Potentially dumb question:Is there a difference between 2 stroke and 4 stroke power?

[Flinch]

I've got a 2stroke 40 hp that I want to replace with a 4 stroke. The boat can plane (15' Livingstone V hull, not the twin hull) with the 2 stroke, and I want similar performance with the 4 stroke.

Is there a difference between power ratings for 2stroke vs 4 stroke, given the same HP rating? Like the difference between diesel and gas engines in trucks: same HP can give you very different 'power' (informal term so keep flames low!).

For example, can you generally replace 2stroike engines with lower/higher rating 4stroke HP rated engines to get same performance, or is HP = HP between the two?

Thanks

Flinch

 

[wifisher]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

In a word, no. You are not comparing gas to diesel. 2 vs 4 stroke is still gas vs gas. HP is HP. If you replace your 40 with another 40, you will see similar performance. Gas vs diesel is a whole different story because of the torque rating. The torque rating in gas engines will not be different enough to notice a large difference in performance.

You may notice that the power is generated at a little different RPM, but I wouldn't worry about loss of performance in the same HP class.

 

[HT32BSX115]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Howdy,

It's also going to depend on the year model of both.

If it's an older 2 stroke it might be rated with crankshaft HP.

Newer engines are rated utilizing propshaft HP.

I don't remember what year they all switched over but I don't think all the engine manufacturers started using the propshaft rating at the same time....

The other thing to consider is that a carbureted 2 stroke 40 (propshaft) hp engine on the same boat loaded the same, will probably outperform a 40 (propshaft) HP 4 stroke simply because it weighs less.

Regards,


Rick

 

[5150abf]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Yup, 50 is 50, it's like a pound of lead and pound on feathers, they are the same but different but the amount of push on your boat is the same.

The power delivery will be a little different and the 4 weights more so that would be the only place you would lose any speed but it shouldn't be much.

 

[TOHATSU GURU]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Yes, there typically is a difference between 2 and 4-stroke power. Normally a 2-stroke will have more power available during initial take-off. That's not a 100% rule, however, it is true enough that you would need to compare exact models to find a four-stroke that was equal to or superior to a 2-stroke of the same size. As the RPM increases the difference eventually fades away...Prop shaft hp is measured at PEAK RPM. How individual engines get there there varies depending on technology and selective tuning by the manufacturer.

 

[DJ]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Crankshaft rated HP to Prop. rated HP happened in 1983.

As TG states, you may notice a difference, particularly in acceleration.

At top end, a 40 is a 40. The potentially differing factor of weight is there.

 

[Frank Acampora]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Because the 2 stroke has one power pulse every revolution per cylinder while the 4 stroke has on every other revolution, all things being equal--and that includes displacement--the two stroke has more low end torque than the 4 stroke. This can mean that under certain load/hull combinations, the 4 stroke will not be able to make enough horsepower at low speeds to climb out of the hole and acheive its rated horsepower--the hull will not plane!

Personally, if I were buying a 4 stroke, and had the extra cash, I would get a larger horsepower engine than you have on there now--up to the limit the hull can carry. I think a 40 on a 15 footer--if it is not a tin boat-- is on the lower end of the HP range. If the boat is rated for it a 50 would probably be more satisfactory.

 

[emoney]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

I think the biggest thing to consider, if it hasn't already been said, is the weight difference. A 4 stroke is quite a bit heavier, so the performance will be different, only because you're adding more weight. You didn't mention, but is your boat newer or older? I ask because you want to make sure the transom is strong any time you add additional weight.

Whichever you get, based on everything I've seen, read and experienced you'll be really pleased with the new 4 strokes. They are, if nothing else, a lot quieter and seem to get better fuel economy. If I ever get the money, that will be my next investment (unless they really do perfect this new 2 stroke idea I've heard rumblings about), but they get pretty pricey in the 140hp range.

 

[Silvertip]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

On the smaller 4-strokes you will notice that lower unit gearing is slightly different as well. That is partially to offset the lack of hole shot with a four stroke.

 

[V153]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

For average everyday use it's hard to beat the 4 strokes. Quiet efficient, but boy the service intervals not to mention initial cost. Phew!

Not to say the new 2 strokes are a whole lot cheaper by comparison but 'least there's less moving parts involved. And the new Etecs, etc are considerably more efficient than older models.

Think if I had a boat over say 21' long I'd have me a 4 stroke. Shorter boat'n that a 2 stroke'd be more fun ...?

 

[sschefer]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

The two stroke motor makes power on every stroke and throughout the power band. It therefore inherantly has more torque than a 4 stroke and fewer moving parts equating to less friction.

The four stroke motor has more moving parts and makes power every other stroke. It used to be the that it was like comparing apples to oranges but not so much anymore. Two stroke technology has changed dramatically to comply with Fedral smog regulations and that has changed the game.

The two stroke is still king of the hill for all out performance but it comes with a price tag and you won't find the options in your smaller motors. Merc is now producing a Pro XS in the 150hp models but you might as well buy an E-Tec for the price.

Four strokes are fast becomming the norm in small engines 115hp and below. It won't be long before the two strokes are gone altogether. Yamaha is phasing them out now and Merc will soon follow suit. Tohatsu makes most of the 4 stroke small engines for many of the major players. (not suzuki or honda).

