modifications,where to find info

[dirtbag]

I have a 73 18hp evinrude,and a minimost,I would like to get the most I can out of the engine,I have a 18pitch prop comeing for it and I would like to modify it so it can push the prop ,I cant seem to find much info on modifying outboards,can anyone steer me in well any direction(aside from burning the minimost,or useing the engine as an anchor) because I am lost

[Mark75H]

First there isn't much out there on modifying motors like yours because it is by far easier to use a different motor that makes more power, that would be something like a Konig or Yamato. Second, more power will make your boat so much less safe that it is not a good idea anyway. Sorry.:(

[dirtbag]

umm guys are running 30hp on these thats about what I would like, "safe" is no matter,the same engine was sold in a 25hp model ,which would be nice,I am more just asking where to start,I am ready to start going through and porting it and put a larger carb on.just thought I might ask to see if someone could give me a basic idea of what to shoot for,instead of running blind,I imagine I have a rev limiter how do I disable this,I realize I might be looked at as a clown with my boat and motor on this sight but I am curious to full race mods to the motor and cant seem to find any info ,this boat world is a bit different than most everyother toy I have had anything ealse seems easy to find info ,but boats it seems are all highly guarded secrets,is this the way or am I missing somthing,how do guys get 45mph out of a 9.9,what do they do to them aside from props

[Mark75H]

OK, I double checked the racing rules on your motor in the Mod division ... there isn't any special information on porting your motor because porting isn't allowed, ditto on raising the compression. There aren't any "secrets"

The thing that gets 45 mph from 6 hp in J class is the more refined boat and the special racing lower unit (gearbox). With the restrictor out and running the full 15 hp they run 55+ Again, the magic is the lower unit, not the powerhead.

[dirtbag]

I guess my question is before I do anything to the engine, what people here might suggest to get more performance out of it, keeping costs down to a minumum as this whole project has been done rather resonably so far

[tthibodaux]

I guess my question is before I do anything to the engine, what people here might suggest to get more performance out of it, keeping costs down to a minumum as this whole project has been done rather resonably so farIf you were going to change the down housing and lowerunit,(for racing purposes) you could do alot of port work and deck the intake side of the front half. (For crankcase pressure) along with cutting windows in the pistons to match the ports. BUT you would be wasting time and money with a gear shifting lowerunit.

[dirtbag]

is there anything I might do to the lower unit to open it up,I had the thought to trim off the exhaust spout comeing down behind the prop and trim the exhaust back so when it gets up on plane its dumping into free air,and whats the largest carb I could resonably get away with

[tthibodaux]

is there anything I might do to the lower unit to open it up,I had the thought to trim off the exhaust spout comeing down behind the prop and trim the exhaust back so when it gets up on plane its dumping into free airThe biggest thing it's gonna make is noise. Being realistic, 25hp and 30 to 35 mph is what your working with. :(

[dirtbag]

well then I would be happy,I do have a mini nitrous system w/5 and 10hp boosts,guys have gps these boats 40+mph,I am shooting for 50 with nitrous it might be unrealistic ,but one has to dream,

[tthibodaux]

well then I would be happy,I do have a mini nitrous system w/5 and 10hp boosts,guys have gps these boats 40+mph,I am shooting for 50 with nitrous it might be unrealistic ,but one has to dream,Now your getting out of my league. Naturally asperated engines are the only way I fly.:D

[Skoontz]

Assuming you know how to install the nitrous, I would recommend you start stocking pistons, rings, engine blocks and rods. Nitrous is a great bolt on tool to make alot of power with the push of a button, but, the engine has to be completely remade to handle the extra HP the nitrous makes. Ever watch the movie Madison? As cheesy as they made the pushing of that nitrous button, there is alot of truth to what they over dramatized when they blew off the sponson. Having been at that race with my dad in 1971, I was dissapointed in how they made the movie, but it was still fun to see all those vintage boats.

Mark 75 said what I would have. Until you do something below the water line (gearbox) you are not going to see a great gain from the powerhead. Something else. Unltil you do a great deal of work up top, that motor is never going to swing an 18" wheel and get any kind of respectable RPM. We had a 25 on a 10' GW Invader years ago and Michigan Wheel recommended a 17 as a box stock package. We could not ever see beyond 4800 RPM when Red line was 5500. It was so doggy out of the hole it took nearly an 1/8 mile to plane off and lugged very badly on the turns. There is a hole thread here on
OMC 25 HP that you would benefit reading from. Check it out.

