can I increase outboard hp?
O.K.: Urban myth or not?I've heard over and over again that everything is the same on a 7.5
h.p. Johnson as on the 15 horse, except the carb. I also heard it's
not even the carb, it's just the jets. I have a 1980-ish 7.5 seahorse that just does not have the guts to pop
my inflatable up on plane with two people and a full tank aboard. I
added those wings, which do help, but still just need a little more. Anybody actually done this carb or jets upgade, or at least seen it
work? Every dock rat seems to have heard of it, but nobody's done it. Failing that, any ideas on how to squeeze a bit more out of a
two-stroke (short of going 100:1 on the mix)? Thanks! |
4ever
Jun 19
2006
|
| considered thoughts to the readers of rec.boats:
Whoa.... Hi, Larry!.. welcome back to the newsgroup! How is life in
Charleston? Ma and I are about ready to zip back down for some good
low country food! How's the Hundley restoration coming?
|
Gene
Jun 21
|
| DO NOT screw with the jets!!! That is not the answer. You need more
AIR and FUEL!!! Not more fuel. The differance is that the bore (throat)
and throttle plate are bigger in the 15hp carb. Take note they are
not 'Twice' as big. Air flow goes up as the square of the diameter.
If the hole was twice as big it would flow four times as much air.
The differance between the 9.9 carbs and 15 carbs is remarkably small.
Remember also it takes 3 to 4 times as much power to go twice as fast.
Changing your oil mix and getting 7.7 horsepower out of a 7.5 isn't
worth the risk of a scored and ruined engine. I ski behind my 70 h.p.
and I always run 40:1 instead of 50:1 because I run my engine hard.
It's a 1981 and never been apart!! If you want to get 7.7 out of that
7.5 switch to synthetic lower unit gear grease!! It helps! Bless and best wishes. |
hexter
Jun 21
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| Try a flatter pitch prop. That will have as much or more effect than a fin
when trying to plane an inflatable. David |
David&Joan
Jun 21
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| Don't know about the 15 hp, but I do know that incresing fuel/air flow
increases hp for car engines.
I like the idea from Robb suggesting trying a carb off a larger motor.
Changing the mix won't increase hp. Variable Ratio Oil (VRO) pumps adjust
the mix from 50:1 to 100:1 based on manufacturer speced lubrication
requirements for the operating range RPMs.
Going to a 100:1 will burn cleaner, but not properly lubricate the engine
thru the RPM range.
MMC <4ever@usa.com> wrote in message |
MMC
Jun 20
|
| Thanks to all for the info. I will leave as-is for now, then add the
nitrous injection when I mount the supercharger...;-) I got the changing the mix for added boost bit from motorcycles. Some
(amateur) racers I know cut down on the oil during time trials and
races to boost performance. I think it's only a couple of percent,
though. Could be yet another myth? Hey, speaking of myths, maybe I'll try the cow-magnets, the Pro-lube
additive, Marvel Mystery Oil... or maybe I'll just go on a diet to
improve the weight:power ratio! Ah, who needs to plane a dink anyway? |
4ever
Jun 20
|
| There must be something to it ... although probably dangerous for the health
of the engine. Ever notice how a chain saw engine starts to have more throttle response and
revs higher just as it is running out of gas/oil? Eisboch www.eisboch.com |
Eisboch
Jun 20
|
| My brother had a Kawasaki 185 Enduro once. It had an oil reservoir and an
auto-oiler system for lubing the fuel for the 2-stroke. One day we were
riding and he was really excited! "Wow! The bike's going REALLY fast
today! I wonder why?" About 5 minutes later the motor seized up ...the
tube from the auto-oiler had fallen off. So yeah, running short on oil
gives you extra power ...for awhile. Race motors are pretty well abused
anyway, so making them temporarily 'do' with a little less oil for a race
wouldn't surprise me, especially in the class races where the motors all
have the same displacement and/or the races are shorter. Brian <4ever@usa.com> wrote in message |
Brian
Jun 20
|
| Messing with the output of an outboard motor is an exercise best left to
someone with expertise, especially with MODERN outboards with strange
metallurgy. |
Harry
Jun 20
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| For sure, and I was not suggesting that he do so. I was simply indicating
that there may be something to the racing trick and it may not be just a
myth. Eisboch www.eisboch.com |
Eisboch
Jun 20
|
| the idea is that these motors use the same engine block ...do the researce
if that is true then you can do it but i suspect the cost will be more than
what is resonable
<4ever@usa.com> wrote in message |
jim
Jun 21
|
| I looked at this for my 75 Mariner vs a 90. The motor guys are smart.
