Merc 75 4stroke converted to 90 or 115?

[1860Angler]

Originally Posted by Prince of Worms

Interesting project.
With this modification, will the BIA hp hull limit be exceeded?
If so, will it affect resale?

If an accident, will liability be unlimited?

Unintended consequences?

115, what 115? This is only a 75!! Motor still says 75, part # on the ECM is still a 75, so for all intents and purposes the motor is still a 75. I'd pay more for a 75 that turned out 115!!

[Lunddude]

Originally Posted by Prince of Worms

Interesting project.
With this modification, will the BIA hp hull limit be exceeded?
If so, will it affect resale?

If an accident, will liability be unlimited?

Unintended consequences?

YES (Under rated already IMHO remember I live @ 5000' elv.)

Doubt It (Don't plan on selling but if I did I really don't think it would hurt the sale)

Never had a claim in 30+ years of operating boats out west so to me it's not even an issue.

[Lunddude]

Originally Posted by 1860Angler

115, what 115? This is only a 75!! Motor still says 75, part # on the ECM is still a 75, so for all intents and purposes the motor is still a 75. I'd pay more for a 75 that turned out 115!!

My thoughts exactly

[REW]

Lund,
Just remember, that if you try to change your motor from one hp to another hp, the changes that you are likely making is the ability to spin the motor faster.

You are not changing the bore or stroke of the motor. So, the only way to get more hp, from a given bore and stroke is to spin the motor faster.

So, this motor change will basically change the motors red line from 5300 to 6000 or so. But, then when you do this, you will also likely have to change the prop to let the motor spin up to this new rpm.

It is tough to beat cu inches for making power.

-----------
That is why, if you compare real world performance differences between motors of different hp, that use the same block and crank and pistons - so have the same displacement - you will often find that the real world performance of these different hp rated motors is much smaller than the hp differences might normally dictate.

But, if you do decide to go with the change, post your performance differences so that we can all learn from your investment.

Be safe
REW

[Lunddude]

Originally Posted by REW

Lund,
Just remember, that if you try to change your motor from one hp to another hp, the changes that you are likely making is the ability to spin the motor faster.

You are not changing the bore or stroke of the motor. So, the only way to get more hp, from a given bore and stroke is to spin the motor faster.

So, this motor change will basically change the motors red line from 5300 to 6000 or so. But, then when you do this, you will also likely have to change the prop to let the motor spin up to this new rpm.

It is tough to beat cu inches for making power.

-----------
That is why, if you compare real world performance differences between motors of different hp, that use the same block and crank and pistons - so have the same displacement - you will often find that the real world performance of these different hp rated motors is much smaller than the hp differences might normally dictate.

But, if you do decide to go with the change, post your performance differences so that we can all learn from your investment.

Be safe
REW

Yea I'm aware of all that but my 75 does not redline @ 5300 rpm, it's more like 6000 rpm. In fact I was told by a Merc Tech that the rev limiter on the 75 does not even kick in until 6200 rpm. Your right in what your saying because the 115 is made to run @ WOT @ 6400 rpm. With the new mapping it will not hit the rev limiter until 6800 rpm according to Eric. At 5000' elv. this new boat/motor combo will never see 6800 rpm. I'll be happy if I can hit 6000 rpm while spining a 17P prop. With my current set up I'm down to a 13P prop while seeing 5800-5900 rpm @ WOT & IMHO that is to small of a prop for my Alaskan so I'm currently running a 15P prop but only seeing 5300 rpm @ WOT. Bottom line is altitude just kills these outboards. I have no doubt that my 75 4S @ 5000' elv. on a 90 degree summer day is like running a 50 4S on it. I'm hopeing by making it into a 115 I might see similiar results from guys that are running the same rig with a 90 4S @ or just above sea level.

[moose-hunter]

Originally Posted by 1860Angler

115, what 115? This is only a 75!! Motor still says 75, part # on the ECM is still a 75, so for all intents and purposes the motor is still a 75. I'd pay more for a 75 that turned out 115!!

Respectfully I have to disagree. If given a choice of engines, one stock, the other modified, I'd pick the stocker every day of the week. UNLESS... I bought it directly from a knowledgeable engine builder or a crate engine from the factory.

Buying a "home built" hot rod, in my eyes, is a gamble I'm not willing to take as you never know HOW it was put together. I used to build Harley engines and you'd be AMAZED with what came into the shop. What started as simple home brewed, bolt-on mods, sometimes ended up as complete rebuilds.

Will that happen to marine power heads? Is a ECM swap the pin being pulled on the hand grenade? Dunno... but I'm not rolling the dice with my cash.

Your mileage may vary. I'd like to follow this thread since a similar mod is available for my 175 Verado. A box change and I have a 200. But... am I pulling "the pin"?

