Raisen the hp on Mercury 25

I just got a like new Mercury 25 hp 1997 model for a song and i already has a Johnson 25/18 jet on my boat so I am going to switch um between the jet on rivers and prop motor on lakes, a simple switch with the turn of the knobs.I am wondering if I can gain horse power on the Mercury 25,maybe gain to 10 horse with high performance work?Looks like a 25 but rum like a 40 :mrgreen:

Maxem the timing out is all I know about.I used to race em.

Anyone esle ever done anything like this?

Have you looked into changing to high performance reed valves? They claim to help power and give a better running motor all around. Never tried them myself, have gone to 4-strokes.

I sent a email to Boyinsen power reeds to see about raisen HP.The cost of them is $43 so the price is good if the do what they are suspost to do.Now I have to wait till they resond.Anyone ever change to Boyinsen reeds?How much power is gained?

In its simplest form, an engine is just an air pump. To produce more HP, you have to move more air through the motor. 2 ways to attack this: 1)increase the displacement; or 2) spin the motor faster.

That model Merc has plated cylinders, so you can't increase the bore w/o re-sleeving the cylinders = $$. You could possibly find an engine builder to build a stroker crank = $$$$$$. (And a longer stroke will begin working against your RPM ceiling.)

More RPM's? The current motor is set-up to spin between 5500 & 6000 RPM's. I'll guess that you'd have to spin 8000 to get a 40% power increase & you'd have to change the porting in order to feed it. (See "re-sleeving") And unless you intend to never run @ low speed, you won't like the way it runs AND uses fuel!

Have that same motor - several years older - & was briefly intrigued by a 20% power increase, but quickly realized that it was project for somebody w/more time, money & a machine shop.

When this one finally gives up, I'll buy a 40....