boosting the power of an outboard

[Carrington]

are there any ways that i could up the power of my 115hp outboard.  its older and runs great but just doesnt push the boat to fast.

[Way2slow]

Being a 115, merc or johnny rude, there's not much room for any big power gains.  You can spend a lot of money trying and  gain a couple of mph.  Then you have the downside of modifing a motor, you will loose bottom end power (holeshot) to gain more top end horse power.   There are very few things you can do without pulling the motor down and having someone that knows how, go through through porting it.  This is something that has to be done by someone very knowledgeable, a few thousands off in the wrong way and you will end up with a piece of junk.

The big V-6's, like the 3.0 Johnson have room for big gains, 300 hp is easily gained from a 200/225 3.0 motor, but thats not the case with the smaller motors.

You will see much more bang for the buck if you work on your setup, starting with a jackplate to get the motor back a suitable distance for the length hull and lets you get the prop set to the best height (finding the sweet spot).

Then you work on finding the best prop suited for your hull.  A stainless steel is a must, then it's finding out if you need a high rake prop (which usually works best on most bass boats) and getting a pitch that lets the motor turn at it's max rated rpm with your normal load.

Spend $1,000+ on modifications and get 2-3 mph.  Spend a little less on jackplate and prop and possibly gain 6-7 mph with a little work dialing it in.   

Now, this is based on the hull design, some hulls hit their threshold and all the hp and setup in the world will only show minimal gain.

[NHfroggin]

Way2slow i have a 90hp merc thunderbolt, from what i was told buy the mechanic it was the smallest inline 6 they made at that time. He then said it could be easily upgraded to a 115 or 135 hp. Any thought on that?

-matt-

[Way2slow]

I'm not much on the mercs but when you say it's a 90 Thunderbolt, I'm guessing you're talking about a late 60's - early 70's.  If so, back then 135 was about the max hp from their race motor.  It's possilbe you could change out the front half (carbs, intake and reeds) to that of a similar hight hp motor and make it that size motor, but only if they both have the same port map.

Usually, when you get the lowest hp rated motor for a given CC size, it's very common the only thing they did was put smaller carbs and intakes on them to restrict airflow to limit hp.   Sometimes they will have a different port map.   For instance, the merc 2.5, V-6 is the same bore and stroke motor used from 200 hp to their 300hp race motor.   The 200/225's are steel sleeved motors with a conservative porting and intake system that makes peak hp around 5,600 rpm and designed to turn no more than 6,000 so they can get a heavy boat up and on plane .  The 260 - 300 are ported to turn way more rpm,  up to 8,500.  However, they would have a hard time getting a heavy bass boat on plane, but will make an Allison or Bullet fly. 

A given size motor can only make so much power, it just depends on where you want that power, up high or down low, you have to increase rpm to increase HP.  Heavy boats need it down low.  Lighter boats can take advantage of more raidical porting and increased rpm.  Low end torgue gets them up and moving, top end rpm makes HP for more speed.   It's the old robbing Peter to pay Paul when you start modifing.   Robbing too much from Peter and paying Paul, pretty soon makes for a sore Peter and it's hard to have fun with a sore Peter.

[montrose]

Do hydro-foils really work?

[NHfroggin]

I'm not much on the mercs but when you say it's a 90 Thunderbolt, I'm guessing you're talking about a late 60's - early 70's. If so, back then 135 was about the max hp from their race motor. It's possilbe you could change out the front half (carbs, intake and reeds) to that of a similar hight hp motor and make it that size motor, but only if they both have the same port map.

Usually, when you get the lowest hp rated motor for a given CC size, it's very common the only thing they did was put smaller carbs and intakes on them to restrict airflow to limit hp. Sometimes they will have a different port map. For instance, the merc 2.5, V-6 is the same bore and stroke motor used from 200 hp to their 300hp race motor. The 200/225's are steel sleeved motors with a conservative porting and intake system that makes peak hp around 5,600 rpm and designed to turn no more than 6,000 so they can get a heavy boat up and on plane . The 260 - 300 are ported to turn way more rpm, up to 8,500. However, they would have a hard time getting a heavy bass boat on plane, but will make an Allison or Bullet fly.

