Evinrude 18 hp Convert to 25hp

[perkdp]

I have a 1973, 18hp Evinrude.
I put a 25 hp caruretor on it.
Does that make it a 25 hp now ?
What are the differences in the 18 and 25 ?
Is there any danger to the engine by running the 25 hp carb on the 18hp power head ?

 

[steelespike]

Re: Evinrude 18 hp Convert to 25hp

Certainly more than just the carb.We're talking close to 50% increase in power.My guess is porting,manifild and reeds are changed as well as the timing.
Someone should be along with more specific info.
Generally most of these type changes are not reccomended.Besides the expense there is the chance of creating a Frankenstiein motor.

 

[F_R]

Re: Evinrude 18 hp Convert to 25hp

The 25 has a different cylinder head and different pistons. (makes different compression ratio) Sorry, you didn't make your 18 into a 25. If fact, it is possible it won't even perform as well as it did before.

 

[jimmbo]

Re: Evinrude 18 hp Convert to 25hp

Well the 18 is rated @ 4500 rpm, the 2500 is rated @ 5500 rpm. The carb, and block are definently different. The reeds and intake manifold, cylinder head, as mentioned before, could be different. Max ignition timing is usually the same being that once a certain timing is reached there isn't any advantage for advancing it further even though rpms are increased, however carb to spark synchronization might be different.

 

[F_R]

Re: Evinrude 18 hp Convert to 25hp

A carburetor works by the fact that air is flowing through it, and the speed at which the air is flowing. An engine running at 4500 rpm pulls a certain amount of air through the carburetor. An engine of equal displacement running at 5500 rpm pulls roughly 22% more air through the carburetor. So the venturi (hole through the carb) has to be larger for the increased volume of air to flow at the same speed. Put the larger venturi on a slower or lower displacement engine reduces the speed that the air flows, disrupting the efficiency of the carburetor. That's why just slapping on a bigger carb doesn't work. If it did, you could just put on a 100hp carb and fly!

OK, so you say the bigger carb will let it turn 5500 rpm. Not so. The horsepower isn't there to make it go 5500rpm because you didn't change the other items.

Now, it is true that in some cases, the different horsepower ratings are achieved by just taking a certain engine and putting a smaller carb or restrictor on it. But that is not the case in the motor we are talking about here.

BTW, I spent considerable time checking the differences on this motor and they are carb, pistons, and cylinder head. The block and exhaust are the same.

 

[Dhadley]

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.

 

[R.Johnson]

Re: Evinrude 18 hp Convert to 25hp

My son, and I have both a 57, and 58 18HP engine's. We also run a 1975 25HP for our river fishing. We found that we had to run the smallest prop we could get our hand's on to make the 25 equal either of the 18's.That was after a total overhaul. It is a good little engine for our use, but nothing to get excited about when compared to an 18. A small, light performance boat would propably be a different story. Dhadley know's what it take's to make these little engine's behave. Maybe he will be along

 

[perkdp]

Re: Evinrude 18 hp Convert to 25hp

The 25 carb fits and runs. But seems to lug some at lower rpms and add rpms at the top end. I didn't tach it but it seems that nay extra performance is coming solely from top end rpm .

My main concern is whether the 18hp crank, pistons and head will hold up to 25 carb.

I also sense the opinion that the stock 18 will outperform the 25 for general use.

(this motor is on a 1548, G3 Aluminum, semi v)

Thanks....

 

[F_R]

Re: Evinrude 18 hp Convert to 25hp

The 1973 18hp motor is factory approved to run at a full throttle RPM of 4000-5000. Since you are running 18hp powerhead parts, keep it within that full throttle rpm range and you will not hurt anything. Toward the upper part of the range is better.

FYI, the 25 is rated for 5000-6000 full throttle rpms. It can stand it because it is better balanced.

 

[R.Johnson]

Re: Evinrude 18 hp Convert to 25hp

Put the 18HP carb. back on,the 25 is doing nothing. Maybe making it a little hard to start

 

[racerone]

I believe the 25 crank will fit all the 22 cubic inch blocks.----There are other websites that lists what years parts fit / are the same.-----They do not like seeing the names posted on here.