2 vs 4 strokes, who has more torque?

waterinthefuel

waterinthefuel

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In our FAA approved mechanics school they always taught us that 4 strokes have more torque at low RPM because of the length of stroke. But on another forum a guy is saying that 2 strokes have more torque, and that's why a boat goes faster with a 2 stroke 40 than a 4 stroke 40, for example.

Am I right, or is he right?
 
F

foodfisher

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Re: 2 vs 4 strokes, who has more torque?

I'll guess 2 stroke because it powers every stroke.
 
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robert graham

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Re: 2 vs 4 strokes, who has more torque?

40HP is 40HP,wheather 2 or 4 stroke, but the torque and HP curves will look different, although still quite similar in peak power. I think generally a 2 stroke motor will have a better "hole shot" from a standing start than a 4 stroke, may be a few opinions on this subject.
 
J

JRJ

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Re: 2 vs 4 strokes, who has more torque?

4-stroke for torque. 2-strokes make power with RPM's. Dirt bikes are a good example as well as 2-stroke Detroit Diesels vs Cummins or Cat. Once the revs drop off, its time to down-shift, where a 4-stroke will get down and grunt :D
 
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Bamby

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Re: 2 vs 4 strokes, who has more torque?

It's my opinion that it depends on the application or what kind of boat the engine is mounted on. I feel that four stroke is superior on applications such as displacement hull boats such as pontoon boats. At low RPM peddling around cruising such as often done with such boats four strokes seem to perform and handle such running "better".

For planing style hulls and fast rapid out of the hole at running at "speed" the two stroke may perform better or maybe have a slight edge in performance. A lot of it has to do with personal opinion and consideration of what hull design and how you intend to run your boat.
 
bruceb58

bruceb58

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Re: 2 vs 4 strokes, who has more torque?

waterinthefuel said:
In our FAA approved mechanics school they always taught us that 4 strokes have more torque at low RPM because of the length of stroke. But on another forum a guy is saying that 2 strokes have more torque, and that's why a boat goes faster with a 2 stroke 40 than a 4 stroke 40, for example.

Am I right, or is he right?
Click to expand...
4 strokes have more torque at a lower RPM but a 2 stroke of the same HP is much lighter and that is why it can probably go faster because of the power to weight ratio.
 
bruceb58

bruceb58

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Re: 2 vs 4 strokes, who has more torque?

foodfisher said:
I'll guess 2 stroke because it powers every stroke.
Click to expand...

Since the 2 stroke powers every stroke, the 2 stroke engine will be a smaller displacement, hence less weight, for the equivalent HP.
 
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drrpm

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Re: 2 vs 4 strokes, who has more torque?

Horsepower=Torque x RPM /5252. An engine designed for max power at high RPMs will have less torque than one designed for max power at lower RPMs. Two cycle engines usually are designed for high RPM use so their torque would generally be lower. However, there are large 2 cycle diesels that operate at low RPM and their torque is tremendous.
 
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rbh

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Re: 2 vs 4 strokes, who has more torque?

Someone mentioned the torque curve for a 2 stroke vrs a 4 stroke, the way I remember it, the 4 stroke has a very flat curve compared to the peaky 2 stroke.
and as for the 2 stroke detroits, yuck, hard on fuel, they can run backward's and if there was no supercharger on them they would be dogs out of the gate.
 
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H20Rat

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Re: 2 vs 4 strokes, who has more torque?

have to compare apples to apples...

For IDENTICAL engines, same bore/stroke/displacement, a 2 stroke will generally have far better torque as well as much higher overall horsepower.

if you are changing bore/stroke, then the comparison makes no sense at all anyway. As someone else mentioned, you could have a 2 stroke diesel with a massive amount of torque compared to a honda high strung 4 stroke with no power down low. Even if you limit it to identical horsepower, its still entirely possible to have two vastly different engine designs.
 
bruceb58

bruceb58

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Re: 2 vs 4 strokes, who has more torque?

The comparison was between two 40HP engines so the displacement of the 2 stroke would be less.
 
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JRJ

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Re: 2 vs 4 strokes, who has more torque?

