power vs displacement?

E

Eldiablo

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Aug 10, 2002
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I have a johnson '84 25hp that im reviving from the dead. i noticed that the only difference between a 25 and a 35 is the carb. the block and displasment is the same. what am i missing?<br /><br />if it is just a jet, why don't ppl pull out from the dealer and into the shop to replace the jets?
 
JB

JB

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45,907
Re: power vs displacement?

Hi, Eldiablo.<br /><br />The myth persists that changing jets or a carb or some other trivial modification will produce 50% increases in hp.<br /><br />The carb is a visible difference. Changing the carb will disappoint you with the small change in performAnce.<br /><br />Good luck. :)
 
F

Forktail

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Feb 11, 2002
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977
Re: power vs displacement?

JB's pretty much right on. But there may be some other non-visual minor differences with the two like timing, compression, and throttle limiter position.<br /><br />FWIW I have a 122 hp ski-doo snowmobile that picks up over 10% power with just a jet change. The difference is night and day. But reliability does come into play.
 
O

orca

Chief Petty Officer
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Jul 12, 2002
Messages
422
Re: power vs displacement?

G'Day. I think you will find that the 25 lower unit is not the same as the 35.
 
J

jfbiv

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Re: power vs displacement?

I don't think I believe that "carb jets increase horsepower" is a myth. I don't know if carb jets can give you a 50% increase, but the specs speak for themselves - carb jets change horsepower. Also, lower unit change shouldn't impact the engine horsepower, just gears it different and will impact max RPM, top speed, hole shot, etc. depending on the prop/gear ratio/engine-HP combination.<br /><br />For example: The 84/85 Merc 18XD and 25XD have IDENTICAL bore, stroke, reed valves, carb model, ignition system and lower unit gear ratio. Only difference that can be unearthed between these two engines is the high speed carb jet. 18XD has .052 and the 25XD has .080. The Mercury engineers have obviously measured 7HP increase with this configuration. I should disclose that 18XD has a listed max RPM of 5500 and 25XD is 6000. Still, the net HP increase is 38%. Not quite 50%, but not too shabby.
 
JB

JB

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Re: power vs displacement?

Howdy JFBiv.<br /><br />Only if a new size jet produces an improved stochiometric ratio of fuel/air would torque and hp increase. Therefore, a larger jet requires more air (larger carb venturi or improved dynamics in the charge flow)) or it will produce an over-rich mix and reduce hp.<br /><br />Having messed with 2 stroke engines, including racing engines for 50 years, I have experimented with jet sizes many times. Increasing jet size reduces hp as often as it increases it. It depends on barometric pressure at the time. Altitude also affects mixture.<br /><br />That is why both high and low speed jets in outboard carbs were adjustable for many years. When they began to use multiple carbs they had to go to fixed jets because nobody could get two or more to work together with adjustable jets.<br /><br />You are welcome to "YaBut" the old man and go learn it for yourself at your own expense.<br /><br />Good luck. :)
 
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Dhadley

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Feb 4, 2001
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Re: power vs displacement?

Right on JB. Been there done that too. And we cant forget about port timing. And lets not even get into port area!<br /><br />Good luck!
 
J

jfbiv

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Re: power vs displacement?

Hi JB - <br />Good answer. Makes sense. Made me do some further research and now I see that, although the service manual lists the same Model carb, they are actually different sub-model numbers on MercuryPartsExpress. Now this could just be because they each have different jet sizes, but you've made a believer out of me, (must've been all those big words like stochiometric), so I have to guess that maybe the carb for the 25XD has a larger venturi, etc.<br /><br />How about this then: Is it possible that the 18XD and 25XD are in fact exactly the same engine except for the carb? If this is true, a guy with certain models of larger HP engines might find an inexpensive HP upgrade by buying some carbs meant for a like-model larger HP with same powerhead specs.<br /><br />Anyway, the carb jet thing has been a nagging question for me. Thanks for the insight.
 
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jfbiv

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Re: power vs displacement?

Uhhh...port timing and area? Better get me some technical manuals on 2-cycle outboard theory. Any suggestions? DHadley? JB?
 
J

Jango

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May 15, 2002
Messages
132
Re: power vs displacement?

TO any of you, who have increased the HP of a 2 cycle, can a 85HP Force be inceased to 90-100HP realiably from real world 70-75? Would a good Porting Job and increased comp. along with jet increase do the job (looking for 33% increase)<br />I asked this question on another Forum and was told "can't increase because it wasn't enginered that way"
 
E

Eldiablo

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Aug 10, 2002
Messages
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Re: power vs displacement?

Sloopy, I think you missed the point of the whole thread. The reason i brought this up was because the specs for a johnson 20, 25, 30, and 35 are mostly identical. this holds true for many classes of engines.. 15 - 220 mid 80's black max's, many smaller outboards as well, its easy to scoff this off in sarcastic remarks like yours or because ive been doin this a long time so just trust me. I trust no opinions, just facts, and i posted here to learn something. so far ive learned that you not only nee more fuel via larger jets, but you also need a larger venturi ring to allow more air via a new carb (? $50 for a 35hp carb). this seems to be all. <br />let me reiterate<br />1. same block<br />2. same piston and stroke<br />3. same compression (see 2)<br />4. same reed config and size<br />5. same porting<br />6. same ignition (and mostly timing)<br /><br />alas a differ carb was fond on jfbiv's merc, so, what do they do in the factory as these blocks are coming down the assembly line side by side that make them different besides the carb. :confused:
 
E

Eldiablo

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Joined
Aug 10, 2002
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Re: power vs displacement?

i mean the 150 - 20 mid '80 black max.. put away your swords
 
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