Good luck with your project!!

[Lunddude]

Originally Posted by moose-hunter

Respectfully I have to disagree. If given a choice of engines, one stock, the other modified, I'd pick the stocker every day of the week. UNLESS... I bought it directly from a knowledgeable engine builder or a crate engine from the factory.

Buying a "home built" hot rod, in my eyes, is a gamble I'm not willing to take as you never know HOW it was put together. I used to build Harley engines and you'd be AMAZED with what came into the shop. What started as simple home brewed, bolt-on mods, sometimes ended up as complete rebuilds.

Will that happen to marine power heads? Is a ECM swap the pin being pulled on the hand grenade? Dunno... but I'm not rolling the dice with my cash.

Your mileage may vary. I'd like to follow this thread since a similar mod is available for my 175 Verado. A box change and I have a 200. But... am I pulling "the pin"?

Good luck with your project!!

This will be far from a "Hot Rod".

What people don't understand is a 75, 90, & 115 are all the same motor just different ECM programing and a little restrictor ring in the 75 air box is all that seperates the 3 motors. Look up the specs on all 3 and you'll see they are the same.

The only reason the 75 & 90 have the lower gearing is so they can spin the same prop as the 115. By removing the restrictor ring in the 75 and swapping out the ECM I could have a 90 HP or 115 HP. All I'm doing is reprograming the ECM so it will run to the 115 HP specs. I could also buy a new Mercury ECM ($$$) for a 115 HP plug it into my 75 HP & presto! my 75 is now a 115!!

This is totaly different from what your talking about by "Hot Rodding" my motor.

[115 optimax]

Originally Posted by Lunddude

This will be far from a "Hot Rod".

What people don't understand is a 75, 90, & 115 are all the same motor just different ECM programing and a little restrictor ring in the 75 air box is all that seperates the 3 motors. Look up the specs on all 3 and you'll see they are the same.

The only reason the 75 & 90 have the lower gearing is so they can spin the same prop as the 115. By removing the restrictor ring in the 75 and swapping out the ECM I could have a 90 HP or 115 HP. All I'm doing is reprograming the ECM so it will run to the 115 HP specs. I could also buy a new Mercury ECM ($$$) for a 115 HP plug it into my 75 HP & presto! my 75 is now a 115!!

This is totaly different from what your talking about by "Hot Rodding" my motor.

NO KIDDING.

[moose-hunter]

Originally Posted by Lunddude

This will be far from a "Hot Rod".

What people don't understand is a 75, 90, & 115 are all the same motor just different ECM programing and a little restrictor ring in the 75 air box is all that seperates the 3 motors. Look up the specs on all 3 and you'll see they are the same.

The only reason the 75 & 90 have the lower gearing is so they can spin the same prop as the 115. By removing the restrictor ring in the 75 and swapping out the ECM I could have a 90 HP or 115 HP. All I'm doing is reprograming the ECM so it will run to the 115 HP specs. I could also buy a new Mercury ECM ($$$) for a 115 HP plug it into my 75 HP & presto! my 75 is now a 115!!

This is totaly different from what your talking about by "Hot Rodding" my motor.

I understand what you are doing completely. Really I do! Reclaiming High altitude power loss.

All I'm saying is that by doing the tuning and removing the restrictor you are changing the engine from factory specs regardless of the fact it's all the same underneath. All the documentation still points to your engine being a 75. NOT a 115. And THAT is what the "average joe" will see. That's where the "hot rodding" comment came from. That's what the "average joe" will see in an engine that has been altered to gain power and not understand the "why" behind the mods.

As I said, my 175 is mechanically the same engine as the 200. And with a simple ECM change... I have a 200 with a 175 cowl. Just as your engine is the same as others that share that same base assembly.

For me it would be the truest meaning of the phrase "hot rodding". As in to make the engine more powerful for power's sake. However, in your situation, I see those mods in the same light as rejetting / reclutching a snowmobile for use in the mountains. You may (or may not) be raising the hp over the factory specs but regaining the power lost from high altitude. And if you get a bit more, no worries since your engine shares the same assembly as a 115.

I know what you're doing. You know. And it looks as if 115 optimax also knows. But what of the "average joe"? They don't know squat when it comes to easy, simple mods. They will see you as having changed the engine to gain power. Nothing more. ie: pulling the pin on the hand grenade. BUT... If you're never going to sell it... I say... HAVE AT IT!!

Sounds like a decent mod and I honestly wish you the best with your project. It's so tough to respond to threads like this with a keyboard. It would be so much easier TALKING shop... well... in the shop.

[BW]

I wish they had this for Etecs somewhere?? The 150, 175, and 200 are the same block also! If I was made of money, I would just buy a 200 and slap my 150 hood on there and go!!!! Now, that would be a sweet setup!