4 stroke to 2 stroke differences

[Trevor9751]

I just bought a new four stroke merc 115 HP I am kind of new to the four stroke outboard thing I checked the wieghts and it is only 20lbs heavier than the 115 2 stroke they say it burns cleaner and uses less fuel I have been looking for tourque specs on them but cannot find any I would assume that since they are rated on HP and that is the same you could expect significantly more tourque out of the 4 stroke but cannot find any #'s to support this or dispute it and what other differences should I expect to see with my new motor compared to a normal 2 stroke. I am upgrading from a 1995 120 force so I know this will be a step up no matter what but I am just wondering how much.

 

[steelespike]

Re: 4 stroke to 2 stroke differences

I'm not a 4 stroke expert or fan but they do have their good points.
It will certainly be smoother and quieter.And compared to the Force the low end power may be a little less as you are 5 hp down and 4 strokes are not known for the best hole shot.It may be a little tougher to prop right.It will use less gas.It will surely burn much cleaner than the Force even allowing for the 2 strokes fuel mix.It will require more routine maintainance.

 

[rob711]

Re: 4 stroke to 2 stroke differences

i have a 2005 115hp merc 4 stroke on my 18 ft trophy walkaround....this is my first season with it and so far absoluly no complaints...it will pull skiers up no problem...as far a maintanence...changing the oil seems simple, havent done it yet but i have a manual and it seems simple and straight forward...and inspecting the timming belt is really not much of a chore as well...i do miss that sweet smell of 2 stoke oil though!! like most things with some proper maintanence i think it will last a while

 

[Cricket Too]

Re: 4 stroke to 2 stroke differences

You will see a big difference from that Force, but in general 4 strokes provide less torque that 2 strokes, simply because of excatly what they are called.

There is a power stroke every time the cylinder comes up in a 2 stroke, hense it being called a 2 stroke, an intake and power stroke every time.

A 4 stroke wastes 2 strokes per cycle, in order to intake a fuel/air charge and then another to scavenge exhaust, then only has a power stroke on every other revolution.

Any given 2 stroke will have a better power to weight ratio over a similar 4 stroke, which is why DI 2 strokes are on the cutting edge and becoming very popular.

 

[JB]

Re: 4 stroke to 2 stroke differences

Well, I don't buy that. 115HP is 115HP. It is calculated from the same Torque and rpm formula for both engines.

Typically, 4 strokes need to get to higher rpm to make peak HP, but the notion that they don't make as much torque as a 2 stroke rated the same is just plain mythology.

Example. My 1980 BW Montauk with a Suzuki DF70 would stay even with my son's identical 1999 BW Montauk with a Merc 90 2 stroke up to about 38mph. His boat would continue to accellerate to about 40.5-41mph. As far as I am concerned the Suzi made better low and mid speed torque than the Merc rated 20 HP more.

 

[Trevor9751]

Re: 4 stroke to 2 stroke differences

I also disagree for the same displacement a 2 stroke may make more peak tourque but when rating on HP the tourque curve of a 4 stroke should definatly be alot more impressive than a 2 stroke rated at the same HP the 2 stroke peak tourque may come out on top but I would not even bet on that it would be interesting if someone would publish a tourque curve for these but I cant even find anywhere that publishes the tourque ratings of outboards nevermind curves.

 

[Silvertip]

Re: 4 stroke to 2 stroke differences

If four strokes had the same low end torque as a two stroke they should be able to be propped the same and geared the same and hence provide the same hole shot and top end. Fact is that is generally not the case. Four strokes tend to have different displacement, gearing and slightly less prop pitch, or the same gearing but less prop pitch than the same HP two strokes. Four strokes of any given HP also have more displacement than the same HP two stroke to compensate for the torque difference. The extra displacement is required to provide acceptable hole shot. Two strokes are generally faster as well. Four strokes are getting better, but still have poorer hole shot and slightly less top end. A 75 HP EFI four stroke Merc (2005 specs) has 1596cc displacment while the same 75 HP two stroke is 1386. Both have 2.33:1 gear sets but I happen to own one of these and it not only revs higher but will beat the daylights out of a 75 four stroke on the same boat. The 225 HP 4-stroke is 3352cc and the 225 EFI 2-stroke is 3032cc. The Verado is only 2598cc but it's supercharged and it also much heavier.

 

[Bigjohn1]

Re: 4 stroke to 2 stroke differences

Here is the biggest difference you will see in the comparison of your new motor versus your older one - you will not be able to run near as much pitch in your propeller. As already stated, you have less low-end power so to partially make up for this, you will need to run a lower pitch prop. Your engine has a 2-07:1 forward gear and for the life of me, I think Mercury is wrong for gearing it like this. 2-33:1 would be more appropriate in my view but I'm no Mercury engineer.

Your [Mercury] prop choices are going to be somewhat limited for maximizing the performance potential of your engine. Think along the lines of the Vengeance series in 15-18 pitch range dependent upon your particular boat and loading setup. I'm sure others can give better prop recommendations for other prop brands. My inout comes from my running the same engine for 4 years now.

 

[Trevor9751]

Re: 4 stroke to 2 stroke differences

yes I did notice that already my old force had a 19 pitch prop on it and this one they recomended a 17 for my boat arangement but another thing I noticed is this new motor is rated for a way higher RPM than my old one the force manual said 4800-5200 recomended 5000 mercury says for this four stoke WOT is beetween 5800-6400 recomended about 6100 so the extra rpm I would assume should make up for any top end losses from having to prop down. am I correct in this thinking.

 

[JB]

Re: 4 stroke to 2 stroke differences

I think so, Trevor.

 

[roscoe]

Re: 4 stroke to 2 stroke differences

Follow ALL maintenance schedules to the T.
It more than just changing the oil and inspecting a belt.