HP or cc .

M

munster1967

Well-Known Member
Joined
20 May 2024
Messages
126
Visit site
Hi folks.
I'm looking at getting a slightly bigger outboard for my sib. I currently have a 2 HP 50 cc Suzuki.
The motors I'm looking at are 2.5 HP 4 strokes. . Which vary widely in their cc displacement. Ranging from 68 cc to 85 cc. Which is more important HP or cc. ?
I presume the more cc equals more power. ??
I don't want to go to a 4 HP because of the weight.
 
HP is power, and 2,5hp is 2,5hp regardless of engine capacity ... displacement will affect torque but on this size engine it will be marginal.

It is common to restrict or limit HP and therefore provide more engine variants from the same base engine. E.g. Mercury 4hp,5hp and 6hp are all the same engine, as are the 10, 15 and 20hp.

If your larger capacity 2,5hp has a bigger brother 3,5hp then it is a larger capacity (heavier) engine series and may be de-tuned to 2,5hp to fill that market niche.

Basically 2,5hp is 2,5hp regardless of capacity - IMO go for the lightest and read some reviews from the sailing press.
 
I am out of date with outboards but when I needed a new one to power my later model Redstart I swapped the old 2.2 Suzuki for the 4-stroke 2.5. As well as being more economical it had much more oomph at moderate speeds and although bulkier was somewhat easier to move around.
 
Baggywrinkle said:
HP is power, and 2,5hp is 2,5hp regardless of engine capacity ... displacement will affect torque but on this size engine it will be marginal.

It is common to restrict or limit HP and therefore provide more engine variants from the same base engine. E.g. Mercury 4hp,5hp and 6hp are all the same engine, as are the 10, 15 and 20hp.

If your larger capacity 2,5hp has a bigger brother 3,5hp then it is a larger capacity (heavier) engine series and may be de-tuned to 2,5hp to fill that market niche.

Basically 2,5hp is 2,5hp regardless of capacity - IMO go for the lightest and read some reviews from the sailing press.
Click to expand...
Ok. Thank you. So HP is far more of an indicator of power than cc. ?
 
johnalison said:
I am out of date with outboards but when I needed a new one to power my later model Redstart I swapped the old 2.2 Suzuki for the 4-stroke 2.5. As well as being more economical it had much more oomph at moderate speeds and although bulkier was somewhat easier to move around.
Click to expand...
Thank you. So you noticed an increase in power with the 4 strokes 2.5. ?
 
munster1967 said:
Ok. Thank you. So HP is far more of an indicator of power than cc. ?
Click to expand...
HP is a much better indicator (see other posts re torque). As an example, I had a 3 litre V6 car in the 1980s (Ford Capri) that produced 150bhp and it did about 25mpg. Until recently, I owned another 3 litre V6 car (Porsche Macan S) that produced more than double the power (350bhp) and had better fuel economy (30+ mpg).
 
wonkywinch said:
HP is a much better indicator (see other posts re torque). As an example, I had a 3 litre V6 car in the 1980s (Ford Capri) that produced 150bhp and it did about 25mpg. Until recently, I owned another 3 litre V6 car (Porsche Macan S) that produced more than double the power (350bhp) and had better fuel economy (30+ mpg).
Click to expand...
Thank you. That explains it perfectly. Basically the more HP the more power regardless the cc.
 
The cc of an outboard is fairly irrelevant as all are sold by their rated hp. Sometimes their cc can be somewhat elusive to find . Their weight and number of cylinders can also be items of interest.
Motorbikes are more commonly sold by their cc. Its hp can sometimes be a little elusive to find !
A 2 stroke is far more powerfull than a 4 stroke of the same cc as a 2t is firing every cycle whereas 4t every 2nd cycle.
A 4t can be much smoother, quiter and economical.
My favorite small outboard is the tohatsu 3.5 2t. Superb power to weight ratio.
 
munster1967 said:
. . . That explains it perfectly. Basically the more HP the more power regardless the cc.
Click to expand...

Exactly so. The clue is in the name HP = Horsepower.

The cc (cubic centimetres) is just telling you how big the hole in the middle of your engine is.


Rappey said:
A 2 stroke is far more powerfull than a 4 stroke of the same cc as a 2t is firing every cycle whereas 4t every 2nd cycle.
Click to expand...

Not necessarily. Each individual firing stroke of a 2 stroke is less efficient than that of a 4 stroke, generally speaking.

A 2 stroke is also, for the same power output, thirstier, far more polluting, and smelly! They have other advantages, though.
 
It's already been mentioned, but the bigger cc engines generate the same horsepower at lower peak revs. So in identical conditions they will be less noisy. Secondly well loaded at displacement speeds, the engines may not have the power to reach peak revs. Basically there comes a point when they can't turn the propellor any faster. For a bigger cc engine this point will probably be closer to its peak power, and so it can push the boat along a little faster if that's needed.

Of course you might simply prefer to slow down and not soak everyone and everything with the spray.
 
johnalison said:
My art teacher told me that he was at a funeral with a friend, and when it was over the friend said "Home, James, and don't spare the hearses". He thought it was rather funnier than I did.
Click to expand...
I looked at a video testing 2.5 HP outboards. And two got a 2.7 meter sib up on plane. The Yamaha and selva. But Honda and Suzuki could not.
James_Calvert said:
It's already been mentioned, but the bigger cc engines generate the same horsepower at lower peak revs. So in identical conditions they will be less noisy. Secondly well loaded at displacement speeds, the engines may not have the power to reach peak revs. Basically there comes a point when they can't turn the propellor any faster. For a bigger cc engine this point will probably be closer to its peak power, and so it can push the boat along a little faster if that's needed.

Of course you might simply prefer to slow down and not soak everyone and everything with the spray.
Click to expand...
Ok. Thank you. I looked at a video testing 2.5 HP 4 strokes , they tested the Yamaha, Suzuki , Honda, and an Italian make. The only two that got the 2.7 meter to plane were the Yamaha and the Italian one.They both had 85cc, the others only had 68 and 57 cc. So the higher cc outboards got the boat to travel much faster.
 
Just to add to the confusion, it is not the engine that moves the boat but the propeller and most small outboards have very inefficient propellers for displacement speeds. On the other hand electric outboards have propellers designed for displacement speeds (larger diameter flatter pitch) and therefore achieve much the same speeds with typically have the power in KW as a petrol engine. Thrust is a much better measure of suitability at displacement speeds (up to around 4 knots). However higher power outputs of petrol engines can allow a dinghy to get on the plane where the propeller starts to work as designed with its higher pitch.
 
Worth mentioning that many 2.5HP OBs are mechanically identical to the 3.5, the difference being simply a restrictor plate and a jet. In my book, that makes the 3.5 the better one to own for the odd time when you're fighting a tide.
 
munster1967 said:
I looked at a video testing 2.5 HP outboards. And two got a 2.7 meter sib up on plane. The Yamaha and selva. But Honda and Suzuki could not.
Ok. Thank you. I looked at a video testing 2.5 HP 4 strokes , they tested the Yamaha, Suzuki , Honda, and an Italian make. The only two that got the 2.7 meter to plane were the Yamaha and the Italian one.They both had 85cc, the others only had 68 and 57 cc. So the higher cc outboards got the boat to travel much faster.
Click to expand...
It is also possible that the props on the larger cc outboards were better matched to the load they were pushing.
While uncommon in very small outboards, larger engines often have a “high thrust” version available, which develops more thrust at low speed but with a theoretically lower top speed.
 
You must log in or register to reply here.

Share this page

Top