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2 Stroke vs 4 Stroke Torque / Power Curves

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leachy

Contributing Member
Hi All,

I was just wondering if anyone knows of a site or has ever seen torque / power curves comparing 2 stroke and 4 stroke motors. Im interested to see how they differ typically around the 75 - 100hp range.

It would be interesting to know if a 60hp 4 stroke produces similar torque to 80hp 2 stroke at 2/3 throttle, seeing HP is related to engine RPM not many outboards are running at max HP.

Regards
Leachy.
 
leachy said:
Hi All,

I was just wondering if anyone knows of a site or has ever seen torque / power curves comparing 2 stroke and 4 stroke motors. Im interested to see how they differ typically around the 75 - 100hp range.

It would be interesting to know if a 60hp 4 stroke produces similar torque to 80hp 2 stroke at 2/3 throttle, seeing HP is related to engine RPM not many outboards are running at max HP.

Regards
Leachy.
Click to expand...
HP is related to both torque and RPM. Takes both of them to make power.

You can have lots of torque but no power. If you have power you have to be having torque.
 
leachy said:
Hi All,

I was just wondering if anyone knows of a site or has ever seen torque / power curves comparing 2 stroke and 4 stroke motors. Im interested to see how they differ typically around the 75 - 100hp range.

It would be interesting to know if a 60hp 4 stroke produces similar torque to 80hp 2 stroke at 2/3 throttle, seeing HP is related to engine RPM not many outboards are running at max HP.

Regards
Leachy.
Click to expand...
It would also be helpful to know to the thrust output of given propeller. One propeller with less HP being applied to it might be more efficient than a different propeller with more HP being applied to it. That is, a faster boat with less power.
 
From my experience with 2 stroke bike engines there isn't a lot of torque but they have power as they get going due to increased RPM. Just reading some specs I guess they would be similar for outboards as said above torque and RPM give you power rating, and the power ratings are very similar at the same RPM. The more interesting part is the the area under the curve on a dyno graph on the way to maximum HP. I would have assumed that the 4 stroke has more torque in the lower RPM, however it always seems that the 2 stroke accelerates faster???. As Boscoe says it could also have alot to do with the prop and its efficiency. It could also be that the 2 stroke motor is a bit lighter.

The crap I think about at 1.00am in the morning, could be time to put a graph together.
 
leachy said:
From my experience with 2 stroke bike engines there isn't a lot of torque but they have power as they get going due to increased RPM. Just reading some specs I guess they would be similar for outboards as said above torque and RPM give you power rating, and the power ratings are very similar at the same RPM. The more interesting part is the the area under the curve on a dyno graph on the way to maximum HP. I would have assumed that the 4 stroke has more torque in the lower RPM, however it always seems that the 2 stroke accelerates faster???. As Boscoe says it could also have alot to do with the prop and its efficiency. It could also be that the 2 stroke motor is a bit lighter.

The crap I think about at 1.00am in the morning, could be time to put a graph together.
Click to expand...
So if a 2 strk accelerates faster doesnt that equate to more torque? Also a 2 strk dirt bike engine has NOTHING in common with an outboard except they are both 2 strks. And if what you are saying is "TRUE" how come 4 strk Moto cross bikes had to be 450 cid to compete with a 250 cid 2 strks.
 
Take this for what its worth. A guy I work worth spent 8 grand on a new Mercury 4 strk 60 hp. He is already talking about getting a bigger motor to pull his kids skiing. Which is awkward because I was pulling them up no problem with my 70 hp evinrude.
 
I agree with all those points, but there are some inherent design flaws and advantages of both types of motors. Its probably a shame the DI 2 stroke didn't get developed more. Im guessing the lower revving 2 stroke outboard design unlike a higher revving motorbike engine gives a far different torque curve.

Its often hard to compare acceleration of a boat as well due to the different prop you can run and the 3 / 4 blade differences. I guess both motors at WOT are going to use a similar amount of fuel.
 
There is so much more to it than hp and or torque curve. I bought a new yamaha 2.5hp for my canoe in 2006. By 2014 it was dead..blow by pressuring crankcase and blowing oil all over. If I could have bought a new evinrude 3hp 2 stroke it would still be running fine and would be an easy rebuild if needed. By the time I priced all parts to rebuild the 4 stroke I could buy a used evinrude and rebuild that. And that 4 stroke vibrated something awful.

I feel that 4 strokes are a necessary evil for outboards....Yea none of this pertains to your question...I'm waiting for catalytic converters on outboards..that will be a good time.

