Drilled Exhaust - Real World Results and Weigh-In On Benefits

[Gordon02]

Guys, I want to get some real-world answers about "drilled exhaust". I've seen this done for years and while I know that noise is one downside result, is there a real benefit? I have a 20" Mercury 2.0L V6 that I'm building for myself and a later project. If I'm going to drill the exhaust, I need to do it now before I start painting.

My gut says Yes - I say this because my 2.5L Mercury V6 ProMax SS (short shaft) is the best running/performing outboard that I've ever ran. I've owned this ProMax SS for 20 years now and it's acceleration characteristics are unlike any other outboard I've ran. The transition from idle thru hole-shot has no hesitation. There is no blubbering, no hiccups, no delay that many outboards experience when "clearing" the water from the exhaust cavity. Since the SS has a huge, open hole that vents the exhaust above the cavitation plate and does not require all of it out through the gearcase and the thru-hub system, the engine is free to spin up from idle.

I know the openings wouldn't help WOT operation, but my thoughts are that they can't hurt idle and should aid (as do the reliefs) and they will help to build rpm quicker to clear the exhaust cavity when transitioning from deep in the water to on-plane.

OK, if the answer is "Go", I'll place 6 1/2" holes in the outer leg (3 on each side), but where do I drill the inner exhaust liner (shown below)? Thanks, Gordon

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[Captain75]

Interesting question, hope to see a nice pro/con debate!
On my looper I experience a better off-idle accel the more exh I relieve above the gearcase, despite the fact that the prop vent holes has less and less effect the more exh is relieved...

[tlwjkw]

but where do I drill the inner exhaust liner (shown below)? Thanks, Gordon

442452

you don't..

you throw tha liner ona shelf or in tha trash...

[mrichartz]

Are you doing any other work/mods to motor?


Michael

[Jay Smith]

Post #3 is my suggestion , while powerhead is off you have perfect opportunity to drill 5 1/2” holes at the base of your bass midsection like Mercury sent all the 260 bass mods in the early 90’s , any gains ? It would be minimum IMO but LOTS louder that once done irreversible... Think it out ...

Jay

[FORBESAUTO]

A lot depends on motor and it’s characteristics. I had a cust call me this morning regarding a Yamaha I just built and modified mid to relieve exhaust. He wanted to know if we could relieve more, because he noticed while taking off and the exhaust reliefs cleared the water the motor would instantly spool up, so in this application it apparently is helping. I’ve also noticed on modified carb 2.5’s, the more exhaust I relieve above water line, the better they idle without loading/puddling fuel. In long idle zones you’d have to periodically bump up accelerator to keep it from loading, much better to all but gone by allowing more exhaust out above water line.

[rock]

Remove the liner but keep it. Drill from the top down like the 260s. All you have to do to reverse it is re-install the liner.

Rock

[Dave Strong]

Remove the liner but keep it. Drill from the top down like the 260s. All you have to do to reverse it is re-install the liner.

Rock

:iagree: But unless you have done major mods to the 2.0L doubt you will see any difference other than more noise.

Dave

[Cervelo777]

I will agree with above comments, my motor does idle much better with it all opened up. (260 with offshore mid, a drag racer cut of the end, and drilled holes every where) Real increased performance numbers will probably not be quantifiable in my opinion, unless other mods are made to take advantage of added breathing in the mid and top range. If it makes you feel better that the motor idles better, and you like the sound, then I could see how it would be fun, but that is about it. Most of my friends think I am stupid for taking the little step in performance, but I kinda like it :)

[Gordon02]

Guys, you've confirmed It's On! I'll get that 20" stock housing drilled over the weekend.

Thanks for the inputs and advice. I'm not after the noise, but don't mind it since I've gotten used to my 15" ProMax's 4" hole under the housing (I've had that little baffle plate removed for 20 years too). I give that above-water relief the full credit for how quickly this motor spools-up on my Allison. Never a hiccup, cough, or pause - it responds instantly and always immediate. Gordon

[rock]

Earlier you mentioned 3 holes each side. Like Jay said the 260s had 5 each side and Mercury felt it was a good idea.

Rock

[58Evinrude]

Its not a Merc, but this is my solution on my 3 litre OMC's. 3" holesaw, thru the mid and liner.
:reddevil:

[mrichartz]

Its not a Merc, but this is my solution on my 3 litre OMC's. 3" holesaw, thru the mid and liner.
:reddevil:

What year is that motor?

