Need 59ci Merc port advice for first timer

[97yamaha]

Putting together another 59ci Merc and decided to take it up a notch and try my hand with the exhaust ports. Before dropping the sleeves back in, hoping someone can give me a little advice. According to Jennings, you can gain plenty in just widening the ports, and that these elliptical exhaust ports can be "safely" widened out to 70% of bore width. So, right now my plan was to leave the stock port height and chamfer, and just widen. Can y'all confirm this or is widening alone even worth doing? Not trying to build a race motor, just a fishing motor. Gonna be running 60 carbs with steel reeds as I seem to have better luck with those over fiber. Thanks guys

[FLATBTM]

I must be the only guy outside of Louisiana with one of these engines haha. Can't help ya on the porting. I'm having to pay that done, but I like learning all I can on these so I'll subscribe. I've been trying to get in touch with the Sabine River Gang down there. Those guys ain't on the forum much anymore, but they pretty much wrote the book on making 100hp+ with these 59ci engines.

[97yamaha]

I must be the only guy outside of Louisiana with one of these engines haha. Can't help ya on the porting. I'm having to pay that done, but I like learning all I can on these so I'll subscribe. I've been trying to get in touch with the Sabine River Gang down there. Those guys ain't on the forum much anymore, but they pretty much wrote the book on making 100hp+ with these 59ci engines.

Yeah man, people are very secretive with info on these. I’m struggling trying to find info on these motors and how they’re getting so much power out of em. So I’ve read through the Jennings book and the Graham Bell book several times. Sense the head and block is all one cast, there’s not much to do to get compression up that I can think of. You can’t really tune expansion chambers on basic outboards, so all the power has to be in the porting and intake modifications. I’m not confident enough in myself to mess with port timing so I was going to play with port width first. Not trying to get one of those 100hp SRG motors yet, just trying to get more than a 60 mod 40 Merc.

[FLATBTM]

You get some custom pistons to get that comp. up. The SRG was using some wisecos that someone was making. Ill try to find that info. If I'm not mistaken they are readily available and put things around 160psi

[97yamaha]

You get some custom pistons to get that comp. up. The SRG was using some wisecos that someone was making. Ill try to find that info. If I'm not mistaken they are readily available and put things around 160psi

That’d be good info if you figure it out. I figured they were using some secret pistons to get more compression. I know standard wisecos for these don’t up compression so I was assuming it was some they were having made.

[FLATBTM]

Yea it's a goofy looking think. Like a reverse dished top. Found this link, but I will keep looking.

http://www.********************/forums/showthread.php?18347-Any-up-date-on-high-compression-pistons-in-59-cu-In-Mercury/page2

[FLATBTM]

Well I guess I gotta try PM'ing that to ya...

Man, hell. This site won't even let me pm you an outside link.

[FLATBTM]

Look up Jeff Dunn on facebook. He has been making the pistons for a few years now.

[97yamaha]

Look up Jeff Dunn on facebook. He has been making the pistons for a few years now.

That makes sense. I’ve heard of Dunn pistons but never knew much about em.

[FORBESAUTO]

Haven’t played with those motas yet, and y’all right most wont talk much about these. On the compression deal, I know some of the guys around here running those things are raising compression by welding in the combustion chamber and machening them back down. And they do run a ton of compression on them. Typically the exhaust is your weak link and anything you do to increase flow helps, this has many variables though depending on engine design. From a general porting experience I find widening the ports at the lower end / widest port of the port increases mid range power, which is good for a fishing boat. Usually not a jaw dropping increase but usually helps, again depends on efficient the motor was to begin with. If ya straighten the port out at its highest point without raising overall height you will see more of an increase in upper rpm power and possibly loose a little on low end. Good rule on how far you can flatten top off is around the same length as the transfer ports. Again this is in general and not specific to this mota. I generally widen ports to at least the casting of the block though.

[Dave S]

I think they could be cleaned up in front of the reeds......

