How to make a 25 hp mercury outboard faster.

[Tripp1845]

I have an early 90's 25 hp mercury outboard on a 14' V-hole boat. The motor is quick but not quick enough for my liking. I was woundering what I could do to the motor to make it have a faster top end speed. I have already changed my prop to a stan. 15 pitch. What else can I do.
Thanks

 

[Harker]

Re: How to make a 25 hp mercury outboard faster.

Play with the tilt and height a little..probably only talking about 1 to 2 mph though. I had mine on a 14-50 jon boat and it would do about 30 gps.

 

[moser42]

Re: How to make a 25 hp mercury outboard faster.

I'd also like to know of anything for these motors, I have a 90's Mariner 20hp and it hauls ass on a little aluminum boat rated for a 9.9! What parts or other things can be done to get some more power other then:
-props
-playing with the tilt
-cleaning carbs
-decarboning (seafoam)
-regular maintenane
-running premium 91octane fuel


Thanks, dont mean to steal your thread Tripp1845 just wanna get some more power out of the ole' 20!

 

[WillyBWright]

Re: How to make a 25 hp mercury outboard faster.

There isn't a whole lot you can do to up horsepower on the Merc. 25 was tke max from that block and there is no racing stuff available for them to my knowledge.

Running Premium is probably reducing your horsepower. Stick with Regular. Not sure about legality in Canada, but in many of the United States, operating an overpowered boat is illegal. No matter how you look at it, hanging DOUBLE the rated horsepower on any boat is very poor judgement.

 

[moser42]

Re: How to make a 25 hp mercury outboard faster.

Do the merc 20 and 25 have the same block? What could u put on a 20 to make it a 25 or have more power? Are there any parts you can get for the exhaust to change the sound?

Thanks

 

[Silvertip]

Re: How to make a 25 hp mercury outboard faster.

This question gets asked all the time and the answer is always the same. Go to Mercury Marine web site, click on Parts Express, and then go down the parts list for the 20 and 25 HP engines. Check part numbers for Carbs, jets, intake, pistons, exhaust tuner, head, and block (porting may be different). Any part numbers that are different between the two engines are what differentiates a 20 from a 25. You may find that the cost to gain 5 HP is prohibitive.

 

[casi77]

Re: How to make a 25 hp mercury outboard faster.

i worked on a motor that i noticed from 9.9 to 15hp, the intake/carb/reed cage was different, where the block /pistons were the same. the intake plate where carb mounts to, had a large diameter, carb was different (prob. with different jetting i didnt check), reed cage looked the same.

any upgrade going to cost.. better to sell and get another one with more power.

 

[bhive]

Re: How to make a 25 hp mercury outboard faster.

The difference between a 20 and a 25 is the carb, every thing else is the same. The carb sells for around $300.00 Cnd

 

[jimmbo]

Re: How to make a 25 hp mercury outboard faster.

There is no law in Canada that prohibits using a motor greater than the capacity plate rating. Some peace officers have made noise about it, but most of the time they don't even know what jurisdition lakes and boats fall under.

 

[rvbilly]

Re: How to make a 25 hp mercury outboard faster.

25 Hp engine can`t be upgraded , but the 20 Hp can by installing a 25 Hp Exhaust Tuner and the 25 Hp carb ,, have fun

 

[achris]

Re: How to make a 25 hp mercury outboard faster.

Do a search on 'Frankenmerc' or 'Frankenstein'... When you find those posts you'll realize why it is a really BAD idea to swap parts for one particular horsepower engine to another engine in an attempt to boost its horsepower. It might be ok for you, now, to remember which parts have been changed, and where they came from, but in a few years, or after you've sold the boat/engine, the next poor bastard who's got to work on it and finds all these parts that don't belong in that engine is going to be so pissed! Believe me... Been there (too many times), done that!!!

If you want more horsepower, sell the engine you have and buy the one you want... It will be easier, cheaper and cause far less dramas in the future. And put away the fantasy of 'It would be a nice experiment'... it won't....

Chris........

Edit: Billy... You been dumpster diving again? This thread is over 18 months old!! Why'd ya drag it up now?

 

[mthieme]

Re: How to make a 25 hp mercury outboard faster.

Speed cost money. How fast do you want to go?
If you looking for bolt on parts, your best bet is to seek out a 25XS.
Good luck, they are like chickens teeth.
Next is machine work, such as 1:1 lower gears.
You might find some aftermarket racing reeds, you could rejet the carb.
You would be better off starting with another motor like a Yamato, which (no offense) would be wasted on a 14' aluminum boat.

Not sure about legality in Canada, but in many of the United States, operating an overpowered boat is illegal. No matter how you look at it, hanging DOUBLE the rated horsepower on any boat is very poor judgement.

Somebody forgot to tell DNR this.
When I was a teenager, they ran Whalers with 115 Johnsons.

 

[Grunter]

Re: How to make a 25 hp mercury outboard faster.

You can do a bit:

Here's what I've done so far....

First here's what Ive got:

2005 Princecraft Yukon(206lbs) plus me(252lbs) plus 3 gal gas(18lbs)
2006 Mercury 25hp (116lbs)

Before speed: 27mph (GPS)
Now 32.6mph (GPS)

Lift motor on transom(1"), use second hole for tilt, use two washers between transom and motor..this should have you with a perfectly parallel cavitation plate to bottom of boat with cavitation plate 1" below boat bottom..
Modify exhaust diffuser
Increase inlet size under rear entry(where you remove the cowl) for better breathing...
Use dremel and clean away screw protrusion in butterfly plate, round of sharp corners on carburator inlet as well as plate side(requires carburator removal)
Re-jet from 0.076 to 0.081(if not you'll smoke her!!!)
Use 10.375 x 13Pitch prop(revs at 6040rpms) better accel than 14P and same speed!!

Result: I hang with much faster/larger boats now - I'm impressed anyway...

I did a few more things since - will report back only if it works...
I'm shooting for 35mph (GPS) and I believe I'll get there soon!

Have fun...

 

[Grunter]

Re: How to make a 25 hp mercury outboard faster.

Now 33.4GPS....alone 27GPS with 4 people, 3 tackle boxes, lunches, anchor, minnows, rods, oars, 4 full back seats, fish net...