[mrichartz]

Guys, you've confirmed It's On! I'll get that 20" stock housing drilled over the weekend.

Thanks for the inputs and advice. I'm not after the noise, but don't mind it since I've gotten used to my 15" ProMax's 4" hole under the housing (I've had that little baffle plate removed for 20 years too). I give that above-water relief the full credit for how quickly this motor spools-up on my Allison. Never a hiccup, cough, or pause - it responds instantly and always immediate. Gordon

So are/did you do any other work/mods to motor?

[Dave Strong]

Its not a Merc, but this is my solution on my 3 litre OMC's. 3" holesaw, thru the mid and liner.
:reddevil:


Drill holes in a Merc mid like that it will crack, there is a reason the Merc engineer's did the 5 half inch holes each side where they did.;)

Dave

[58Evinrude]

What year is that motor?

93 block on a 92 mid.

[Gordon02]

Quote - So are/did you do any other work/mods to motor?

Yes, well sort of. I want to turn it a little higher and want durability. Performance wise, just mild - I am smoothing the exhaust chest and will match the base of the block to the gaskets where the exhaust goes through the adapter plate.

Top-pinned and balanced pistons, rods balanced and beams polished, metal rod bearing cages, and I've got a BP tuner that I was going to use based on the old Bass&Walleye Boats article, but I've got some advice not to run the BP tuner from a very reputable builder. I've got the nice WMV 1.3" carbs and plan to run velocity stacks. I'm not sure what boat this will be put on, I just need to be busy, so I'm building a 2.0! I'd love to find a nice Glastron GT150/160 or perhaps a classic "Cadillac finned" 18~20 foot runabout from the 60's to rebuild. Gordon

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[Gordon02]

Does anyone have a photo/image of the 5 holes? I'd like to get an ideal of spacing and location.

Asking a lot here, I know......

Thanks, Gordon

[mrichartz]

Better to ask alot than to screw something up..
Yes with other work your doing drilling will make a difference more than just noise..

[FORBESAUTO]

Don’t have pics handy. But when you remove the liner and look down into mid, the holes are drilled on each side of sealing lip in floor for liner, evenly spaced from front to back, 5 on each side. I use a long drill bit with an extension on it and drill downward from inside. You can make you a little template to lay on each side from wood or something to make it easy to keep em nice and straight.

[FORBESAUTO]

Which mid do you have, the newer fishin style with rubber hose on above exhaust water relief or other style with reliefs in the slots? You can get trick tuning the boot style and not be noticeable externally. When you remove the liner, you’re already deleting one the loops/baffle the exhaust has to go through to reach the upper relief, so it’s gonna help idle. If you still need more relief at idle to help loading up, remove the boot from mid and there is a second wall behind opening that’s part of a channel that forces exhaust to make a loop through adapter plate before exiting. You can drill a hole in that wall to allow exhaust straight out to opening, you a can adjust the size of the hole depending on how mean of an air pump you have to lower back pressure enough to help prevent fuel puddling/ loading up:thumbsup:

[Gordon02]

The block/carbs are a 1996 powerhead replacement. Everything else is a 1989 SeaRay 135. I'm going to paint and decal it in Mariner colors - leaving all numbers off of it. I like clearing over the decals once installed using only urethane.

This is the mid section with the exhaust tube removed.

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[tux974]

What has also helped in idle once liner is removed even on the O/S mids is drilling a couple of 3/8" holes in the rectangular slots under the adapter plate.

[tlwjkw]

http://www.bbcboards.net/showthread.php?t=20163

[big tom]

So if performance gain is little is there a happy medium?. Like just pulling the liner out?. Don't really want the noise from the twins like I have had with dry pipes on big blocks before.

[Rigaud]

The benefit from exhaust releif is hole shot no H.P. gains.

[big tom]

The benefit from exhaust releif is hole shot no H.P. gains.


What i meant by happy medium was noise level. With twins and motors hanging on back unlike I/O's with exhaust coming out under swim platform. Theres usually people in back seat 6 feet away.

[FORBESAUTO]

No real world test, but I would think removing just the can would help idle and just off idle acceleration performance by removing one loop/baffle the exhaust has to make before reaching above water relief and having a larger area. It will change the tone and be just a tad louder at idle because of this, but no where near obnoxious. Shouldn’t be much difference up and running in noise, but will be a little louder due to exhaust coming out of water drain in lower.

