High performance modifications for a 25hp Mercury

[jrober64]

I fish on a lake that has a Horse Power limit. I have already done the "buy a bigger engine swap out decals" I have to deal with the engine I have. I want to get more speed out of the 25HP mercury that I have. I understand that it may not be cost effective. I just want some suggestions as to what I can do regardless of the cost.

 

[JB]

Re: High performance modifications for a 25hp Mercury

There is probably performance to be gained by fine tuning your rigging and setup, jrober.<br /><br />What rpm are you getting at WOT?<br /><br />What prop are you using?<br /><br />Where is your anti-ventilation plate relative to the bottom of your boat?<br /><br />How does the boat/engine perform when getting up on plane?<br /><br />Please give us a detailed description of your boat, how it is usually loaded, the max HP rating and what it weighs ready to launch.<br /><br />Your thread in Merc troubles has been deleted because of your hostile response to cautions about the legality of what you are doing. Not many are interested in helping you casually defy the law.

 

[phatmanmike]

Re: High performance modifications for a 25hp Mercury

aw man, i wanted to see some of those repsonses. dang it, i missed it again!?!?!?!

 

[deejaycee_2000]

Re: High performance modifications for a 25hp Mercury

You can always modify the engine, but I would not suggest that, a modified engine don't idle and you have the risk of something going wrong with it as any boat racer will tell you. A 25 hp merc you can boost up to run with a 40hp if you blueprint it ....

 

[CFronzek]

Re: High performance modifications for a 25hp Mercury

Two-strokes are very free breathing and can be modified to produce amazing increases in HP. But, what you come up against is reliabilty and large increases in gas consumption if the modifcations are taken too far. The one thing that makes two-stokes muscular is a tuned expansion chamber exhaust system which is totally impractical on an outboard motor.<br />Also, increasing the HP of a motor without increasing the dispacement invovles moving the HP curver higher in the RPM band. For your touble you get more heat, more wear, and poorer performance in the bottom of the RPM band.

 

[Chinewalker]

Re: High performance modifications for a 25hp Mercury

There is no easy fix for what you want to do. That motor doesn't lend itself to being modified easily. You can't mill the heads - they're part of the block. You can't easily change the intake ports - they're cast into the cylinder walls. and rely on the piston skirt shape for intake timing. Take any meat out of the ports and the pistons can start getting sloppy. <br /><br />Probably the easiest thing to do would be to get a smaller boat, lose the fishing buddy, and fish with less gear. How big a lake are we talking here? A 25 on a lightly loaded 14-15 foot aluminum hull should flat out fly!<br /><br />- Scott

 

[jrober64]

Re: High performance modifications for a 25hp Mercury

Upon the excellent assistance of "Myoutboards" the conclusion I have come to is that even if I increase the horsepower on the top end of the engine it can not be converted to the prop. It is only geared to turn so much HP. What I have decided to do is put a small Jack Plate on, change out the prop, and make an attempt lightening the load. I have marine grade plywood casting decks on the front and I am going to replace those with aluminum which will make a huge difference in the weight load. Wish me luck and I will repost as I do it and let all know how it turns out.<br /><br />PS. JB ... Relax man I'm not smuggleing drugs in this I am fishing!!!! LOL

 

[Pony]

Re: High performance modifications for a 25hp Mercury

PS. JB ... Relax man I'm not smuggleing drugs in this I am fishing!!!! LOL

If that was true, then would you really care about the MARGINAL (at best) hp increase on a 25hp motor???

 

[deejaycee_2000]

Re: High performance modifications for a 25hp Mercury

Good job on the restoration, i'm impressed

 

[waterinthefuel]

Re: High performance modifications for a 25hp Mercury

Without going into what I did, I broke the law and low and behold got caught. I did 2 years in a federal prison and am now a born again christian, I found Jesus in prison. <br /><br />Please don't break the law, even if it's just turning screws on your outboard. You can get caught, and fines aren't fun. Jailtime is even worse.

 

[jrober64]

Re: High performance modifications for a 25hp Mercury

I really do not know why I am even responding to this but I feel compelled. The county in which I live has an old law on the books that does not allow larger than a 10 hp motor on this paticular lake I fish. This law has no fine associated with it if they did write you a citation and, further more it is not even patrolled by the authorities. It is the same law that states that you can not remove any trees or even cut the grass 20 feet up from the bank. In the last 15 years a sub-division has been developed on this lake with $500,000 and up houses that is full of doctors and lawyers. All of which have cut out the trees blocking there view of the lake and cut the grass right down to the water. None of which have had a word said to them. In the same right no one says anything about fudgeing on the motor size as long as you do not abuse it. The only reason I inquired about speeding up my boat is because we have little Friday and Monday night tournements and because some of the more mechanically minded guys who fish in these tourneys I suspect have beefed up there motors somehow. The reason I suspect this is that they keep beating me to my favorite fishing holes. Therefore I came on here to inquire about a little advice on how I could make my little 25hp motor run a little faster so that I could get to my favorite hole a little quicker. So guys please relax on the law breaking stuff. Like I said in another post if you have some advice for me on how to speed up the little john boat I would love to hear it.<br /><br />PS. I think if you read my previous post I too came to the conclusion that it would not be cost or effort worthy to do major mods to get the small amount of increase in HP I might get. That is why I am trying other things. Have a great day all!!!

 

[phatmanmike]

Re: High performance modifications for a 25hp Mercury

i told you what to do. ditch the 25hp, and get a late 80's omc 35, and the put a 25 hood on it!

 

[jrober64]

Re: High performance modifications for a 25hp Mercury

I will eventually do that but for now I am installing a small jack plate and changing the prop and lightening the load. I would like to do that down the road. (and get electric start on the new motor) I fished with a guy yesterday who knows alot more about motors and set ups than I do and based on the size rooster tail I am throwing he seems to think that the plate is going to help alot. He also looked at my prop and said that the difference in with the new prop will be substancial.