A given size motor can only make so much power, it just depends on where you want that power, up high or down low, you have to increase rpm to increase HP. Heavy boats need it down low. Lighter boats can take advantage of more raidical porting and increased rpm. Low end torgue gets them up and moving, top end rpm makes HP for more speed. It's the old robbing Peter to pay Paul when you start modifing. Robbing too much from Peter and paying Paul, pretty soon makes for a sore Peter and it's hard to have fun with a sore Peter.

Thanks Way2slow!! I think i will look into the carb, intake, and port map. The boat is already over powered by 20hp so we will see how she goes before doing these mods=).

Thanks again,

-matt-

[burleytog]

Do hydro-foils really work?

If you have trouble getting on plane, yes.

[Way2slow]

Port map is not something you can just swap out, that's how they refer to the porting of the cylinders port height and width.  That has to be changed with a die grinder. 

Yes, hydro foils do work for getting on plane, but that's about all.  They can greatly improve the hole shot.

[NHfroggin]

Port map is not something you can just swap out, that's how they refer to the porting of the cylinders port height and width. That has to be changed with a die grinder.

Yes, hydro foils do work for getting on plane, but that's about all. They can greatly improve the hole shot.

But i would have to check to make sure they are the same as a higher hp motor correct? and then all it would be is carb, intake, reeds?

-matt-

[CFFF 1.5]

Do hydro-foils really work?

Yes, especially if you are having a hard time getting on plane. I bought one for my Carolina Skiff that was having a hard time getting on plane and it dramatically decreased the time to plane the boat.

[Carrington]

thanks for the help guys, i ordered a stainless steel prop for it, and also saw somewhere else to put a certain kind of sparkplugs in it, cant remeber their name but ima try those since they are not super expensive.

[Way2slow]

I would strongly try and discourage you from just buying a Stainless prop. They are expensive and there are many different kinds. You should try any prop before you buy. Some stainless won't give you any more speed increase than an aluminum. I'm not up on merc props but you should look at a three blade, high rake prop first. These will give you more bow lift to help get the nose up and the hull running on the back part of the pad. The more hull you can get out of the water, the faster you're going to run.

Just to give you an example of the difference a prop can make, my Javelin R20 runs 72.3 mph with a stock 26" Raker. With a custom tuned 26" raker by Bob Lipton, it runs 76.8. With a stock four blade renegade, it runs 70.1 mph.

Most dealers will let you try before you buy or have demo props. If all that fails, get on some of the sites and start asking others about you same rig and see what works best on theirs. Of course, you may have to sort through the junk because not many people get into optimal setup with the smaller boats/motors like the guys running the big rigs and big V-6's.

[Carrington]

there is a guy who lives here and races boats and he told me which he though was the best, i got the style in aluminum, i meant to say the current prop on the motor he told me was worth around $500 and just needed to be fixed up so we are going to give him a little bit to fix the dents and such in it from the previous owner.  thanks for the help.

[fishfordollars]

Speed costs...How fast you wanta go?

[Carrington]

id love to just see the boat hit 50+ atm its hitting 42mph in perfect conditions.  i just want 8 more mph.  its a mental thing.

[Way2slow]

As I first mentioned, the very first thing you need to get is a good jackplate. On most boats you can get a 6" plate on without having to change steering and control cables, however, that's not all boats. You shop around and find a good 6" Detwiler or other good brand for $150. If your steering cables will give you the slack, that's the best $150 you can spend. If you have to go to longer steering cables, a pair of those can cost close to $300, and that's you installing them.

No way on gods green earth is an aluminum prop going to give you the performance a good stainless prop will. The blades are simple not strong enough to keep from flexing under a load.  Also, your standard stainless, made in the same shape as the aluminums, are only general purpose props, they will not give you the lift you are looking for to get the hull up.