[smittythewelder]

Dirtbag, don't feel out of place here with your Minimost. A lot of the older guys had boats like that to start with.

If you MUST hop up that nice fishing motor, see if you can get a little more torque, so you can swing a prop with a lot of pitch. First do web-search on the subject of squish (also try it under quench, squishband, quench area, and compression). Check the various 2-stroke sites (look for Kevin Cameron, Dr. Dirt, MacDizzy). What you'll find is a way to increase compression without causing detonation. This involves milling the deck surface of the block, or head, or both, so that the piston has about .035" clearance from the head at the tightest point. You might have to use the high-octane pump gas. If you disassemble the engine, you could lightly bevel the sharp edges around the ports, but do not knife-edge them and don't open them up. If you disassemble the carb, you could bore the venturi on a lathe, opening it up maybe 1/16", but then you have to re-jet unless you have adjustable needle valves on that engine. You could experiment with spark timing. You should go at least one heat range colder than the standard spark plug.

Have fun.

[dirtbag]

right on this is what I am looking for ,so I have a old lark 40 the carb looks like it will bolt up, would that be foolish,and what could I do to my bottom end ,or should I just attack it with a hole saw ,ha ha,on rc boats they run a tuned exhaust and put the end right below the water,or run a baffle,I am guessing I would have a hard time finding a race lower unit or would I ?

[Mark75H]

Racing lower units are easy to find, but expensive

[tthibodaux]

Racing lower units are easy to find, but expensiveWouldn't he need to find the whole down housing? What type of gearcase is there for a stock midsection? I have a B1 complete but I dont think it would work with a tiller handle.

[Mark75H]

Correct, that makes it even more expensive; unless you can do it yourself. Like some people say ... speed=$ squared

[Skoontz]

For some things, it's the tinkering and not the purchase that is the fun. But, I agree David L6 on this one.

For the carb question, the 18-20-25 manifold studs are about 3/8" too narrow, and, the inlet hole in the manifold would need to be opened or it would act as a restrictor plate. Second, size. 22 cu. in. with the same RPM as you would have with a 36 cu. in. You would have rejet the low end, and increase RPM on your 18 by 40% in order to efficiently burn all the juice that the 40 HP carb would throw into the engine. To do that, the crank and rods would need replacement, forged units put into their place, preferably H beam rods. Pistons also might need and probably would need forged units.

Dollar and cents of the deal, plan on $1,500 for a crank, $500.00 per rod, and, whatever you can work with pistons. Then you are going to adapt a hot dog gearbox. Cost of the hot dog, at least a grand, then $500.00 a prop, and, and morped midsection, or custom made section. Plan on at least a grand. Then, you have a motor that at best, will need alot of fine tuning and dedication to dial in, when you cqan just find a very decent motor to do what you need to do for the combined cost, that will need very little to do what you want to achieve.

I'm not telling you not to do what you are inquiring about, matter of fact I'm intrigued by your interest. Just trying to lay out the carpet you will be walking across. I've been playing with engines since I was about 3, I leearn every time I read or touch something, and I could see this project taking me a considerable amount of time, so, if you have limited mechanical abilities, or are unsure of how to proceed, I would probably start you playing with a smaller project.

[smittythewelder]

I haven't looked at a motor like his in a long time. Is it like the older OMCs, where the water pickup is a screened-off slot in front of the exhaust stub?

Dirtbag, you can see that nobody here is very optimistic about your chances of waking up that engine very much, and generally expect you to wreck it as a useable fishing motor, and maybe render it permanently non-functional. But so what? It's your motor, and so far as I know it has no collector value, so go ahead and carve on it! You'll have some fun, maybe learn something. But don't expect us to spend much time helping you on the project, or to give you specific directions. YOU have to do the work, study the manual, and read the books and magazine articles and website stickies on engine hop-ups. "Pay your dues," in other words. And come to some races! Get a better idea of what we're about. And good luck! You can have a lot of fun with a Minimost.

