Converting a 75 or 90HP Mercury to a 115

[CBintheNorth]

First some background...
I apologise now for the long-winded review.
11 years ago I bought a Legend 17Xtreme and at the time didn't have a ton of experience with boats, having only owned a 12ft tinner and been in a few others.
I wish somebody would have told me the value in maxing out a boat's rated horsepower. Long story short, due to finances at the time and little experience, I opted to put a 90 horse Optimax on the back even though the max rating was for a 115. Something I have regretted since I figured out what "planing" meant.
I love that engine, it's been bullet-proof, I just wish everything was a bit faster; planing time, top end, the usual...
To trade in my engine for a 115 made zero financial sense , so I've lived with it for 11 years. In that time I've gained considerable experience on the water. Both big and small, good and bad, which makes my original decision sting that much more.
Hopefully not anymore...
I have always known Mercury built their engines in*"Family" groups. For example, the 75, 90, and 115 share the same displacement and several of the same parts. I just didn't realize how many.
After countless hours of research I learned that basically the only difference was the air intake size (a restrictor), a slight gear ratio change, and different programming of the ecu.
Enter Simon Motorsports via this thread:
http://www.outdoorsmenforum.ca/showthread.php?p=3295201
Huge thanks to 67ZL1 !!! The timing was really perfect to stumble onto that thread.
I called down there and spoke with Eric, a very knowledgeable man. We talked about some of the concerns I had read about and some that I had, all were put to rest.
I removed my ecu, which took me about twenty minutes, and mailed it down there. 2 days later it arrived to Simon and the next day they called to say it was ready, collect $650 USD, and ask how I wanted it shipped. Very professional.
Time to rip out the restrictor...
First you gotta find the carb!
*It's nestled in behind the oil tank on this engine. You've got to rip most of the casing off the unit to move the tank, but when you do, presto, out goes the restrictor. Roughly 1.5 hours to perform this step.
Now I wait...
In 2 days my ecu is supposed to arrive and next weekend (weather permitting) I'm headed out to give it a test run.
Currently my boat will hit 37.2 mph with a full tank of fuel, my wife, and 2 boys in the boat. The prop is a 13x19 3 blade aluminium and wot is 5300 RPMs.
I was finally a passenger in my own boat and while snooping back there I realized that my engine is sitting about 2" too low in the water while trimmed right out. I'll correct that after this weekend, as I want to keep it apples to apples.
Stay tuned...

[Tfng]

I'll be following along CBinthe north. I'm curious to see how that tuned ecu works out for you.

[Kingfisher]

Sounds like an interesting project. I sure hope it works out well

[67ZL1]

Eric runs a tight ship down there. They have awesome customer service and return your calls or emails promptly which is pretty rare these days. Let us know how you make out apples to apples. My guess is you'll gain 5-7 mph once propped correctly with your engine height set properly.

[CBintheNorth]

Originally Posted by 67ZL1

Eric runs a tight ship down there. They have awesome customer service and return your calls or emails promptly which is pretty rare these days. Let us know how you make out apples to apples. My guess is you'll gain 5-7 mph once propped correctly with your engine height set properly.

That's what I'm hoping for.
If I can pick up some hole-shot and 4-5 mph with the reflash, I'll be really happy. I figure I should get another 1-2 mph after a re-prop and a height adjustment.
Thanks again for putting me on to them.

[Walleyedude]

Subscribed.

I'm looking forward to hearing about your results.

[PerchBuster]

A 4 blade stainless prop would help quite a bit as would a good Hydrofoil mounted on the cavitation plate. I put a Hydrofoil on my 150 Merc and not only does it get up on plane in half the time, I am convinced it has also helped my fuel economy by about 15%/20%. If the motor is too low that is major performance stealer as well. With the added HP going up to 115 hp I don't think you will see great improvement on the hole shot and getting it up on plane that much quicker but add a Hydrofoil and you will be in business! Good luck!

[CBintheNorth]

Originally Posted by PerchBuster

A 4 blade stainless prop would help quite a bit as would a good Hydrofoil mounted on the cavitation plate. I put a Hydrofoil on my 150 Merc and not only does it get up on plane in half the time, I am convinced it has also helped my fuel economy by about 15%/20%. If the motor is too low that is major performance stealer as well. With the added HP going up to 115 hp I don't think you will see great improvement on the hole shot and getting it up on plane that much quicker but add a Hydrofoil and you will be in business! Good luck!