[Crestliner2008]

Do hydro-foils really work?

Not unless you have an adjustable jack plate. Yes, the foil will get your butt out on plane quickly, but it will also slow you down, if it's not out of the water when it does get up on plane. Dragging around a foil under the surface of the water will slow your top end by a few mph - guaranteed. However, if you can adjust your trim & tilt with a jack plate, then you will experience the best of both worlds.

Another overlooked "tuning" aspect is your prop pitch. Some dealers will let you "try" a new prop (as mine did). By decreasing the pitch you will go from a standard duty prop to a heavy duty prop. This will get you more top end speed - normally.

Remember one more thing. You'd better have a good tachometer when you start to fool around with top end speed. If you over-rev your engine, you're just asking for trouble. JMO.

[GLADES]

id love to just see the boat hit 50+ atm its hitting 42mph in perfect conditions. i just want 8 more mph. its a mental thing.

You need to buy a different boat.

[fishn hard]

If your boats will start and RUN LEAVE THEM ALONE AND GO FISH when you start doing HP mods to your motor it cost time and money Look here it is May and i haven't been on the water yet > Yes I rebuilt my power head this winter 2nd time in 3 years I got the motor all put together and check compression and way to much 155psi so here i set waiting on my heads to arrive in the mail  and waiting on my lower unit to get finished and back to me. You guys think you want to go fast but its not worth the time and money and yes I dont care how good the motor builder is A modified 2 stroke is gonna blow so just go hook up the boat take it to the lake and go fishig with a big smile on your face wile i set here with a 100mph Allison in the garage that wont run :(  

[Wayne P.]

Crestliner, lowering the prop pitch to a "heavy duty" prop will decrease your speed "normally" and increase the engine rpm's as compared to the suggested prop for the craft and engine. You are correct about the tachometer if you start dropping the prop pitch.

A hydofoil, when the boat is on plane, is at the surface and will not have any underwater drag. It may reduce speed 1 or 2 mph due to surface drag.

I ran an Attwood hydrofoil for several years on a late 80's model V-6 OMC and with that design, there was no reduction in speed and I didn't use a jack plate (my motor was installed at the optimum height for best performace). I used it to get on plane quickly in the shallow water of the Potomac River.

[Way2slow]

Actually, a "PROPERLY" modified two stroke will get better fuel milage and live longer than a stock motor. The problem is, there are only a few that know how and they are expensive. Most people don't know how to properly build one but think the motors are so simple, anyone can build one, those are the ones the that don't live.

I built a 200hp 3.0 Johnson six years ago that's making approx 300hp @ 6300 rpm and pushing a Stratos 201 79 mph with two people, gear and full live well. The motor now has over 500 hours and still going strong. Cruising at 4,600 rpm, it gets 4.6 mpg. That's about as good as a DFI motor will get on that heavy of a boat.

Granted now, if you're running a 100 mph Allison, you're probably running 2.5 merc and turning it 8,000+ rpm and the ring life in that's about 100 hours max.  I'm assuming that because the merc is much better suited for an Alison and to get 100 mph from it, you're going to be reving the crap out of it.  Also, if you keep fresh, 93 octane, you can run those heads, there's just not much room for octane loss if the gas is a month old before you get it.

[fishn hard]

150psi cranking the motor over on the bench with new rings a pistons is way to much for for 93octaine you know after the rings seat the compression is goana come up 5 to 10 psi and i like to shoot for 145psi after break in. I can build a fishin motor to last for ever well almost but you are rite a 260 2.5 nick motor is only good for 100 hours or so and there is no better sound then my 260 spinning 9000 rpms  8-)

[Wayne P.]

fishn hard, those motors sound good just idling, LOL

[fishn hard]

Mine has the offshore mid and its really loud sounds like a drag care idling ;D ;D in the water