You end up replacing a lot of stuff before you are done and without a
whole engine to steal parts from this would cost a fortune. If I could
have found a 90 with a thrown rod or something that I could take all
the manifolds, adapters and various other plumbing from to get the
carbs to bolt up it would work.
Priced at the dealer the parts are shockingly close to the difference
between the cost of the engines, new. |
gfretwell
Jun 20
|
| @p79g2000cwp.googlegroups.com: > I've heard over and over again that everything is the same on a 7.5
> h.p. Johnson as on the 15 horse, except the carb. WHOA! Look more closely at any outboard motor! That's not a 7.5 horsepower outboard motor! It's a Johnson 7.5. 7.5 is a model number or "trade name". It doesn't say "Horsepower" on it
for the last 50 years! They USED to say HORSEPOWER on them, long ago,
until someone pointed out that was a lie....maybe the FTC? I owned an Evinrude 55, 3-cylinder outboard on a tri-hull runabout. A
friend had an Evinrude 70, 3-cylinder outboard a few years newer. We couldn't find any difference... I call it "Numeric Inflation"....same phenomenon as in consumer stereo
equipment. |
Larry
Jun 20
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| Going 100/1 on the mix won't get you anything. All you need to do is get the parts list for both outboards and compare the
lists.
All the answers come to those that do the most homework. -W <4ever@usa.com> wrote in message |
Clams
Jun 20
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| Where'd you get that BS? Changing the mix will get you a scored cylinder. |
pacman
Jun 20
|
| Even if the displacement is the same, there will be enough differences in
parts that the parts bill will exceed the cost of selling the one you have
and buying a 15. Additionally, your 7.5 will not have any more value at the
end of the exercise. Your call.
Steve <4ever@usa.com> wrote in message |
Steve
Jun 20
|
| Just thinking out loud here, but I'd "guess" this is an urban myth. It
seems easy enough to check, though. Changing just the jets in the carburetor is not rocket science but it is
tricky - and you can end up with a useless carburetor if you make a
mistake. Instead, why not try installing a used carb from a 15hp motor
(larger jets and as you claim identical parts otherwise), seeing what
happens, and if you don't like the results, switching back to the
appropriate, original size. I wouldn't be too optimistic though. The reason I don't think it would work is that horsepower is a function
of the amount of gasoline converted "efficiently" into smoke. It is the
pressure from the smoke that drives the pistons during the power stroke.
There is a physical limit to how much fuel a cylinder of a given size
can burn properly. Adding more than that would result in incomplete
combustion in the cylinder, fouled plugs, back firing and higher exhaust
manifold temperatures. While it may be possible to increase the performance of your engine
slightly by using slightly larger jets then what are currently installed
(but perhaps not ones spec'd for twice the rated horsepower) my guess
is that you would also need to somehow adjust the timing as well as the
air entering the carob manifold, to maintain the proper air/gas ratio. You could possibly achieve this by adding a blower (turbo) type
arrangement and adjusting the dwell, but I think we've now strayed in
way more effort than you had originally hoped for. Oh, BTW, one side effect of "peaking" your engine performance,
generally, is reduced engine life. So what you gain in additional hp
tends to add stress and strain not originally intended for that hardware. Hope this helps. Cheers, Robb
4ever@usa.com wrote: |
AMPowers
Jun 20
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| Actually, it might have more value in the end for people who boat on
lakes with a HP limit. Some of the ones around me have a 10hp limit and
a 7.5hp engine that actually puts out 15hp would be great. Steve |
Steven
Jun 20
|
| You've heard wrong. That engine is a 6/7.5/8 hp. The conversion you
are wishing for is the 9.9 - to - 15 hp, those two are nearly identical
except for the carb and, on earlier models, the exhaust tuner. Rob |
trainfan1
Jun 20
|
| That is the same thing that I heard (9.9 and 15 hp nearly identical). Bill |
wfyehl
Jun 20
|
| ====== I have heard a few stories about people swapping out the motor cover
on a 15 or replacing the engine decals with a 9.9 |
Wayne.B
Jun 20
|
| All of this is true. Now imagine that an engine manufacturer wants to offer a range of engine
sizes, but wants to minimize the cost of doing so. Make the 15 HP
motor, and install a smaller carb on it, and voila! you have a 9.9 hp
motor. This was the tactic followed by Johnson/Evinrude for a long time
( and is a common practice in the automotive industry as well... how
many V8 engines were sold with small 2 BBL carbs?). Changing the jets will only screw up the mixture. The ratio of gas to
air is critical, and is carefully established by the jet size. The
AMOUNT of fuel/air mix is controlled by the size of the carb. But Steve's advice about just getting a 15 HP motor is well worth
considering... bob
s/v Eolian
Seattle |
RW
Jun 20
|
