no problem paying cash money though if It would be better to take it to a professional.

[Moonshot180]

Your ProMax is probably a 20” shaft and the exhaust is pretty much closed off and pushes the exhaust out through the gear case. A short shaft (15”mid) or offshore midsection has more of an open exhaust.
You can relieve the exhaust on a 20” midsection, but you want to be careful how you do it… done wrong you can weaken the midsection and this can lead to cracks.
Basically open exhaust and MORE Rpm’s is that SWEET sound you are hearing. :D


hmmm...I love a challenge:D ....I'm not sure what to do now:confused:

[Jay Smith]

To beat the excess weight and expense of an OS housing you need to find a "BASS" mid section 20" and feel under the ledge and get an idea of where to drill the holes so that integrity won't be compromised.. You will have to remove the powerhead and the inner lining or can as some refer to it and simply drill 5 1/2" holes on each side of the housing in the bottom ledge..It will take a couple of hours to remove and re install but the drilling should take no more than ten minutes..;)

Good luck,

[Moonshot180]

Thanks for taking the time to help me out guys, :)

The louder the better IMO!:D

[Raceman]

I want to add a little to what Jay said here. My Red/yellow/orange STV had a NON-offshore 260 on it when I bought it. This engine comes from the factory with the mid drilled in the bottom of the fat section. A previous owner had the mid break off through the factory holes, losing his gearcase and prop.

After I bought this boat I had one of all three mid sections on it at one time or the other................ a factory drilled high perf mid, a STOCK mid that was put on in place of the factory drilled one because of fear of another failure, and a 20" offshore mid. In addition, I converted the boat to a 15" Merc mid which of course has a snout similar to an offshore mid.

I could NEVER tell any difference in the top end numbers. It was NOT measurable on a high perf speedo. I'd say that the ONLY difference in any of the 4 besides for the neat looks was the noise.

On SOME boats, like V bottoms especially, tha relieved exhaust will help the boat get on top better, but if you spend the time and money expecting top end gains you're gonna be disappointed.:(

The bottom line is that the passage through the gearcase is adequate to handle the exhaust output of a 2.5 liter Merc. Noise and MAYBE slight acceleration are all you'll see.

As far as "louder the better", unfortunately we have tons of beareacrats (never can spell that word) just lickin' their chops to make NEW restrictions on our hobby. These little boys with big egos jump at ANY excuse to make rules. The more we antagonize people on public waterways that make these people's phone ring, the more we'll pay the price with regulations. If you're plannin' on running your boat in any populated areas I'd think long and hard about opening the exhaust. Hell, I've got more snouted mids than I can count and it makes me mader'n hell when a bass fisherman blows through here at first light with the exhaust relieved at first light, and I've got neighbors a lot grumpier than I am and they DO call the DNR and bitch.

[Moonshot180]

I want to add a little to what Jay said here. My Red/yellow/orange STV had a NON-offshore 260 on it when I bought it. This engine comes from the factory with the mid drilled in the bottom of the fat section. A previous owner had the mid break off through the factory holes, losing his gearcase and prop.

:eek: :eek: :eek:


I'm pretty happy with the Venture now with speed, 78gps @6400. If I don't see any speed gain I won't be disappointed, just as long as it's ridiculously loud.

I hope I do this right, having my gearcase and mid break off and settling at the bottom of 'chick' lake would make me.. VERY SAD!:(

[Jay Smith]

I wouldn't worry about the gearcase breaking off , I ran a stock 20" BASS mid section with the 10 1/2" holes drilled from the factory as I decribed for 3 years in Modified Production winning High Points championships and World Titles so I HAD a harder hitting 2.5 than most and NEVER had issues with ANY fatigue and or cracks and belive me I punished it ..

IMO Most stock from the factory or an exact duplicate of OEM parts if it breaks its a freak thing and FAR from the norm.



Good luck,

[Raceman]

I know of 3 that broke off. One on this lake (relatively smooth water), and found out about 2 others chattin' about this particular one. All three were Merc drilled high perf mids.

Jay it may very well be that you don't log the hours that the average lake boater does, even though there's no questioning your success with yours, or the stress put on it, albeit for relatively short periods of time.

If I just HAD TO have the noise, it'd be with a Merc factory offshore mid.