I just bought a 4 stroke 60hp Merc and I've very impressed with it. It is a 2011 and weighs in at a meager 248lbs. It pushes my 16' heavy gauge welded aluminun fishing boat nicely although I haven't run it all the way up since it's not fully broken in yet. I love the fact that it's fuel injected and not affected by altitude changes. It starts on about a 1/2 revolution without any choke and no smoke. I run it on 87 octane fuel which it sips. I don't have to worry about carrying oil, I just fill it up and go fishing.

 

[fishndirk]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Negetive, a 4 stroke with the same power rating will NOT produce the same performance as a 2 stroke with the same rating, especially in the range you are talking about. They may be rated at the same piston power produced, but have very different performance characteristics. Simply put, a 4 stroke cannot cycle as fast as a 2 stroke (thus a loss in top end). If a boat is rated for a 40hp max, it will definately go faster with a 40hp 2 stroke. There are advantages to both however; a 4 stroke offers great low end acceleration, quiet, smooth performance and better fuel economy while a 2 stroke offers generaly a simpler design, and higher engine cycling speed thus higher top end. Because a 4 stroke requires more parts they typically weigh more too but can still push and manuever a boat very well especially at the boats max rating. It all depends on what you expect out of a boat.

 

[TOHATSU GURU]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Although I prefer 2-stroke to 4-stroke....You are incorrect.

 

[fishndirk]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

How so,...I own a 20hp 4str and my buddy owns a 25hp 2 str. My 20 can beat him right off the bat, probably because its geared different. But once we're both up on plane his 25 slowly pulls ahead mostly because the 2 stroke is just cycling faster. My 20 pushes my boat at about the same speed even with a heavier load, it has the torque to get to its top speed very fast but then it stops accelerating because it simply cannot cycle any faster. All 2 strokes ive seen take a little longer to "get there", but then once they do they pull ahead. We borrowed a friends boat one time (14ft mod-v welded hull). He had a 25hp 4 stroke Tohatsu and my buddy with the 25hp 2 stroke Tohatsu was always "dying" to try his motor out on this boat cause we thought it would "fly". We were supprisingly shocked when we got to try it one day. We took off the 4 stroke and put the 2 stroke on, the boat took FOREVER to get on plane but did go a little faster once it got there. We were very supprised

 

[TOHATSU GURU]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Your wrong because your incorrect....You are basing your assumptions on a very small personal sample rather than the engineering aspects of outboards and their technologies. Do a search on iboats and you should find a lot on this subject. Concentrate on propping, weight, (engine torque is meaningless) and RPM.

 

[fishndirk]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Both motors ran the same exact stainless steel prop pitch and diameter same. The 4 stroke is obviously much heavier so we figured if anything would decrease hole shot but nope, the 4 stroke got that boat on plane much faster that the 2 stroke. 2 different motors, both with no problems being tested on the same boat the same day, and an obvious difference in performance. I dont see how that expariment could have got any simpler than that. We even played around with every trim setting on the 2 stroke to get it to plane the boat out as well as the 4 stroke but couldnt do it. And yes gear ratio does have a lot to do with a motors torque output.

 

[pootnic]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Seeing that a 2 stroke makes it power every stroke,I would assume it would make it's power quicker...better takeoff.
It must be a gear ratio thing with the 2 outboards you were trying out or they may have not been the same displacement at all?
I think your the first person I've heard say,that a small 4 stroke is better out the hole then a 2 stroke.
Top end speed is normally about the same, a 2 stroke should get you there quicker.

 

[fishndirk]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

I think gear ratio is the main factor here. Believe me, I am well aware of the differences of engine cycling between a 2 and 4 stroke and yes you would think that about a 2 stroke. The only thing I can think is that the 4 strokes are geared to focus their power on the initial take off. Im not crazy on this and its not just that situation that im basing this on. I run into a lot of other boats out on the water that have comprable size motors and hulls that are 2 strokes and my 4 stroke gets em on the hole shot every time, just as my friends 25hp 4 stroke got the 25 2 stroke

 

[ChampionShip]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

In your case I would look into etec for the weight. yeah it's still 2 stroke but as for efficiency and emissions they're about as good as a 2 stroke will ever get. I also agree that if you change to a 4 stroke check the transom thoroughly and go up maybe 10 hp or so if you can to push the extra weight.

 

[TOHATSU GURU]

Re: Potentially dumb question:Is there a difference between 2 stroke and 4 stroke pow

Both motors ran the same exact stainless steel prop pitch and diameter same. The 4 stroke is obviously much heavier so we figured if anything would decrease hole shot but nope, the 4 stroke got that boat on plane much faster that the 2 stroke. 2 different motors, both with no problems being tested on the same boat the same day, and an obvious difference in performance. I dont see how that expariment could have got any simpler than that. We even played around with every trim setting on the 2 stroke to get it to plane the boat out as well as the 4 stroke but couldnt do it. And yes gear ratio does have a lot to do with a motors torque output.

And again, the engine's torque has no bearing on this. The four-stroke can't develop the RPM fast enough to compete with the two-stroke so the torque is not an issue. Any potential "torque" advantage is off set by that lack of RPM. Also, I've run both of those two Tohatsu models more than I want to even think about They aren't that close...That 25C should eat your 20C normally. Chances are there is some sort of glitch in your comparison...Might be the prop or something else. Believe me when I say that I would like to agree with you I loathe 4-strokes.