[dirtbag]

this is what I was looking for just opinions,thanks for the info ,it sounds like I am best to build another boat and find a racing engine,that has been the lingering thought for awhile

[jeff55vDSH]

What are the differences between the 25mod OMC and the 18hp powerhead? I would think the two powerheads are similar in design. Therefore, what's good for the 25 should be good for the 18. I think the little 25hp powerhead is popular in the 25mod class. There should be a wealth of info to be found about the 25mod OMC. Can anyone here help direct this man to an engine builder for the 25mod class?
If you stick with the stock towerhousing and transom height, Do not cut the exhaust snout off. You will lose power. The snout runs below the waterline and the passing water creates a vaccuum. Thus, "sucking" the exhaust from the motor. The power gain from this action outweighs the drag caused by the snout. Take it from me, I learned this lesson already on a stock KG-7 Merc. Unless of coarse you are able to jack the motor up on the transom. Then the lesser drag from the lower unit will FAR outweigh any powergain from that vaccuum exhaust action. I know it sounds strange, but it's true.
Best of luck to ya with your little boat! Have fun! :)

[tthibodaux]

What are the differences between the 25mod OMC and the 18hp powerhead? I would think the two powerheads are similar in design. Therefore, what's good for the 25 should be good for the 18. I think the little 25hp powerhead is popular in the 25mod class. There should be a wealth of info to be found about the 25mod OMC. Can anyone here help direct this man to an engine builder for the 25mod class?
If you stick with the stock towerhousing and transom height, Do not cut the exhaust snout off. You will lose power. The snout runs below the waterline and the passing water creates a vaccuum. Thus, "sucking" the exhaust from the motor. The power gain from this action outweighs the drag caused by the snout. Take it from me, I learned this lesson already on a stock KG-7 Merc. Unless of coarse you are able to jack the motor up on the transom. Then the lesser drag from the lower unit will FAR outweigh any powergain from that vaccuum exhaust action. I know it sounds strange, but it's true.
Best of luck to ya with your little boat! Have fun! :)Depending on the year there are cubic in differances. As long as the down housing is stock, no amount of port or exhaust work is going to help. We could build a hell of a race motor for a hydro or runnabout but he's not going to be able to run it at the lake due to the elbow and horn for one thing. Those powerheads are just about maxed out stock for local lake play purposses.

[Mark75H]

. I think the little 25hp powerhead is popular in the 25mod class. There should be a wealth of info to be found about the 25mod OMC. Can anyone here help direct this man to an engine builder for the 25mod class?
It is very popular, but Mod does not allow porting the OMC in 25, just adding exhaust, I checked the rule book already and posted that higher up on this thread :(

[beaver]

25 hp head, carb or later mod 80& UP 75 CARB intake,exhaust from a 25. cut off exhaust on l/u and plug it up open it up right above where it was. put h2o pick up in front. look on the powercat web site go to pic and look at my 25.BEAVER TYLER

[RichardKCMo]

Of that OMC 22 ci i mean lot of nice boats tho', but there old too,well oh well.

[JohnsonM50]

What are the differences between the 25mod OMC and the 18hp powerhead? I would think the two powerheads are similar in design. Therefore, what's good for the 25 should be good for the 18. I think the little 25hp powerhead is popular in the 25mod class. There should be a wealth of info to be found about the 25mod OMC. Can anyone here help direct this man to an engine builder for the 25mod class?
If you stick with the stock towerhousing and transom height, Do not cut the exhaust snout off. You will lose power. The snout runs below the waterline and the passing water creates a vaccuum. Thus, "sucking" the exhaust from the motor. The power gain from this action outweighs the drag caused by the snout. Take it from me, I learned this lesson already on a stock KG-7 Merc. Unless of coarse you are able to jack the motor up on the transom. Then the lesser drag from the lower unit will FAR outweigh any powergain from that vaccuum exhaust action. I know it sounds strange, but it's true.
Best of luck to ya with your little boat! Have fun! :)

The 18s are great runners, good power & dependability but I think they are ported non radical for those charicteristics, A 25hp 22ci would bolt on and go faster but all the others that say Lower Unit are right reguardless of the powerhead 50 is optimistic at best. Ive tried a pals mini max 1nc and splashed then stuffed it, dont know if theyre alike tho.
There is a releif plate on the lower unit, remove it , copy it onto a 3/16 - 1/4 pc of alum.. drill it out then drill 1 more hole where theres room, tap a 1/2 pipe thread in and get a fitting and a tube. Fasten the tube to the transome securely and run a hose between and you now have a substitute low water pick up so you can raise the motor. You can lose alot of weight off the motor if you want too, [cowling and pan] bring it down to around 65 from 85. If you want to put a 25hp carb on just match the manifold hole to it and itll work if you look at my posts Ive written some crap on these carbs B4. A bigger one than that probly wont do it. I rebuilt a 22ci 25, enlarged the ports 1/8 then traded it to a pal who put it on a merc 1 to 1 Qwicky he did 60 in hydro [ lucky on the ports Im sure] but as is you can hope to get 7 or so mph. ;)