Mercury 2 stroke Outboard injector question

[GXPWeasel]

Okay, so I've got a wild hair up my ass about pinching a little more umph out of my engine.
I currently have a 90hp 2 stoke Mercury ('05). I was told by several other Mercury owners locally that a 90hp, and a 115hp Merc. are the same engine, with different injectors.

Can anyone confirm this? If this is the case, how hard is it to change injectors? If this was a car, I would dive in, heads first, no questions asked. However with boats, I'm not quite as savy, yet.

Let me know what you guys think.

( I'm already getting a bigger prop next year, as my 17 is WAAAAAY to slow, and I hit red line rpm, 5,500, at about 2/3 throttle. )

Thanks in advance.

[jimrs]

These stories go on and on, I remember when I was told that my Johnson 85 was the same as the 115, except the carbs. There is more than that there are changes in everything from the flywheel to the prop to get the extra horsepower. They just share the same bore and stroke. If you want more power your best bet is buy it in a new motor.

[slingshot]

No, your 90 is a 3 cylinder and that 115 is a 4 cylinder.

[mikeherb]

I asked this question on a mercury mechanic site where you can ask any question you want about outboards and certified mechanic answers. I can't remember the site but I basically remember the answer:

You can covert a 90 HP to a 115 by swapping out the CDI and the intake cam. Number of cylinders, bore, stroke is all the same. There is a variance in the gearing from the factory when you go from 90 to 115, so you would not have a "true" 115 as it would come from the factory.

I asked him a follow-up on cost and he did not respond. He also stated he had never done this but confirmed it could be done.

I would think this would only be worth it if you were going from 75 to 115 but even then it would be a stretch to justify it. But I would not attempt it just to get 25% more horsepower.

[brumbyvet.1044]

You can covert a 90 HP to a 115 by swapping out the CDI and the intake cam.



I didn't know 2 strokes had a cam. Please explain.

[GXPWeasel]

I didn't figure it would be a simple as injectors, but worth asking.
I haven't needed the boat to go any faster than it would go last year, and probably never will, but thought if it was as simple as I had heard, than it would be worth the couple extra dollars.

Oh, and as for the cam response... Doesn't every engine of some sort have a cam to open intake and exhaust valves? Every engine I ever took apart has anyway.

[brumbyvet.1044]

No, two strokes have reed valves. No need for a cam,lifters,intake/exaust valves. That's the differance between two strokes and four strokes. At least that's the way it's been for the past 57 years that I've been around....

[wed]

brumby,

To ask a similar question, what is the difference between a 90 HP four stroke and a 115 HP four stroke? My boat gets similar MPH numbers as yours. I get 39 MPH GPS @ 6000 RPM. The problem is that my motor will actually turn about 6200 RPM with a very light load on board (just me and fuel). I did this very briefly and pulled back to 6000 RPM (no alarms from the engine management system). The 115 and 90 share mechanical specifications but the 115 is rated at 6500 RPM. My question to Mercury Marine was about running the 90 beyond 6000 RPM causing mechanical damage. I guess it was a question about HP rating at a given RPM vs. mechanical design of the assembly.

The Mercury tech said I could run the 90 HP 4 stroke at 6000 RPM constantly but do not run it past 6000 RPM for any length of time. By the way, he told me I should check my tach, I will get around to it one day.

[brumbyvet.1044]

Wed
My boat was delivered with a 15p aluminum prop and it was also turning 6200-6300 rpm @ wot. The Tracker Marine Center service manager told me this was the perfect prop for this boat. I ordered a 16p Quicksilver stainless prop and that made a whale of a differance. My wot dropped to 6000 rpm and I picked up 3 mph . My boat will run 39.2 mph with my wife and I, and a half tank of gas. I actually had it up to 41 mph once with just me in the boat and a good wind at my back. I can't answer about the differance between the 90 and the 115. I would guess the computer settings are probably differant but have no idea about anything else.
If you haven't already, I would definately suggest a stainless prop.
Mike

[mikeherb]

You can covert a 90 HP to a 115 by swapping out the CDI and the intake cam.



I didn't know 2 strokes had a cam. Please explain.

The mechanics exact answer from JustAnswer.com:

"Hello.. The only mechanical differences between a 90 and 115 is the intake camshaft and the ECM. Block and all other parts are the same".

I called the ECM the CDI but there you go. That's why I sell software for a living instead of servicing outboard motors.

[brumbyvet.1044]

I still stick with my original answer that a 90HP 2 stroke does not have a cam. They must have thought you had a 90HP 4 stroke Merc. If a certified Mercury mechanic tells me differant, I will have learned something.
Is your motor a Optimax or a 90 elpt ?

[mikeherb]

Here was my exact question:

I have a 2003 90 hp Mercury ELPTO. Is it possible to "make" this motor into a 115 HP by swapping the computer? Other than the gear ratio I can't find any difference in the specs of a 90 vs a 115.

You've seen his answer in my previous post.

Here are his credentials (they don't give you their name):

Marine Mechanic
Certified Technician Mercruiser Mariner
Mercury Johnson Evinrude & business owner

He has 100% positive feedback on 852 questions asked of him.

Take it for what it's worth cuz I don't know if they have a cam either

[LTB Racing]

A 90 ELPTO is a 3 cylinder conventional carburated 2 stroke...it does not have cams, valves, injectors, or a "computer" and cannot be converted to a 115. Which is a 4 cylinder completely different engine. There isn't much u can do with a 90 to get more power...it is what it is.

The mechanic at justanswer was talking 4 stroke which are a different breed altogether.

[jimrs]

LTB Racing is right on.

[brumbyvet.1044]

Thanks LTB for the backup.