From 18 to 25 HP, the only difference?

[restornator]

I've been looking at the parts diagrams from a 1969 18hp and a 1969 25hp. Powerheads, intake, carburetor, gearcase and gears are identical. The only difference i see is the piston. 25hp part no. 383524-6, 18hp part no. 383476-8

Even standard props are the same.

How does the 25 put out 1000 extra rpms and 7hp?

does it really just boil down to the piston?

 

[tashasdaddy]

Re: From 18 to 25 HP, the only difference?

there is a lot more to it. too much to go into.

 

[restornator]

Re: From 18 to 25 HP, the only difference?

there is a lot more to it. too much to go into.

well it doesn't appear to be that in depth, crankshaft and connecting rods all the same. same power head, same displacement, bore, and stroke. You see why i am puzzled? how does the 18 become rated 25hp at 5500 rpms?

 

[1946Zephyr]

Re: From 18 to 25 HP, the only difference?

From what I was told, it's in the carburation, pistons and head. Other than that, I can't see any differences.

 

[rjezuit]

Re: From 18 to 25 HP, the only difference?

I believe some exhaust tuning also. The motors are rated at different RPM's. The HP would probably be similar if rated at the same RPM. Usually HP is a function of the ability to put the fuel mix in and out plus the efficiency (compression ratio) of the system. Rick

 

[bob johnson]

Re: From 18 to 25 HP, the only difference?

I think a bigger question is how they can ask more money for the exact same motor...just by calling it more HP... and even if it does have one or two DIFFERENT parts... those parts dont cost any more or less money to make they the other hp's version of those parts!!

looks like compression on your examples
heads and pistons combos have historically made plenty of difference in motors!!



bob

 

[F_R]

Re: From 18 to 25 HP, the only difference?

The 22 cubic inch powerhead was made in three horsepowers. 18, 20, and 25. They are NOT alike. The differences are mainly in compression, and they did it by using different combinations of pistons and cylinder heads. You can't just slap a 25 head on an 18 and make a 25 out of it. In addition the 18 crankshaft is part number 315286 and the 25 crankshaft is part number 383182. The difference??? The 25 is balanced to withstand the higher revs.

Yes, the prop choices are the same, but remember, the 25 turns at higher RPM for the same load because there is more horsepower to turn it.

 

[restornator]

Re: From 18 to 25 HP, the only difference?

The 22 cubic inch powerhead was made in three horsepowers. 18, 20, and 25. They are NOT alike. The differences are mainly in compression, and they did it by using different combinations of pistons and cylinder heads. You can't just slap a 25 head on an 18 and make a 25 out of it. In addition the 18 crankshaft is part number 315286 and the 25 crankshaft is part number 383182. The difference??? The 25 is balanced to withstand the higher revs.

Yes, the prop choices are the same, but remember, the 25 turns at higher RPM for the same load because there is more horsepower to turn it.

Thanks. I thought the difference in pistons initially was to create more compression, but didn't know how it resulted in higher rpm's

 

[Dhadley]

Re: From 18 to 25 HP, the only difference?

The 18 E-rude and the 20 Johnson are identical. The blocks are the same as the 25. The 18/20 piston has a lower wrist pin height. The 25 crank is factory "balanced" hence the higher rpm. The 25 is rated for 25 hp at a higher rpm. The 25 head is different.

Putting a 25 head on an 18 will cause a hole in the piston if you use anything less than 93 octane, maybe even if you use 93.

No need to ask how I know that.....