2 stroke vs 4 stroke outboard

[macd]

I always thought that 2 strokes had more torque not less for the same power. Anyone know the right answer?

Power and torque are not independant of each other: in imperial units bhp = (torque x rpm)/5252. (One corollary of which is that torque and bhp are always numerically identical at 5252rpm (assuming the engine in question revs that high)). So any two engines of any type producing the same power at 'x' rpm will also produce identical torque at 'x' rpm.

2-strokes generally have more torque than 4-strokes of the same capacity, for the simple reason that they produce more power. However, also generally speaking, 4-strokes (esp low-tuned ones) typically produce peak torque lower down the rev range and sustain it for a greater part of the rev range.

 

[CAPTAIN FANTASTIC]

Power and torque are not independant of each other: in imperial units bhp = (torque x rpm)/5252. (One corollary of which is that torque and bhp are always numerically identical at 5252rpm (assuming the engine in question revs that high)). So any two engines of any type producing the same power at 'x' rpm will also produce identical torque at 'x' rpm.

2-strokes generally have more torque than 4-strokes of the same capacity, for the simple reason that they produce more power. However, also generally speaking, 4-strokes (esp low-tuned ones) typically produce peak torque lower down the rev range and sustain it for a greater part of the rev range.

In addition, four strokes will produce torque at a much wider range of rpm for longer

 

[Lakesailor]

All that's spiffing, as long as one doesn't have to lift the thing to stow it or take it home, and the boats' trim / performance doesn't mind more weight at the stern if left there, which can lead to boat pitching, transom dragging or displayed engine being pinched...

Boats having to leave the engine in a well due to weight - in the past largely due to daft oversize engines, now maybe standard 4 strokes - make me cringe, not only thinking of the drag but also the corrosion.

Roll on electric jobs !

Yes, well. All that's spiffing...

...except that if you read the OP you will see he is replacing an existing outboard and not in the market for the drum you bang about how rubbish outboards are.

 

[BarryH]

I have an 8hp Mercury 2 stroke of the "Saildrive" version 1999 vintage. It looks a bit used. It has a low pitched big bladed prop on a longshaft leg. It sits on a lifting bracket on the stern, it won't lift.

It runs very well i the wheelie bin at home.........never used it on the boat as since I've had the boat its never been in the water. I will, in due course, tell you if its any good. Or to put it another way, if its not I'm in trouble!

 

[VicS]

I have an 8hp Mercury 2 stroke of the "Saildrive" version 1999 vintage. It looks a bit used. It has a low pitched big bladed prop on a longshaft leg. It sits on a lifting bracket on the stern, it won't lift.

It runs very well i the wheelie bin at home.........never used it on the boat as since I've had the boat its never been in the water. I will, in due course, tell you if its any good. Or to put it another way, if its not I'm in trouble!

Before you do launch your boat have a read of this post on the iBoats forums http://forums.iboats.com/showthread.php?t=158086 It may suggest some items to check on your outboard.

 

[BarryH]

Read that Vic, thanks for the link, always handy to have. I'm not a 2 stroke novice. I used to build my own engines on my race bikes. I currently looking at stripping the wheezing Merc giving it some port work and fitting some expansion chambers. I reckon I could get over 15hp out of the thing. It will be all or nothing though............

 

[Lakesailor]

Remember you will be starting it with a cord. (Unless you are posh)
I ported and padded a Lambretta once. Went like sh*t off a stick, but what a bugger to start.

 

[macd]

In addition, four strokes will produce torque at a much wider range of rpm for longer

In other words, "4-strokes (esp low-tuned ones) typically produce peak torque lower down the rev range and sustain it for a greater part of the rev range." So why the "in addition"?

 

[stevensuf]

Four strokes are always heavier but drink much less fuel!

 

[VicS]

Four strokes are always heavier but drink much less fuel!

My 2 stroke 6hp Evinrude is 27.4kg
Earlier, #17, the weight of a 6hp Tohatsu 4 stroke has been given as 25 kg

"Always" is perhaps the wrong word.

If weight is a consideration you have to check it in the manufacturers literature!

 

[catalac08]

I switched from a 30HP Tohatsu 2 stroke to a 20hp tohatsu 4 stroke. Both are I think excellent engines and they weigh about the same. The 2 stroke never missed a beat in 6 seasons and I averaged about 1 gallon per hour on a cruising catamaran. The 20 hp 4 stroker is only just run in but I think
I will average about 0.5 galls per hour, but this is not comparing like for like as one is a smaller engine, nevertheless is still more economical. I love the power tilt as I can get it up or down quicker so I tend to sail more in variable winds.
The 4 stroke is much, much quieter the result being I have more usable power without being deafened and embarrassed by the noise.
I like the simplicity of the 2 stroke as something that is more fixable on the hoof although I conciousely avoided a fuel injection model of engine with the 4 stroke-did not want to get into ECU's, engine management , code readers as done that with cars not aleays with success!

 

[stevensuf]

OKay I will rephrase that, 2 stroke are usually lighter than four stroke engines!

 

[andrewbartlett]

My son has replaced his 5hp 2-stroke with a Yamaha5 4-stroke in the well on his Achilles 24 and it is much smother and quieter (a relative term). It can be laid down any way except head-down.

I hope I'm not being too dim but could you spell out what you mean by 'head down'. I have a similar problem when they say 'the first four coaches' on a train. I'd like to know because the same would probably apply to my 6 hp 2 stroke Selva o/b engine.

 

[Tranona]

In some positions a 4 stroke will drain oil from the sump. The instructions will therefore say how you can lay the engine down to avoid that. As a 2 stroke does not have oil in a sump it is less critical how you lay it down. The "head down" means the power head lower than the leg.

 

[andrewbartlett]

in some positions a 4 stroke will drain oil from the sump. The instructions will therefore say how you can lay the engine down to avoid that. As a 2 stroke does not have oil in a sump it is less critical how you lay it down. The "head down" means the power head lower than the leg.

got it , thanks:d

 

[JimC]

One thing that I don't think has been mentioned is the nature of the noise produced. Two strokes howl while four strokes throb. At 5 knots The Isle of Man is 12 hours away from my home port of Glasson and I've several times done the trip under engine in o/b powered boats. I find the the lower-pitched sound of a four stroke less disagreeable.

 

[VicS]

In some positions a 4 stroke will drain oil from the sump. The instructions will therefore say how you can lay the engine down to avoid that. As a 2 stroke does not have oil in a sump it is less critical how you lay it down. The "head down" means the power head lower than the leg.

Just to add to the above
You should not even put a two stroke down with the power head lower than the gear case because there is a chance of water remaining in the leg entering the engine via the exhaust port.

There are no hard and fast rules about which way a 4 stroke can be carried or stored, on its left side, right side, front or back, except to say upright is always acceptable, upside down is never acceptable . . In all cases read the manual and take note of what it says.

 

[neale]

My 2 stroke 6hp Evinrude is 27.4kg
Earlier, #17, the weight of a 6hp Tohatsu 4 stroke has been given as 25 kg

"Always" is perhaps the wrong word.

If weight is a consideration you have to check it in the manufacturers literature!

'Nearly always' may be correct but the 6hp is a special case. The four strokes are now often single cylinders at this size whereas the old two strokes would have been twins.

I had a 4hp Evinrude 2 stroke which weighed around 15kgs. My 4hp Suzuki four stroke weighs 25kgs.

9.9hp - 2 stroke 34kgs (27 if you got the 9.8 Tohatsu) 4 stroke typically 40 kgs

I am note 100% sure, but I can't think of any other sized outboard apart from 6hp,where four stroke weighs the same or less than two stroke.