Crank It Up

[joehomey]

First of all I realize this is a 10 year old thread but I thought I'd give it a try anyway. I have a 1959 Fastwin 18. I have done a bunch of work to it and it runs great. That being said it is a bit under-powered when I have someone else along with me. It is running fine, just that 18hp with my boat and gear and 2 people is too much for this motor to get up on plane. It does plane with just me and runs about 16 mph.

I am a mechanic by trade so I am familiar with ways to get more power from a motor and this thread fascinates me. In the automotive world it has been common to do these mashup engine configurations, perhaps the most famous being the 383 stroker made from a chevy 350 block with a 400 crank so it is not unreasonable to think that this can be done with an outboard. The thought of combining a balanced 25 crank with my current pistons and a head from a 25 horse to increase compression makes perfect sense. However, I can not find any reliable resources that list compatibility of components or even a comprehensive list of years of manufacture and HP models offered for that year. If I were able to cobble together compatible parts and got it running well I figure the end result would be an 18 horse motor (and its relatively light weight) putting out well over 30 hp.

My questions are:
Would my 1959 block accept a 25 crank from a later year. I know from reading different places that there were changes at some point to these engines as far as bearings go so I don't even know if a crank from say a 1969 25 would fit my 59. Is there a balanced 25 crank out there that would fit? What about a 25 cylinder head? Is there a combination someone could recommend?

Are there any resources online that contain some sort of compatibility list?

Thanks in advance.

 

[GA_Boater]

joehomey Your new thread courtesy of the FORUM WEB POLICE. Please read the Help Tip on top of the page. Starting a new topic/thread is preferred over reopening a 10 year old thread.

Moved the post out of this thread - http://forums.iboats.com/forum/engi...tboards/194241-evinrude-18-hp-convert-to-25hp

I believe the answer you're looking for is in post #6 in that ^^^^ thread.

Re: Evinrude 18 hp Convert to 25hp

The 25 crank is a balanced crank. Will the 18 / 20 crank turn the same rpm? At the rpm we're talking here, no problem.

The heads are different. The 25 head is "tighter".

The pistons are different. The odd thing is that the wrist pin height is lower in the 18 / 20 piston. That means that it'll come up higher than the 25 piston.

If you put the 25 head on an 18 / 20 you better fatten up the jetting and run hi-test fuel. Otherwise you'll burn a hole in the piston. Don't ask me how I know that.

You can open up the reed stops some too.

You can also use a carb off of a 2 cylinder 40 / 50 or a 3 cylinder 60 / 65 / 70 / 75.

When it's all said and done, you still have the same gear ratio and gearcase (not very efficient). And the same style prop.

On standard type runabouts (back in the day) we had good luck using the 25 carb and playing with set up. The prop selection was kinda limited so we couldn't raise the motor a whole lot but we did find that the prop was buried a lot further than it needed to be.

Now, if you build one with a 25 crank, 18 / 20 pistons, a 25 head, the good 50 hp carb and put it on a Champion Hot Rod mid section & lower unit -- you'll have a screamer.

 

[jimmbo]

The difference regarding the wrist pin relocation mentioned by Dhadley. That would effectively change the port timing, it also required the 25 head to be tighter to maintain the same mechanical compression ratio as the pistons for the 25 remained deeper in the block, it also set the piston deeper into the crankcase, raising its compression slightly to improve scavaging. So using 18/20 hp pistons with the 25 heads is an detonation problem waiting to happen. Interesting, I learned something

 

[thatone123]

Good post - old or not. I would like to see more complex posts like this as they are very informative. The 18 does seem underpowered to begin with. Can a 25 carb be used on an 18? Probably not or it would have been mentioned.

 

[jimmbo]

Intake is different between the two as the reeds. It will bolt on the 18/20 intake but the opening is smaller.

 

[racerone]

Are you sure the reeds / reed plate / reed stops / intake manifold is different between the 20 and 25 hp models.

 

[jimmbo]

I was reading one of the archives about a year ago. There was a long post about the differences, it had pics of the intakes and the reed assemblies. I thought I read the reeds had a larger radius, so the reed plate was different too The intake off the 25 had a bigger intake hole, and it a larger plenum, upstream of the reed plate. It didn't mention the difference regarding the wrist location. Of course it was a year ago...
Service parts lists might supercede the 18/20 manifold to the 25 manifold. It would still work, the carb becomes the restriction by itself and it shrinks the number of replacement parts
Racer, just out of Curiosity what part of Ontario are you in?