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Author Topic:   Converting Mercury to 90-HP from 75-HP
mflysaround posted 06-20-2009 11:46 PM ET (US)   Profile for mflysaround   Send Email to mflysaround  
While doing web research on converting a merc 75 to a 90, I ran across davej14's past threads. I'm very interested to hear how he came out on that project. If anyone knows [how to convert a Mercury to 90-HP from 75-HP], please [reply in this thread].
jimh posted 06-21-2009 07:47 AM ET (US)     Profile for jimh  Send Email to jimh     
The notion that you can easily convert a motor of a particular horsepower to one with more horsepower when the displacement and basic configuration of the two motors is the same is often pursued but seldom attained.
jimh posted 06-21-2009 09:10 AM ET (US)     Profile for jimh  Send Email to jimh     
[Moved to REPAIRS/MODS from another discussion.]
davej14 posted 06-21-2009 11:08 AM ET (US)     Profile for davej14  Send Email to davej14     
Well, although I have thoroughly researched the conversion I have not actually tried it. I would also be most interested in knowing if anyone out there has made the changes. My research shows that the only difference between the 75 and 90 ELPTO motors is the high speed jet and timing. These would be inexpensive and easy changes to make, I just have not had the time to try them out. Maybe later this year, right now I am more focused on Walleyes.

Any other comments out there?

pglein posted 06-22-2009 12:46 PM ET (US)     Profile for pglein  Send Email to pglein     
Are you sure the carburetors are the same? Generally, in order to attain more horsepower, you need to increase the volume of fuel and air being delivered to the engine. This would usually mean a larger bore carburetor. It is certainly conceivable that the motors would have identical bore and stroke configurations, but I would not assume that the pistons are necessarily the same. Another way to increase horsepower is to increase compression through the use of a different shaped piston. This also often necessitates stronger conecting rods.
Tohsgib posted 06-22-2009 12:56 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Len put 90hp carbs on his 70hp Merc and it did zilch.
mflysaround posted 06-22-2009 02:31 PM ET (US)     Profile for mflysaround  Send Email to mflysaround     
Well, I've just spent quite a bit of time talking to a local merc mech. He says they have done conversions in the past but don't mess with it anymore. His comments were that really more displacement is all that really makes the difference. He said that a converted 75 will add only about 1 mph to the top end, and doesn't help hole shots enough to matter. His angle was that making sure you have the proper wheel match is what matters most. If (he says) you are turning the max rpms with your prop setup, then you won't gain enough to make it worthwhile considering the cost, all the messing around getting it tweaked in with sync, timing etc., plus you will burn more gas.
Sounds (to me) like the 75 vs 90 hp. ratings are more a sales tool for Merc than a real difference in performance. My conclusion is that I will be sure my prop is right, then live with what I've got. Guess if I really want more performance, I'll have to bite the bullet and get more displacement in a bigger engine.
Thanks for your input guys!

P.s. I also have a Honda 4 stroke 8hp kicker. Same deal here, virtually the same as the 9.9 mechanically, and a couple of years ago I looked into upgrading that one. Basically got the same story. Ya think I would have learned....but nooooo.

Buckda posted 06-22-2009 04:26 PM ET (US)     Profile for Buckda  Send Email to Buckda     
This is very easy to do. In the Summer, you will need to travel to Chicago. In the Winter, you will need to travel to Fort Lauderdale. Sell your motor to LHG. Repurchase the motor from him.

You will then have a 90 HP 75 HP Mercury, which is almost as good as just buying the 115 HP 90 HP Mercury, minus a few ponies.

davej14 posted 06-23-2009 10:51 AM ET (US)     Profile for davej14  Send Email to davej14     

Did Len also advance his timing to the 90 hp specs? Did he change the prop as well? Going to 90 hp you should be able to spin a higher pitch prop at WOT. With 20% increase in hp there should be a noticeable difference. I wouldn't expect much change in performance at lower rpms. I also wouldn't expect milage to suffer except at WOT.


The motors are identical except for the high speed jets and timing. I went through the Mercury parts manual that covers both the 75 and 90 page by page. Where there is a difference the parts are charted so it was relatively easy to determine.

pglein posted 06-23-2009 10:57 AM ET (US)     Profile for pglein  Send Email to pglein     
I have a hard time believing that 29% power increase can be obtained simply by changing out the high speed jets, and advancing the timing a little bit. But if so, I would definitely say do it.
Newtauk1 posted 06-23-2009 11:41 AM ET (US)     Profile for Newtauk1  Send Email to Newtauk1     
General Specifications on Mercury 2001 75hp and 90hp 2 stroke:

Full throttle rpm range:
75hp: 4750-5250
90hp: 5000-5500

75hp 56
90hp 67

# of cylinders:same (3)

Piston Displacement: Same 84.57 cu. in. (1385.8cc)

Cylinder Bore: same 3.500 in. (88.9mm)

Stroke: same 2.930 in. (74mm)

Spark Plug: same NGK BUZHW-2 or BUHW-2

Gear Ratio: Same 2.3:1

Is the difference only at the carbs?

L H G posted 06-23-2009 05:17 PM ET (US)     Profile for L H G    
By some standards and interpretations, you probably already have a 90HP engine.

Regarding a 17 Montauk and other 16/17' hulls, I have seen the following top end speed reports here:

1. 75 Merc 2-stroke - 41-42 MPH
2. 90 Yamaha 2-stroke 42 MPH
3. 90 E-tec, average of many reports, 41-42MPH

Another way of looking at it would be that all of these engines put out about 82 HP. They all seem to run about the same.

Buckda posted 06-23-2009 05:38 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Told you so!

davej14 posted 06-23-2009 09:11 PM ET (US)     Profile for davej14  Send Email to davej14     
Not so fast Binkie (pun intended). Until I see a comparison of a Mercury 2-stroke 75 and 90 ELPTO on the same hull I will not be convinced. I know that marketing plays a part, but even Mercury would not charge more for nothing.
fishgutz posted 06-24-2009 11:31 AM ET (US)     Profile for fishgutz  Send Email to fishgutz     
Didn't the Whaler website have a comparison a few years ago of the 75 ELPTO and the 90 ELPTO? Back when you could get a 75 on a Montauk 170. Or am I dreaming? I'm betting a difference of at least 5 MPH from a 75 to a 90.

Simply put, more gas and air will give you more horsepower. Hence the reason that Suzuki and Honda are going to 4 valves per cylinder.

Newtauk1 posted 06-24-2009 12:08 PM ET (US)     Profile for Newtauk1  Send Email to Newtauk1     
don't the Optimax's 135/150 compare the same as well?
davej14 posted 06-25-2009 11:24 AM ET (US)     Profile for davej14  Send Email to davej14     
The problem with newer motors is that you would need to change the engine computer. The manufacturers will not legally sell you this upgrade. With the older carb'd 2-strokes you can still make mechanical modifications yourself.

Fishgutz, good to hear from you. Some day I will try this out because I also believe a 4-5 mph change is realistic. I would prefer to see someone else do it first :-)

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