Mercury 15hp to 18hp

[Sykes705]

I have just taken delivery of a new mercury 15hp 2 stroke.

Reading the manual I have noticed that all the specs are the same as the 18hp.

Can I convert the 15 to an 18?

[Fenlander]

Do you mean as in a brand new one? If so I assume it is based on the Tohatsu. In the Tohatsu range the 18 and 15 seem identical except the 18 has a larger bore hence a larger capacity.

Oddly the currently available new 15hp Mercury 2-stroke called a Super 15 seems to have the larger bore and capacity of the Tohatsu 18.

Is this the outboard you have... a brand new 15 with 294cc?

Edit: I've never seen a 2000 on Mercury 2-stroke in 18hp rating.... only the 15. I've just looked to find a USA manual for the Mercury 9.9/15/18 from 2008 which are obviously Tohatsu clones and like Tohatsu the capacity of the 18 is larger.

Puzzling until we know what it is you have.

[Sykes705]

Yes it is a brand new one, for commercial use.

Have just checked the manual again and i missed the difference in bore. it says that the bore/stroke is 55x52 for the 15 and 60x52 for the 18 but the capacity of both is 294cc!

[Fenlander]

Obviously a misprint on the capacity... so no the upgrade isn't possible really.

[RichardinNZ]

I was told by my local Mercury agent that my Tohatsu 18hp was identical to the Mercury super 15hp i.e. including same power.

[Fenlander]

It would be interesting to know what hp/kW is on Sykes OB data plate. I can't think its output would be any different to that stated.
What I could believe was that the super 15 was the same capacity as the 18 with a restrictor or smaller carb.

[RichardinNZ]

You could be right...!
https://www.tradeboats.com.au/tradea...outboard-motor
https://www.tradeboats.com.au/tradea...d-motor-review

[Fenlander]

What great reviews, so much detail. So yes it seems if the OPs engine is a super 15. then upgrade might be possible.

[RichardinNZ]

Seems like this confirms the difference...very little...
https://www.google.co.nz/url?sa=t&so...Zl6wFiKq5xevTQ

[Fenlander]

To summarise that link....

As usual loads of folks posting who haven't a clue or just think they know the differences... the normal situation for a "can I upgrade my X to a Y" query. However at the end a guy who seems to know what he's saying reckons the only difference Super 15 to a the 18 is that the throttle cam on the carb is a different shape on the Super 15 which prevents the throttle opening fully... and seems you can buy that cam from a Tohatsu dealer for an 18 to get the upgrade.

The OP could soon see if this was the case by looking down the carb throat to see if the throttle butterfly only opens part way. The only other thing I'd check would be to see if there are any changes in the ign timing setting.

This assumes the OPs motor is the larger capacity 294cc Super 15 not the standard 15.

[maverick58]

Originally Posted by Sykes705

I have just taken delivery of a new mercury 15hp 2 stroke.



Reading the manual I have noticed that all the specs are the same as the 18hp.



Can I convert the 15 to an 18?

Yes you can, there is a restrictor on the throttle which allows the engine to rev a bit further reaching its full 18hp potential because it’s a tohatsu 18 block rebadged as a merc 15. I had mine done beginning of this year and boy was it noticeable. My mechanic knew instantly what to look for. A full service, winterisation and release of those 3 extra horses came to £80.