Way ahead of you my man....

I installed a Stingray Hydrofoil after the second outing, which was 11 years ago. I have to agree with you, HUGE improvement on the planing time. That's what was looking for when I went to the back snooping, I wanted to see if water was passing over or under it. It was passing over alright, by at least 2". Dragging that thing around plus the extra leg in the water I'm sure is killing me. I'll find out by just how much after I test the ECU this weekend.
Now I wish I would have timed my planing time, as once I get the motor lifted and test performance there, I'm going to rip the Hydrofoil off and run it again.
Just a guess, but I'm going to bet I put it back on.

[Big Sky]

Originally Posted by CBintheNorth

I'll find out by just how much after I test the ECU this weekend.
Now I wish I would have timed my planing time, as once I get the motor lifted and test performance there, I'm going to rip the Hydrofoil off and run it again.
Just a guess, but I'm going to bet I put it back on.

'Butt dyno' should give you a pretty good idea.

[jim summit]

Originally Posted by CBintheNorth

Way ahead of you my man....

I installed a Stingray Hydrofoil after the second outing, which was 11 years ago. I have to agree with you, HUGE improvement on the planing time. That's what was looking for when I went to the back snooping, I wanted to see if water was passing over or under it. It was passing over alright, by at least 2". Dragging that thing around plus the extra leg in the water I'm sure is killing me. I'll find out by just how much after I test the ECU this weekend.
Now I wish I would have timed my planing time, as once I get the motor lifted and test performance there, I'm going to rip the Hydrofoil off and run it again.
Just a guess, but I'm going to bet I put it back on.

If you can, lift the motor up on the mounting holes. Most outboards are mounted flat on the transom, when they should be 2" up.
Free's up the fin as the boat planes off and no extra drag at higher speeds.
As you slow down it comes into effect again.

I picked up 5 mph on my 150 Merc by lifting it 2''. Helps with chine walk too.

[Wes_G]

How do you know if your motor is mounted to low?

[67ZL1]

Originally Posted by Wes_G

How do you know if your motor is mounted to low?

When you can see your cavitation plate running down the lake, you're in the zone!

[CBintheNorth]

Originally Posted by jim summit

If you can, lift the motor up on the mounting holes. Most outboards are mounted flat on the transom, when they should be 2" up.
Free's up the fin as the boat planes off and no extra drag at higher speeds.
As you slow down it comes into effect again.

I picked up 5 mph on my 150 Merc by lifting it 2''. Helps with chine walk too.

Lots of room to go up. It's sitting on the transom bolted through the top holes, just like is done by most people who have no clue.
In their defence however, Cars RV and Marine never claimed to be boat techs.

[CBintheNorth]

Well, today it arrived.
I was like a kid at Christmas waiting to open the box.
I don't know why I was disappointed when it looked exactly the same as when I shipped it.
I guess I expected it to grow muscles..

The motor didn't need new plugs, but I changed them anyway just so I could track the burning condition after the upgrade.
Installation of the ECU was just as easy as removal, had it in and the muffs on in 15 mins.
Turned the key and.....it started!! That was a relief.
But there was a definite miss now. Not spark, as it was too irregular. It did have the odd stutter to it before also, when it was cold or coming down quickly from high revs, but this was happening more often and at all RPM's. Pulled the plugs and they were all firing and all looked good?
Not sure what that was about, but thankfully, after another 3 mins of running it was purring like it always did. Pulled the plugs one more time and all looked good.
It now runs and sounds just like before all this started.

I can't wait for Saturday now. Stay tuned.....

[CBintheNorth]

Originally Posted by Jet

Just a theory, most modern cars need to relearn timing/fuel/idle etc. after changing the ECU or sometimes after battery removal.

I don't know for sure, but being these are modern engines, it might be it had to relearn after the ECU was upgraded, the existing settings likely wouldn't apply, could have been wiped, or zero'd by the upgrade.