A 40 2-stroke may be faster than a 40 4-stroke due to higher RPM's turning the prop, not because of torque.
 
dolluper

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Re: 2 vs 4 strokes, who has more torque?

apples have more torque than oranges 4 stroke engines have more torque but in the case of 2 stroke ship or train engines the ship 2 stroke turbocharged,forced induction {instead of supercharging} has the most torque....because it is rated by it's TERMAL EFFICENCY is the highest .....so now your really confused
 
dolluper

dolluper

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Re: 2 vs 4 strokes, who has more torque?

OK left you confused long enough LOL's .When it comes to regular 2 and 4 stroke engines of the same HP this is how you figger the torque
Torque = hp x 5252 ..over speed {RPM}
so take 40 hp 2 stroke rpm 7200 and 40hp 4 stroke RPM 5300
40x5252=210080 then divide by RPM 7200 = 29.177 ft lbs of torque
now 4 stroke
40x5252=210080 then divide by RPM 5300 = 39.637 ft lbs of torque
EASY EH !
 
JB

JB

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Re: 2 vs 4 strokes, who has more torque?

Check out the 24 Hours of LeMans race currently in progress.

I would say that direct injected, turbocharged 4 stroke Diesels make the most torque per cc at any particular rpm.

Outboards? If that is what you are talking about you are in the wrong forum and asking the wrong question.
 
Bifflefan

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Re: 2 vs 4 strokes, who has more torque?

rbh said:
and as for the 2 stroke detroits, yuck, hard on fuel, they can run backward's and if there was no supercharger on them they would be dogs out of the gate.
Click to expand...

Those were terrible engines. All of them 3 cyl's throught the V8's.
But i don think its cool that they could run backwards. You dont get that with anything else.
 
JustJason

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Re: 2 vs 4 strokes, who has more torque?

Doesn't matter 2 or 4 stroke, it's all in the stroke of the engine that makes torque.

Small bore and long stroke means torque, low end grunting pulling work.

Big bore and short stroke means RPMS, RPMS are a requirement to building HP. But RPMS are not a requirment for tq.

Apples to apples, 2 stroke wins every time in it's power to weight ratio, and volumeteric efficency, they flat out breath better. Where they always loose is in MPG's and emissions.
 
dwco5051

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Re: 2 vs 4 strokes, who has more torque?

bifflefan said:
Those were terrible engines. All of them 3 cyl's throught the V8's.
But i don think its cool that they could run backwards. You dont get that with anything else.
Click to expand...

They can run backwards. I once was pulling a converted Greyhound with a 6-71 GMC up on a ramp to do some work under it. I stalled it and it drifted back down the ramp and restarted backwards. Only ran for a few seconds and sounded terrible but it was running till I shut it down.

I also have an old Harley golf cart with a two cycle engine that to reverse it you shut it down shift into reverse which reverses the starting motor and starts the motor running backwards. If I remember correctly didn't Mercury use a similar system years ago on its outboards?
 
J

JRJ

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Re: 2 vs 4 strokes, who has more torque?

bifflefan said:
Those were terrible engines. All of them 3 cyl's throught the V8's.
But i don think its cool that they could run backwards. You dont get that with anything else.
Click to expand...

I ran some Detroit twin turboed and of course blown 8-92's that were loud, but fun :D
 
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TilliamWe

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Re: 2 vs 4 strokes, who has more torque?

"Detroit Diesel, GM's way to turn diesel fuel into smoke and noise!"
But, for an engine that was "only" 736 cubic inches, making 475 hp and 1440 lb-ft of torque is pretty impressive in a semi. CATs and Cummins were many more cubes, (in just 6 cylinders), weighed A LOT more, and were much more expensive.
Growing up around Detroits, I always thought that a 2 stroke would develope less low end torque, because the Cat 3406 and Cummins 444 have more torque than an 8V92. But imagine my surprise when I got into boats and found that two stroke outboards and PWCs are holeshot monsters?! Guess it's all in how you tune 'em and prop 'em?
Also, didn't motocross motorcycles used to be 125cc two strokes and 250cc two strokes? Now I see on TV it's 250 and 500cc four strokes. Did they need the extra displacement to get the torque? I don't know.
 
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