how do i get more hp out of my 20hp mercury

[andrew k]

in nb we do alot of small engine outboard racing. it seems it is easier to get more hp out of the johnson motors. iwas just wondering if anyone has any tips on how to get more hp out of my 20hp mercury.

 

[Chinewalker]

Re: how do i get more hp out of my 20hp mercury

Hi ATK81,<br /> Which 20hp are you referring to? If it is newer than 1981, then you can probably swap in a 25hp powerhead. If it's 1980 or older, then 20hp is about all you'll get out of it in a service motor application. You can get more, but you'd need the Quicksilver lower unit (like from a 25SS racing engine) to take advantage of increased potential. Trying to get much more than 20hp out of the old 22-cube powerhead, while it's still on a service unit is like dropping a tweaked engine in a car without addressing transmission and tires. You need the whole package to get the most out of the motor....<br /> Of course, the easiest way to get more power out of a motor is to get a bigger motor....<br />- Scott

 

[andrew k]

Re: how do i get more hp out of my 20hp mercury

its a year 2000, i considered going bigger but its aboutintegraty i got a few (johnson)guys that wont let it down that they can get by me on the water.

 

[andrew k]

Re: how do i get more hp out of my 20hp mercury

Originally posted by Chinewalker:<br /> Hi ATK81,<br /> Which 20hp are you referring to? If it is newer than 1981, then you can probably swap in a 25hp powerhead. If it's 1980 or older, then 20hp is about all you'll get out of it in a service motor application. You can get more, but you'd need the Quicksilver lower unit (like from a 25SS racing engine) to take advantage of increased potential. Trying to get much more than 20hp out of the old 22-cube powerhead, while it's still on a service unit is like dropping a tweaked engine in a car without addressing transmission and tires. You need the whole package to get the most out of the motor....<br /> Of course, the easiest way to get more power out of a motor is to get a bigger motor....<br />- Scott

 

[andrew k]

Re: how do i get more hp out of my 20hp mercury

i built my own boat it only weighs 110lbs., i get about 42mph out of my motor now according to gps. i've already tampered with the reeds , drilled exhaust, and upgraded to a 25 carb and a 14.5 prop. just need a few more horses to get by the comp.

 

[Chinewalker]

Re: how do i get more hp out of my 20hp mercury

Hi ATK81,<br /> Sounds like the motor is already peaked. Try looking elsewhere. What kind of hull is the motor pushing? Is it a flat bottom Sea Flea, semi-racing type hull with a flat bottom? If it's flat, is it DEAD-ON flat? A flat bottom hull with absolutely no imperfections will be faster than one with a slight hook or rocker.<br /> Is your prop stainless or aluminum? Is it cupped? Where is your motor height? Angle? Where is your RPM? Are you running on the tiller or with a steering wheel and remote throttle? <br /> It's also possible your friends have the 31.8 cube Johnson 20s which use the same powerhead as the old 35hp.. Your 20 is only about 24 cubes. No substitute for cubic inches, and if they've upgraded to 35hp carbs and intake, they've got you pretty much covered no matter what you do. <br /> I've driven 25hp/24 cube looper Mercs at over 60, but that's in full racing trim. The motors have some serious nuts to them if allowed to breathe, and if using the proper lower unit and prop.<br />- Scott

 

[andrew k]

Re: how do i get more hp out of my 20hp mercury

Scott,<br />my boat is dead on flat. and your right they are running 31.8 cube johnsons, and i have got by them a couple times already. Its just that we are so clos and they are getting me out of the hole. my motor got great top speed but its a bit lazy down low. it seems it only holds back until i get the boat up out of the water and planed off, then it really goes i'm running a 14.5 aluminum. do you think i can go stainless and gain down low? if so what size? i cant affor to loose to much top end speed eitherand thats what is holding me back from going stainless. the motor seems like it has a little bog too just when you open the throttle.

 

[andrew k]

Re: how do i get more hp out of my 20hp mercury

scott<br />i run my boat by the tiller, and my prop is cupped. my boat is turned up a bit at the nose. i did this to prevent nose dive, was i right? my rpms are maxed at top speed but it takes a while to get there. my motor sets on the fourth notch, out of five. i set it there because it seemed to be the best performance notch, any more and it spins out and starves for water, any less and i loose top end.what about lower unit. can i get a better gear ratio? what about boison reeds, will they help my problem with getting out of the hole? so many Questions......<br />thanx for your patience!

 

[Chinewalker]

Re: how do i get more hp out of my 20hp mercury

Hi ATK81,<br /> If your prop is a through-hub type, you might want to consider drilling some ventilation holes near the leading edge of the propeller hub. Somewhere here on i-boats there is a description of how to do it and exactly where. The holes ventilate a little bit of exhaust gas into the propeller stream, allowing it to reach top revs quicker. Once you get moving the water flow over the hub effectively closes off the holes, preventing ventilation at higher speeds. If it doesn't work for you, this is a reversible modification as the holes can be filled with JB-Weld or Marine Tex epoxy. <br /> A good stainless prop should help both bottom and top end. If you've got a local prop shop who is willing to lend you a prop before buying, all the better. A stainless prop of the wrong pitch can do more bad than good, so test, test, and test. A tachometer is a must, too. The 25hp 24-cuber Mercs peaked around 6000 revs, so that should probably be yours as well with your mods considered. Any more and you likely won't be making any more real horsepower, just more noise and risk of eating a rod.<br /> You're stuck with that gear ratio if you stick with a service motor chassis. The racing version (25XS) carries a 16:21 direct drive ratio on a 12-inch mid-section. They are rare and pricey. Boyesen reeds may give you a bit more punch - certainly worth looking into as it is another fairly simple mod. Lots of threads here on i-boats on Boyesens.<br /> Hope you've got a kill switch plugged in! 40+ mph on a stick is pretty fast!<br />- Scott

 

[andrew k]

Re: how do i get more hp out of my 20hp mercury

scott, <br />thanks for the help, i'l' keep you informed . and yeah 40+ is alot to handle on the stick, but thats whatit's all about ... the thrill. my boat usually stands onend and jumps in and out of the water for the first 50 or 60 feet. quite a sight quite the fun<br />atk

 

[Chinewalker]

Re: how do i get more hp out of my 20hp mercury

Hi ATK81,<br /> Yep, I know the feeling. If you look at my Avatar pic, it's of my 12-foot Dura-Nautic, running a 1951 Mercury KG7 Hurricane. It'll run over 30mph as it is...<br />- Scott

 

[Nikko]

Re: how do i get more hp out of my 20hp mercury

What a great thread - I enjoyed reading it!<br /><br />Nikko