[Rigaud]

What i meant by happy medium was noise level. With twins and motors hanging on back unlike I/O's with exhaust coming out under swim platform. Theres usually people in back seat 6 feet away.
Yes a lot noisier on the back seat, I`m mostly with one passenger along side.

[Rigaud]

No real world test, but I would think removing just the can would help idle and just off idle acceleration performance by removing one loop/baffle the exhaust has to make before reaching above water relief and having a larger area. It will change the tone and be just a tad louder at idle because of this, but no where near obnoxious. Shouldn’t be much difference up and running in noise, but will be a little louder due to exhaust coming out of water drain in lower.
I started with the can removed, no improvement on hole shot. I opened up the mid on my OMC similar to Mercs, 2 holes drilled through 1-1/2" each. Loud yes but the hole shot is awesome. so much that I no longer use a hydraulic jack. Fixed jack.
1987 Valero, 1987 Johnson GT, setback 5.5" plus the built in setback 7". Coned lower LWP, D.A.H. chopper 27p heavy cupping and I'm 1-1/4" above pad. I get up on step easy 2 people and 4 I play with the throttle to get the sweet spot no problem.

That's the reason for the exhaust relief, relieve the back pressure in the water with quick throttle response to get up on the pad instantly. For me.

https://www.instagram.com/p/Bu4uWwxlLz1/?utm_source=ig_web_copy_link

[FORBESAUTO]

Good info, I’ve never tried just can, always can and drilled. Anytime you lower back pressure, It’s gonna help. The amount of relief is dependent on how well the engine breathes. The healthier the engine the more it will benefit

[Dave Strong]

One side effect of pulling the liner and drilling the mid is the water pump is no longer protected from the heat and exhaust gasses. Worth the slight if any real gains? This mod is a lot like flipping the air cleaner lid in the old days, sounds like it more power but actually is not. I'm stuck with no liner and drilled mid on my Bridgeport but motor was designed to run that way. My Pro Max runs great with the liner and stock mid, in fact it out performs the previous 2.5 245hp carb motor. Nice and quiet with great fuel economy, might gain a tiny bit by modding it, but not worth the side effects to me.
To each his own.

Dave

[Rigaud]

One side effect of pulling the liner and drilling the mid is the water pump is no longer protected from the heat and exhaust gasses. Worth the slight if any real gains? This mod is a lot like flipping the air cleaner lid in the old days, sounds like it more power but actually is not. I'm stuck with no liner and drilled mid on my Bridgeport but motor was designed to run that way. My Pro Max runs great with the liner and stock mid, in fact it out performs the previous 2.5 245hp carb motor. Nice and quiet with great fuel economy, might gain a tiny bit by modding it, but not worth the side effects to me.
To each his own.

Dave
Yes it does effect the water pump and will slowly melt the impeller housing. I made a SS deflector. Zero issues

[David - WI]

I have a couple drilled mids that came with drilled cans... bought them from someone on here but never ran them because all the bushings were shot and the pins broken. :rolleyes:
But, somebody was running them that way (drilled mids/drilled cans).

[tux974]

Never heard, felt or seen any power gain from drilling. It just sounds cool if you like the sound of a 2 stroke singing. Maybe having some noise and vibration makes things feel faster.....:D I like them drilled in the correct location....:reddevil:
The 245, 260 and 280 came with drilled cans and no liner.
Liners where mostly for long idling fishing setups.

[tlwjkw]

Liners where mostly for long idling fishing setups.

and old, irritable f###'s like myself....

[tux974]

and old, irritable f###'s like myself....

:D....I know speed and noise it's still in the blood buddy.....;)
Heck the louder they are the faster they run.....:D

[tlwjkw]

:D....I know speed and noise it's still in the blood buddy.....;)
Heck the louder they are the faster they run.....:D

it still is ta me also but like it over ina 1000 feet or less... :reddevil:

[TwzteD]

I know pretty much everyone is running two strokes here but how does this concept work with 4 strokes? would it make the engine run lean or anything? just curious would be cool to drill some holes in my 60 merc formula race, but i do like the quietness at the same time.

[Gordon02]

This is exactly why I love this place! These are the very reasons for posting/asking the questions...many of us can take advantage of your years of experience.