My friend has a Honda 30hp on his lund something..what a dog that thing is..Maybe the added weight of cam phasers would be helpful..or a 2 speed mid..
 
It is interesting looking at the different perspectives. Im guessing it completely depends on how the boat is used and the RPM range the motor is running at most of the time.
 
Quite a bit of misunderstood physics here. Power is the RATE of doing work. In other words, 1 HP = 33,000 ft lbs/minute, or 550 ft lb/second.
Torque is a measure of FORCE. It is a vector quantity. Force is what accelerates an object along a linear path, torque accelerates around an axis, angular acceleration.
 
Edchris177 said:
Quite a bit of misunderstood physics here. Power is the RATE of doing work. In other words, 1 HP = 33,000 ft lbs/minute, or 550 ft lb/second.
Torque is a measure of FORCE. It is a vector quantity. Force is what accelerates an object along a linear path, torque accelerates around an axis, angular acceleration.
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But there can be a force (torque) without movement? What good is that?
 
If one uses the words power and torque in a sentence then they obviously misunderstand both.

Power has torque in the equation for power. If one uses the word power then there is no need to mention torque. Torque is implied.

At least in case of a piston motor.
 
boscoe said:
If one uses the words power and torque in a sentence then they obviously misunderstand both.

Power has torque in the equation for power. If one uses the word power then there is no need to mention torque. Torque is implied.

At least in case of a
Click to expand...
Power and Torque are not the same thing. A simple Google search could gave saved you some time.
 
It is really all about the area under the curve..

Unit force and unit distance are inversely proportional.

I want the motor with the most area under the power curve..all else being equivalent.
 
boscoe said:
If one uses the words power and torque in a sentence then they obviously misunderstand both.

Power has torque in the equation for power. If one uses the word power then there is no need to mention torque. Torque is implied.

At least in case of a piston motor.
Click to expand...
2nd sentence - no.
it is getting weird…I’m out.
 
It's funny it was a simple question that was answered. Then everybody had to confuse it. A 2 strk will always have more hp and torque given similar cid. Simply because it fires twice as often. You have double the power strokes.
 
flyingscott said:
It's funny it was a simple question that was answered. Then everybody had to confuse it. A 2 strk will always have more hp and torque given similar cid. Simply because it fires twice as often. You have double the power strokes.
Click to expand...
Well now..not always..I had a friend who added a turbocharger to his...
 
Torque is a very misunderstood area of physics. In an application such as a piston engine, the torque developed is a function of stroke, the equation being related to F (force) x moment arm & sin theta. (Can’t find the symbol for theta on this computer).
It increases from the beginning of the power stroke, greatest when the throw on the crankshaft is perpendicular to the bore, then decreasing, to become zero as the piston bottoms. A longer stroke gives greater torque, as the crankshaft throw, (moment arm) is greater. Of course, a longer throw, with the same bore is also more CID.
A longer throw will not accelerate the rpm’s as quickly, hence the short stroke 2 strokes reach max rpm very quickly, resulting in quicker hole shots in the boating world.
Combustion engineering also plays a part. A slower burning mixture that achieves maximum pressure just before theta equals 90* will give higher torque. (It always bugs me when writers speak of the spark plug firing & the mixture “exploding”).
As a young fellow I raced on a Honda Elsinore. The thing reved to 14K, with a very narrow power band, & comparitivley little torque. One often had to ride the clutch on longer hill climbs.
Switching to a Can Am, with Rotax engine, (rotary valve), it only revved to 9600, but had far greater torque from the same displacement.
 
Sunkinmess said:
Well now..not always..I had a friend who added a turbocharger to his...
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So what? Any forced induction can be added to either engine. See everybody has to complicate the question. How many turbo outboards have you seen? It's a ridiculous statement. You sound like the Mercury salesman who sold my co worker his boat. Don't worry he said that 60 hp 4 strk has the power of a 90 hp 2 strk.
 
flyingscott said:
So what? Any forced induction can be added to either engine. See everybody has to complicate the question. How many turbo outboards have you seen? It's a ridiculous statement. You sound like the Mercury salesman who sold my co worker his boat. Don't worry he said that 60 hp 4 strk has the power of a 90 hp 2 strk.
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I was not serious...needed a silly emoji.

The whole thread turned silly quickly..was just adding more silly..
 
The newer 4 strokes with 100+HP have features to offset the low end lag. Here is Yammy dealer doing a shoot on on 60hp's.
 
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