[Jay Smith]

Agreed Norris......;)

[roostertailn826]

this is how you make it really loud like piss off the other boaters loud.

[Raceman]

Exhaust chest work in itself isn't going to make big differences in noise. It's what happens BELOW the powerhead that dictates the sound the most.;)

[1BadAction]

as far as people regulating our stuff, its going to happen anyways so you might as well piss in as many peoples Wheaties as you can. then when the regulations come, just keep doing it. F them. Some GD ignorant dork can blast that idiotic rap at all hours of the night, I'm gonna have my loud trucks and boats no matter if "they" like it or not.

Luke, I pm'd you. :eek: :D

[Talon2.5]

Some GD ignorant dork can blast that idiotic rap at all hours of the night, I'm gonna have my loud trucks and boats no matter if "they" like it or not


WORD!! Damnnnn Right

[T-REX]

I'm wit U boyz on tha noise...I like mine!!!

But, My brother got run off a new lake they juss opened sumwher in north La. and iz Fed. resurve.....NO OPEN EXHAUST......If U have a 280 wit open exhaust, U will NOT fish on this lake!!!....

Az Raceman say'z...Tha burikratik idiots are slowly faze'in out life az we know it!!!....

By tha way, I had a mid break thru tha 10 holes, and Nicks 260 broke, and damm neer lost tha geercase!!!!.....Anythang will break, but tha more holes tha better tha chance!!

[Action Dave]

I had to relieve the exhaust on my OMC to spin an over-hub prop. I just drilled some holes through the gearcase above the cavitation plate. It's nice because it idles quietly but once it comes out of the water, it screams. I'm not sure how true it is but I have been told that it is illegal to alter the exhaust on an outboard motor newer than 1998 though. I have never had an issue but my cousin got a ticket for relieved exhaust before. This was in the Florida keys on the ocean side of Key Largo too. Not near any residences.

[SatisfAction]

My exhaust is relieved above water and is LOUD :) I like it and so does everyone else that turns their head to stare, take pictures and ask silly questions like "Hey man, how fast'that go"? :D

[Cleavor]

Every time I have been in the Keys I have had a problem with the Water Patrol or Customs or the DEA. They stopped me and checked everything, basically making me prove my innocence multiple times every trip. I finally got so fed up with their Nazi tactics, I quit going down there.

[Action Dave]

Every time I have been in the Keys I have had a problem with the Water Patrol or Customs or the DEA. They stopped me and checked everything, basically making me prove my innocence multiple times every trip. I finally got so fed up with their Nazi tactics, I quit going down there.


Really? I'm in the northern keys at least once a month and the only time I was ever stopped by marine patrol was when my boat had no registration numbers on it. They've actually gotten better at harassing the jet-ski duche-bags.

[Riverratt]

Did you guys have to richen up the jets when you relieved the exhaust? My V8 needs the exhaust relieved so I can get out of the hole but I don't want to lean it out and I really don't want the noise either but I need to get the exhaust away from the prop. I am afraid to remove the block off plate. I am thinking of trying to make a block off plate for my offshore V8 mid and just drilling a couple of small hole as apposed to opening the entire opening.

[Cleavor]

I was always in Marathon or lower keys, stopped around Marathon while fishing or out in the ocean coming from / going to Looe Key Reef.

[Action Dave]

I am thinking of trying to make a block off plate for my offshore V8 mid and just drilling a couple of small hole as apposed to opening the entire opening.


A friend of mine did something similar with his V8. It's still loud as hell though.

[Riverratt]

A friend of mine did something similar with his V8. It's still loud as hell though.
Thats what is keeping me from removing the plate. I don't want the noise at idle and I don't want to lean it out but I know the holeshot would be better if I get rid of some exhaust. I don't know what to do.

[rock]

I have OS mids on both boats and I do love the sound. I have been asked many times why my boat is as loud as it is. I tell em it comes from the factory this way. Drilling a standard mid is common and will definitely give you the noise you want. Correct info is already posted on how to get the job done. I have a 1/2" bit I welded onto the end of a peice of 1/2" rod and the project is a piece of cake. Don't make the rod any longer than it need's to be to reach the bottom of the housing. Drill low speeds unless you are a better welder than I and can make it very, very straight. If that ain't enough you could always try this.http://www.youtube.com/watch?v=1DhH3feoLXo
Rock