If that's the case, it might be a little erratic on first startup/idling and when you have it under load for a little while.

I don't think you're too far off.
Balancing fuel, air, and minor timing are something the engine is always doing. In this case it had some major sensing and adjusting to do.
It hasn't done it since. I tried firing it after it was cold again and it ran fine at idle, so I think you're probably right.

[LutherDLG]

I'm pretty pumped to hear the test results this weekend also. My 115 is 35 years old, so no ECU to re-program, but I often think of cheap upgrades like that becaues my engine was a detuned 150 block.

[CBintheNorth]

Well, I'm really happy with how the boat performed today.
I let the boat warm up for a few minutes longer than normal, as I was pretty nervous to drop the hammer.
When the time finally came I've gotta say, I got a bit startled. Not from the power, but the RPM's. Almost instantly the boat hit 4200 and was up on step. It was truly amazing and I felt that ear to ear grin come over my face.
I'd say time-to-plane was cut down to a quarter of the time.
Top end was impressive as well. Top speed trimmed out, and loaded apples to apples, the boat hit 41.4 mph. A gain of just over 4 mph. Not a huge gain, but just what was expected. The surprise was the hole shot. Simply incredible.
Top RPM all trimmed was hovering between 6000 and 6100, right where Eric at Simon said it should be. Maybe I'm odd, but I don't like an engine screaming like that. I'll be going up a couple inches in pitch on my new prop. That should bring my RPM's down to Mercury recommended range and pick up my top end a bit. I think the bottom end can spare it..

Later in the day I raised the motor. I figured I needed 2.25" higher to be perfect, and that's where the second last hole put me. However, lined up on the same plane as the hull, the cav plate was nearly 1.5" above the bottom of the boat. Oh well, that's where the water said it needed to go.
Bolt her down and back into the water.
Surprisingly the speed only picked up about 0.4mph. Went back to check how things looked and water was sliding perfectly under the cavitation plate. It definitely helped the chine walk, but I didn't like the lack of trim adjustment I had. I definitely lost a bit of hole shot with the higher motor, but nothing crazy. Also, when cruising at top speed, minor wakes off of passing boats would cause slight ventilation to occur. That never happened before and it isn't something I want to leave, so tomorrow its getting lowered 1 hole (.75"). I'm also going to take the Hydrofoil off for a round to see how I like it, as it's been on so long I don't remember what's it's like without it.

Overall, some of the best money I have ever spent on the boat, and I've spent a fair bit.
I can't wait to pull the tube again! I'll be coming out of every turn with so much torque they won't know what hit them.
I'll update again after the final adjustments tomorrow.

[CBintheNorth]

Took the Hydrofoil off this morning and slid the boat in the water. Left the height where it was.
Time to plane slowed a fair bit, but nothing to cause discontent. Top speed however climbed to 42.8 mph.
I noticed last night that the 'wings' of the hydrofoil were still dragging in the water, even though the centre was riding above. I guess that was causing a fair bit of drag.
Overall performance also feels better. Cornering is much better without the hydrofoil.
For now I think it'll stay off and the motor height will stay where it is also.
I'll get a look at the back later to see where the cav plate is riding, but it's handling well.
Another huge plus I noticed today is that with the higher HP and higher engine, I can stay on plane at a much slower speed. Not sure which one of these is helping that the most, but it's substantial. 17mph vs at least 25mph before all this started.
Overall, I'd do it all again in a heartbeat! Except next time I'd probably start with a 75HP

[Walleyedude]

Those are great results, both on the speedometer and the butt dyno. I'd be very happy with the upgrade as well!

By your description, I'd say your motor is mounted one hole too high. Everything is a compromise, and the negatives you're describing would not be worth it to me for 0.4 mph. Those negatives will be even more pronounced when it's windy and the water's rough.

What were your RPMs at WOT without the hydrofoil? There's likely some room for improvement in your prop selection after adding that much HP and raising the engine. Adding pitch and trying a few different props should really bring out the benefits of the added HP. Revving it too high at WOT is a sure way to shorten its lifespan, so I'd be reluctant to run 6000-6100.

