1970 18hp evenrude fast twin

[77kingfisher]

What makes this motor an 18? Is this a 15 that's carbed up or a 20 that's tuned down? I wouldn't mind getting more if I can anyone have any ideas?

Model is 18002 and if im not mistaken 18002C

 

[racerone]

This motor started life as a 22 cubic inch in 1957 and 18 hp.---------The Evinrude folks liked marketing an 18 hp model.---------The Johnson folks marketed this exact same 22 cubic inch motor and changed it to a 20 hp in 65 I think.----------Motor was upgraded with some different parts ( crankshaft ) and was also available as a 25 hp from 69 to 76.

 

[77kingfisher]

Realy ok so what kind of parts would I need to get my hands onto make that adjustment?

 

[F_R]

I'd have verify it, but as I recall the cylinder head makes an 18 into a 20. Don't bother, you won't be able to tell the difference. To make it a 25 requires pistons, head, crankshaft, carburetor, and more. In other words, sell your 18 and buy a 25.

 

[77kingfisher]

Hmm so the head has what a smaller chamber and gives it higher compression?? This gives me the extra punch that i know your right I probably wont feel. But now im curious. My other question is will my intake manifold be "restrictive" de tuning it.

 

[F_R]

No.

 

[jimmbo]

If you stick the 25 head on a 20/18 without changing the pistons the engine will self destruct from detonation. The pistons for the 25 had the wrist pin located higher(closer to the crown) in the piston and that reduced the compression, the 25 head with a smaller volume restored the compression. Relocating the wrist pin effectively changed the port timing and moved the torque curve into a higher RPM range. Crankshaft balancing was more precise on the 25 and the induction system was different. As F_R says there may have been more differences

Would you see much difference? On a very light small boat, 1 maybe 2 mph. Otherwise not likely

 

[77kingfisher]

Ok like I said now that I know its basically the same block with different guts and some other stuff Im wondering why omc didn't just go for the "light" 25 to begin with? Not that far in there R and D world?

 

[racerone]

Apparently factories make " stuff " that people will buy.----Today people buy new stuff that some of us say " Runs really nice when new, but can not be rebuilt "-----Not the same as motors designed / built in the 50's / 60's for a different mindset.

 

[F_R]

Ok like I said now that I know its basically the same block with different guts and some other stuff Im wondering why omc didn't just go for the "light" 25 to begin with? Not that far in there R and D world?

The 18 was redesigned from the 15 in 1957. It was a big step forward in it's day, running almost as fast as the old 25. And the old 25 was redesigned into a 35. So, all that stuff was state of the art in it's time. Fast forward, the 22 cubic inch 18/20hp was basically souped up to make 25 horses, mostly through compression and exhaust tuning. The tuning technology grew out of racing and didn't even exist in 1957.

 

[77kingfisher]

The 18 was redesigned from the 15 in 1957. It was a big step forward in it's day, running almost as fast as the old 25. And the old 25 was redesigned into a 35. So, all that stuff was state of the art in it's time. Fast forward, the 22 cubic inch 18/20hp was basically souped up to make 25 horses, mostly through compression and exhaust tuning. The tuning technology grew out of racing and didn't even exist in 1957.

Ok that makes sense. I suppose if you think about it that's no different than motorcycles and cars. Though I agree a lot of this new stuff today is throw away. As and automotive dealer tech I can vouch for that.

 

[77kingfisher]

Ok would a say late 70s 25 power head bolt right up to my exiting O/B?

 

[racerone]

yes it will

 

[77kingfisher]

Really and whats the difference in weight from an 18 to a 25? On the power head any way? Couple pounds or a bunch?

 

[racerone]

No difference.------The crankshaft is balanced for more durability at 6500 RPM.------Do not overthink this.

 

[77kingfisher]

Whats its rated rpm?

 

[racerone]

Right from the 73 brochure----------25 hp at 5500 rpm.-----------Range from 5000 to 6000 rpm.----------- 20 hp at 4500 rpm range from 4000 to 5000 rpm.--------Weights are given at around 86 lbs------These motors are from the 20 BC era.--------That is to say 20 years " Before computers "

 

[F_R]

And then somebody will immediately (falsely) reason, why can't I just use a smaller prop to let my 18 run at 6000 RPM? As I said, that's false thinking. A 25 will rev 1000 RPM higher than an 18-20 with the same prop, because it has more horsepower to turn it faster. Turning the same prop at a higher RPM makes the boat go faster. And as already mentioned, the 25 crank is balanced to withstand the high RPM.

 

[jimmbo]

" Runs really nice when new, but can not be rebuilt

:lol::lol::lol: but also :Cry: and :mad-new:

 

[jimmbo]

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