Hopping up 22 cube Hurricane

[Fastjeff]

Hello!

I'm new here, but not to hydros--I built and ran my first one 45 years ago! A few years ago I built a 'dink' (dingy) to hang off our cruiser, powered by a Merc 110. I soon got the bug, so off went the 9.8 hp and tiller, replaced by a Mark 25. I then got a Merc 200 (20 hp.) and installed a wheel and remote controls. Yeah! Since then I have been challenged to a race by a buddy of mine with his 14 foot inflatable powered by one of those new design 30 hp. Merc (2 strokes of 39.6 cubes). The race will take place next Spring, so I've got all winter for improvements.

I built an adjustable transon plate to move the motor up and dpwn, and installed Smart Tabs to reduce drag. I also got another Merc 200 to use for my 'race' motor. Pulled the LU apart and shaved the teeth off the reverse gear (to gain more rpms) and closed off the top 3 of 7 water inlet holes. I also enlarged the lower 4, to allow running the LU higher up. Finally, I got the stock 2 blade prop repitched from 11 inch to 13 with some cup.

Now to the power head...I got a good running spare unit to play with, a 1968, but I'm not sure what mods will work. I've been told to open up the reed cage/ crankcase and install a larger bore carb, which I will do, but on how to port the thing...I'm at a loss. Some say porting will not help without a header, which I don't care to use (close neighbors). Finally, I'm not interested in mid-range power, just all out top end horsepower.

Any and all ideas will be appreciated!

Jeff

[mercguy]

Hello!

I'm new here, but not to hydros--I built and ran my first one 45 years ago! A few years ago I built a 'dink' (dingy) to hang off our cruiser, powered by a Merc 110. I soon got the bug, so off went the 9.8 hp and tiller, replaced by a Mark 25. I then got a Merc 200 (20 hp.) and installed a wheel and remote controls. Yeah! Since then I have been challenged to a race by a buddy of mine with his 14 foot inflatable powered by one of those new design 30 hp. Merc (2 strokes of 39.6 cubes). The race will take place next Spring, so I've got all winter for improvements.

I built an adjustable transon plate to move the motor up and dpwn, and installed Smart Tabs to reduce drag. I also got another Merc 200 to use for my 'race' motor. Pulled the LU apart and shaved the teeth off the reverse gear (to gain more rpms) and closed off the top 3 of 7 water inlet holes. I also enlarged the lower 4, to allow running the LU higher up. Finally, I got the stock 2 blade prop repitched from 11 inch to 13 with some cup.

Now to the power head...I got a good running spare unit to play with, a 1968, but I'm not sure what mods will work. I've been told to open up the reed cage/ crankcase and install a larger bore carb, which I will do, but on how to port the thing...I'm at a loss. Some say porting will not help without a header, which I don't care to use (close neighbors). Finally, I'm not interested in mid-range power, just all out top end horsepower.

Any and all ideas will be appreciated!

Jeff

Jeff, all the info David Woodall gave you on the other website is 100% correct. Forget about porting, etc on that motor if you are not going to run open tuned exhaust), get a racing lower unit and props and test! You have to do mods that work with each other. Just putting on a larger carb and opening up the reeds will not do much with stock exhaust and you will probably slow down!

[Fastjeff]

Surely...

....the larger carb and some intelligent porting will do SOMETHING! More air in, more power out. Right?

Jeff

[sam]

Only if those components are the motor's choke points. If they aren't - changing them will probably make less power rather than more.

Most outboards have monsterous ports from the factory. KA carbs are probably already optimal, "opening up" and polishing the reed cage might make you feel better, but probably won't make the boat any faster. Darren and David are already pointing you in the right direction.

If you are brave you could drop a Franklin on Ted Miller and have him screw in some pads to increase the compression. Personally I'd spend another chunk after the screws and have them welded. 55 years ago O.F. Christner gave up on screwing in pads because the screws fall out and the pads trash the motor.

[mercguy]

....the larger carb and some intelligent porting will do SOMETHING! More air in, more power out. Right?

Jeff

more air in also means you need to get more air OUT!! That is where the "tuned pipes" come into play..........forget about porting, as you are not going to be turning enough RPM's to notice and you will suffer at low end! Look at the converted 20H...............tuned exhaust!