As for the hydrofoil, I know that not everyone will agree, in fact, it will likely make some posters here really mad at me, but there's a reason why no outboard manufacturer has incorporated them into their design. If they truly provided a performance advantage, one of the OEMs would have adopted them a long time ago. They do have their place, but they are a "band-aid" fix for boats that are under-powered or improperly setup. As you're finding out, they hinder performance, or at best, have little or no effect, when a boat is properly setup.

[CBintheNorth]

Originally Posted by Walleyedude

Those are great results, both on the speedometer and the butt dyno. I'd be very happy with the upgrade as well!

By your description, I'd say your motor is mounted one hole too high. Everything is a compromise, and the negatives you're describing would not be worth it to me for 0.4 mph. Those negatives will be even more pronounced when it's windy and the water's rough.

All those negatives went away as soon as I removed the Hydrofoil, and the speed increased by 1 mph as well. The boat handles incredibly well now that the tail is off and the motor raised. It just feels more stable. Slight waves don't cause it to start porpoising and it rides 'softer', for lack of a better term.

What were your RPMs at WOT without the hydrofoil? There's likely some room for improvement in your prop selection after adding that much HP and raising the engine. Adding pitch and trying a few different props should really bring out the benefits of the added HP. Revving it too high at WOT is a sure way to shorten its lifespan, so I'd be reluctant to run 6000-6100.

RPM's were just over 6200 all out. I'd say a good 2" of pitch coming my way. I might also try a 4 blade for when pulling the tube, which I did yesterday, and let me tell you, there is no comparison between then and now. The low speed planing and power available out of the turns is nothing short of incredible.

As for the hydrofoil, I know that not everyone will agree, in fact, it will likely make some posters here really mad at me, but there's a reason why no outboard manufacturer has incorporated them into their design. If they truly provided a performance advantage, one of the OEMs would have adopted them a long time ago. They do have their place, but they are a "band-aid" fix for boats that are under-powered or improperly setup. As you're finding out, they hinder performance, or at best, have little or no effect, when a boat is properly setup.

2 months ago I would have argued to the death in favour of the "whale tail", but now my opinion has changed. Your paragraph above is spot-on.

.

[Walleyedude]

Originally Posted by CBintheNorth

RPM's were just over 6200 all out. I'd say a good 2" of pitch coming my way. I might also try a 4 blade for when pulling the tube, which I did yesterday, and let me tell you, there is no comparison between then and now. The low speed planing and power available out of the turns is nothing short of incredible.

That's an awesome all around improvement for sure.

You're going to need at least 2" more pitch, like 4" if you stick with a 3 blade.

I'm a 4 blade convert, I'd have a hard time ever going back to a 3 blade prop. The 4 blade is like 4X4 for your boat. Better hole shot, lower planing speed, better all around handling, and with the right prop, nearly identical top speeds.

[67ZL1]

I'd try a 21p 4 blade first if I were you.

[CBintheNorth]

Originally Posted by Walleyedude

That's an awesome all around improvement for sure.

You're going to need at least 2" more pitch, like 4" if you stick with a 3 blade.

I'm a 4 blade convert, I'd have a hard time ever going back to a 3 blade prop. The 4 blade is like 4X4 for your boat. Better hole shot, lower planing speed, better all around handling, and with the right prop, nearly identical top speeds.

Originally Posted by 67ZL1

I'd try a 21p 4 blade first if I were you.

Thanks for the suggestions guys. I was thinking a 21 pitch 4 blade as well. May also see if they have something in a 21 or 22 - 3 blade for comparison. The staff at RPS here in Edmonton are pretty good about letting you test a prop.
If the weather stays smart I may get out for the second weekend in a row!
I'll post the results up here.

[couleefolk]

The hydrofoils depend a lot on your setup. My boat has a stingray 3 hydrofoil, and it makes the boat jump out of the water a lot better, and I can't be without it for tubing and skiers. I also switched to a 4 blade stainless prop on my 150 merc, and I gained 3mph over the same companies aluminum 4 blade prop, which I use when I know there are a lot of stumps or rocks. I only do small lakes (less than 16 miles across) so I'm not concerned about maximum speed. If I needed to go faster, I do believe that I would use a 3 blade from the research I have done. The 4 blade sure sits you back at half throttle and the tubers really feel it.