So, my plan right now is to drill the mid-section as soon as my 12" drill bit extension is delivered. I did make a drilling template from hard oak as one of you suggested (see image). This is symmetrical and just flips for the other side.

I worked on the exhaust chest late Saturday and plan on stopping with what you see - I have rounded the bottoms of the 6 outlets that should help gases flow downward towards the adapter and mid-section. I have also smoothed and widened the channel by some 20%. This is going to be a family motor that I want to install on a classic runabout. I like the idea of being able to turn high 6's if I want with a small prop that delivers awesome holeshots (holeshot is far more important than speed for this application). These mods will be used along with the top-pinned, balanced pistons and balanced rods.

Thanks for all the inputs!! Gordon

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[rock]

Can't believe you made a template for drilling. Very impressive but you may need professional help.

Rock

[Dave Strong]

Can't believe you made a template for drilling. Very impressive but you may need professional help.

Rock

Make sure you put a block of wood where the drill bit exits, sometimes the bit will catch on the way out and make a mark where you don't want it.
Don't ask me how I know. Lol

Dave

[Gordon02]

[QUOTE]One side effect of pulling the liner and drilling the mid is the water pump is no longer protected from the heat and exhaust gasses. Worth the slight if any real gains?

So....I was thinking, hole shot is the gain I'm after with this drilling of the mid. Why not keep the liner and drill holes on the sides just above the new holes in the mid? This would give the exhaust somewhere to go when the lower unit is buried deep in the water and divert the exhaust away from the water pump. The noise level would be somewhere between stock and non-lined, not a bad place. Thoughts?

Rock, I've got lots of rough sawed white oak, so it made the perfect drill guide since I can't get in there with a center punch.. Gordon

442740

[Dave Strong]

[QUOTE]One side effect of pulling the liner and drilling the mid is the water pump is no longer protected from the heat and exhaust gasses. Worth the slight if any real gains?

So....I was thinking, hole shot is the gain I'm after with this drilling of the mid. Why not keep the liner and drill holes on the sides just above the new holes in the mid? This would give the exhaust somewhere to go when the lower unit is buried deep in the water and divert the exhaust away from the water pump. The noise level would be somewhere between stock and non-lined, not a bad place. Thoughts?

Rock, I've got lots of rough sawed white oak, so it made the perfect drill guide since I can't get in there with a center punch.. Gordon

If your holeshot issue is that bad doubt drilling the mid is your issue or cure.

Dave

[rock]

[QUOTE]Rock, I've got lots of rough sawed white oak, so it made the perfect drill guide since I can't get in there with a center punch.. Gordon

442740

White oak is some tough stuff. I am a retired custom cabinet builder and used oak on a daily basis but it was almost always red oak. The one time I remember using white oak I was running it thru the table saw a chunk split off and went thru my bottom lip pinning the bottom lip to the upper lip. Lots of cussing and spitting blood.
Your project is looking good.

Rock

[Gordon02]

What many don't know is that White Oak is far superior to Red Oak in boat building applications. White Oak is the wood that ships have been built of for hundreds of years because it does not wick/absorb water.

This video from a shipwrite shows why - https://www.youtube.com/watch?v=L6t2AZubF8U

I use it to replace the stringers and build pedestal supports in the flooring. The strength is amazing and it bonds extremely well to laminates using US Composite's 3:1 Epoxy. We're still able to buy it "rough sawed" locally which gives me more that 1" of board thickness and widths of 5" to 10" to choose from. The rough attribute gives the epoxy and 1708 bi-axial glass lots of teeth.

[rock]

People would often ask the difference and I would slice an 1/8 inch piece of end grain off each and hold them up to a light. White oak is solid whereas red oak loos like there is more air than wood. Red oak would be useless in a boat.

Rock

[Motv18]

The 60 should be drilled or exit the anodes. Mine bubbles there when the anodes are close to top

[Merc 2.5]

U can take a slice of white and red oak same size , get glass water , blow thru slice of wood into water , red oak will make bubbles, cant blow thru white ,

[carb a rator]

I pulled the bucket out of my 2.4, but didn't drill any holes. On the hose it has a bit more crackle, nothing like open exhaust. On the ramp, same thing a bit more crackle but not much. Hope to take i off the trailer next week. My plan is to drill holes through the cavitation plate in to the can a couple at a time till i get the desired level. Too many years of 260's on offshore downhousing make the stock setup just too quite for me.I figure 4 will probably do it, shall see.