1999 Johnson 15hp upgrade

Calibra

Seaman
Joined
Sep 5, 2014
Messages
71
Hi

I am rebuilding a Johnson 15hp engine. I took the block down to a marine place so they can bore it out.
If you look at the picture below you will see two small whole.

They told me I should get these welded up as this would increase horse power. They said as the piston goes down the cylinders it starts to loose compression before the pistons reach the exhaust outlets.

Is this good advise?

Thanks

James

engine.jpg
 

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boobie

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Those holes make the motor idle better. The factory wouldn't have put them in there if they weren't needed.
 

nwcove

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not sure what the shop is thinking, but id guess you may actually see a drop in performance and fuel efficiency if those holes were plugged. would be interesting to know their train of thought tho.
 

Calibra

Seaman
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Sep 5, 2014
Messages
71
They say the cylinder starts loosing compression when the piston gets to those holes as they are higher than the exhaust holes.

By blocking small holes up the compression stays for longer

I will ignore the shop
 
Last edited:

racerone

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Dec 28, 2013
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Let them talk to you and nod your head , But you should ignore them !!!
 

Faztbullet

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They are idle relief holes and reduces compression about 10psi at idle, on V-6 motors we plug em with a allen set screw
 

racerone

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It makes them idle smoother and quieter.---At full throttle these holes essentially do not exist as there is no time for significant leakage through them.
 

Faztbullet

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We plug em to increase compression on low end and improve hole shot.....
 

Calibra

Seaman
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Sep 5, 2014
Messages
71
The more I read about this, the more I see it being applied to v6 engines. I have not seen anything relating to small 15hp 2 stoke engine.
The only down side I can see if it can make the engine a little harder to start.
The good stuff is a stronger pull up the rev range, approx 10psi extra compression and better idling.

Being a small 15hp pull start engine, I wonder how hard it will be to start.

I certainly don't want to weld the holes, so I am wondering if I can tap in some threads and bung in a small screw.
 

bob johnson

Rear Admiral
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Feb 25, 2009
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4,304
you don't want a 15 hp outboard that you use for recreation, and dependablility to act anything like a racing motor. racing motors idle poorly, because they don't putz around the lake leisurely.... they scream...90% of the time they are running..... did you know that years ago racing guys would inject water into the fuel mixture to increase horsepower!!....try adding that to your civic...hahahaha......just because someone does it....doesn't mean it is good for you...CONTEXT..... what is the CONTEXT of the info?? that's what you need to ask yourself when you hear about "great ideas"

bob
 

Calibra

Seaman
Joined
Sep 5, 2014
Messages
71
you don't want a 15 hp outboard that you use for recreation, and dependablility to act anything like a racing motor. racing motors idle poorly, because they don't putz around the lake leisurely.... they scream...90% of the time they are running..... did you know that years ago racing guys would inject water into the fuel mixture to increase horsepower!!....try adding that to your civic...hahahaha......just because someone does it....doesn't mean it is good for you...CONTEXT..... what is the CONTEXT of the info?? that's what you need to ask yourself when you hear about "great ideas"

bob

You make a very good point. I am just going to forget about this modification :)
There is just something in side of me that if I can change something, I like to have a try. Even with my BMW M3, it's running Velocity stacks.

It was interesting reading up about it though :)

Thanks all.
 
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