[seanp3]

Daren,

Maybe Jeff could use one of your Yamato's

[Fastjeff]

Thanks, all

I like the "choke point" analogy--that makes sense to me. What I can't understand is how all that millions of 'factory' race engines, the KG-7H, Mark 20H, etc. etc.--all without a header--made more power with porting mods. And you KNOW they got ported, legally or not.

Don Muncie suggested improving crankcase compression, which I'm looking into (got a trick ay to do it). I'll forget the bolted in pads idea, especially since I located a pretty good TIG welder (and just up the street).

Again, thanks all; and I'm not trying to be thickheaded on the subect of porting--I just wanted another opinion.

Jeff

[sam]

I'm not sure the KG7H ports were different from the fishing KG7, the Mk30 & 30H ports are the same. The .060 difference in porting on the Mk20H & 55H is not enough to overcome having the wrong prop or being a single tilt pin hole off. If anything it only allowed better accelleration off the turns to top speed because the motors could pull power for another thousand rpm. You are leaving off the fact that these motors all had padded heads. Raising the ports without increasing the compression on a 2 stroke usually lowers power (less closed exhaust port compression time). When replacement H blocks starting getting hard to find some racers WON with unported stock blocks - prop and set up means more than a fingernail's worth of grinding

[Fastjeff]

Got yah! I'll be leaving the things alone.

Tanks,

Jeff

[mercguy]

much better off!

Daren,

Maybe Jeff could use one of your Yamato's

good idea Sean!

[Fastjeff]

One more point before I drop the subject....Back pressure. Merc designed the Merc 200 as an all-purpose motor and not as a race motor. To keep the peace (so to speak) the exhaust system was designed to be quiet as possible. (There's a weird dingus in the top of the tower with odd length tubes coming out of it--pure Rube Goldberg looking--to kill resonaces.) And they succeeded pretty well.

Now, what happens if I reduce back pressure? I can easily make a hole or holes in the rear of the tower, or in the sides just above the cav plate (toilet bowl fashion). Would this increase air flow through the motor--assuming this is the "choke point"? And (rubbing hands with glee) if so, would some intake porting do any good NOW?

Jeff

PS: I recall cutting off the rear of the tower, above the lower unit on my old KG-7 (everyone did!), and it went better.

[sam]

If there is sufficient back pressure, rpm is restricted almost the same as an ignition rev limiter. If you are not hitting the rpm wall (has to be proven to not be the prop load, though), you do not have "too much back pressure" and exhaust relief is not going to change any thing.

The long forgotten drag racing instructions for the Scott McCulloch racing motors called for exhaust back pressure that just barely did not restrict rpm. The instructions said to set up the boat & prop stuff first and then with an accurate tach record the max rpm. Then restrict the exhaust by closing the normal exhaust opening and drilling a new smaller hole - opening it 1/8" or less at a time until the original rpm was again achieved. Any prop or set up change required a complete retest of the back pressure set up. They claimed that this just barely restricted exhaust gave an accelleration improvement.

Outboards come from the factory with super generous exhaust openings because the manufacturers have no direct control over what boat they will be put on, so they must err on the safe side. Same thing for compression, ignition timing and carb jetting - all on the safe side.

[Fastjeff]

Sam, you read my mind. I'm planning on cutting a rectangular hole in the back of the housing, low down. Necxt, I'll make up a series of bolt on plates each with a different size hole. I'll then do a bunch of speed runs and see what happens.

Jeff

[sam]

On top speed you won't see any increase, only a decrease when you are too small. Keep your speed runs VERY short. I doubt there would be any benefit with a smaller motor like yours - aren't you going for top end speed?

[Fastjeff]

Top speed is ALL I care about in this case, even if I have to sit on the bow, feet dangling in the water, to get the sucker on plane.

I have a race scheduled next Spring, see, and the bragging rights are huge! Unless I can sneak a hot motor inside that Merc 200 cowling (and not get caught) I will have to buy the beer for a month! (Like I said: this is serious stuff!) So,any mods that can pick up a bit of speed--without "obvious" cheating (hee, hee) is what I need. Hell, I don't care if the bloody thing blows the hell up at the end of the run, long as I was out front before it does.

Jeff