[97yamaha]

Haven’t played with those motas yet, and y’all right most wont talk much about these. On the compression deal, I know some of the guys around here running those things are raising compression by welding in the combustion chamber and machening them back down. And they do run a ton of compression on them. Typically the exhaust is your weak link and anything you do to increase flow helps, this has many variables though depending on engine design. From a general porting experience I find widening the ports at the lower end / widest port of the port increases mid range power, which is good for a fishing boat. Usually not a jaw dropping increase but usually helps, again depends on efficient the motor was to begin with. If ya straighten the port out at its highest point without raising overall height you will see more of an increase in upper rpm power and possibly loose a little on low end. Good rule on how far you can flatten top off is around the same length as the transfer ports. Again this is in general and not specific to this mota. I generally widen ports to at least the casting of the block though.

Thanks for the info man. That’s alongside the info I’ve gathered. If you’re going to leave port height alone. Try and widen the top without totally flattening it out. Where my confusion is, is with my measurements. I’m getting an exhaust port width of 43mm which is exactly 50% of the 86mm bore. In order to widen out to the Jennings’ recommendation, I’d have to increase up to <60mm. That’s a huge increase. I took a junk sleeve and did a test cut out to 53mm but afraid to go too wide. Hoping someone can confirm these numbers.411432

[97yamaha]

I think they could be cleaned up in front of the reeds......

Yeah I was wondering what could be done up front other than reeds. Which, so far steel reeds seem to run better than boyesen or ccm. I might just leave the steel reeds and try to clean up in front of em.

[FORBESAUTO]

https://www.screamandfly.com/album.php?albumid=2761&attachmentid=380718

Just finished building a 2.5 we opened up the ports about like that, gotta look at my notes but think I’m right at 70%. Had to cut a good bit of aluminum in block to get em that wide. Still breaking motor in, got 12 hours on it with no issues so far. Motor has real good midrange not sure yet how it’s gonna do in upper rpm. Rings are seating in good with no issues. You can expect to rering it more often than normal because they are gonna more around more than normal.
Wish I knew more about that motor. Ain’t but one way to find out, and that’s start playing and trying things and record outcome. It won’t take ya long to get a feel what the motor responds to. Keep us posted. If ya wanna start playing with port timing, that info is like pulling teeth. I suggest get ya a degree wheel and record as much info about ports as you can a stock setup. You can study that a get an idea the design of it from Mercury’s standpoint. Then you can get a game plan of what ya want to change to achieve your goal. Start making small changes and note how it responds. May take ya a few tries, but you’ll figure out what works and what don’t.

[97yamaha]

https://www.screamandfly.com/album.php?albumid=2761&attachmentid=380718

Just finished building a 2.5 we opened up the ports about like that, gotta look at my notes but think I’m right at 70%. Had to cut a good bit of aluminum in block to get em that wide. Still breaking motor in, got 12 hours on it with no issues so far. Motor has real good midrange not sure yet how it’s gonna do in upper rpm. Rings are seating in good with no issues. You can expect to rering it more often than normal because they are gonna more around more than normal.
Wish I knew more about that motor. Ain’t but one way to find out, and that’s start playing and trying things and record outcome. It won’t take ya long to get a feel what the motor responds to. Keep us posted. If ya wanna start playing with port timing, that info is like pulling teeth. I suggest get ya a degree wheel and record as much info about ports as you can a stock setup. You can study that a get an idea the design of it from Mercury’s standpoint. Then you can get a game plan of what ya want to change to achieve your goal. Start making small changes and note how it responds. May take ya a few tries, but you’ll figure out what works and what don’t.

You're right. Ultimately that’s what I need to do. Was just hoping I could get in the ball park the first try. I’d reallllllly be interested in knowing how your 2.5 turns out with those 70% ports.

[FLATBTM]

Sending you a PM with Mr. Dunn's number. He will take care of you if you want to bump that comp up with some customs pistons.