2-Stroke vs. 4-Stroke Torque Output

[SRFARRAR]

Do 4-strokes put out more torque than 2-strokes of equivalent horsepower? Reason I ask is I saw Mercury ran a 26-in Trophy Plus prop on a 4-stroke 115 mounted to an 18-ft boat. I run a 19-in stainless 3-blade prop (no vent holes) on my 2-stroke 115 mounted on a Key West 1720DC (about 2000lbs fully loaded with motor, gas, etc). My 115 will run right at the 5250 redline at about 54 MPH and it gets there very quick. I can't imagine my 115 turning a 26-in prop with any success. Also noticed the 4-stroke has a smaller bore and longer stroke than the 2-stroke motor. Planning to buy a 115 4-stroke Merc for my Key West because the 2-stroke has been nothing but frustrating. Never wants to start, won't run at slow speeds more than 5 - 10 minutes, and currently will not go above 4500 RPM. Looking for any info on what I should expect for performance & prop selection.

 

[JB]

Re: 2-Stroke vs. 4-Stroke Torque Output

Howdy, SRF.<br /><br />It is widely believed that 2 strokes make more torque at low and mid rpms than 4 strokes. I am sure it is true for some 2 strokes compared to some 4 strokes, but I also believe it is the opposite for some cases.<br /><br />I would expect an ideally rigged 115 Merc 4 stroke with a correctly selected prop to perform as well on your boat as your 115 2 stroke at its best, but not better. My guess is that you will stick with the 19" pitch, but might want to go to a 21".<br /><br />Good luck.

 

[RJS]

Re: 2-Stroke vs. 4-Stroke Torque Output

4 strokes have a broader torque profile throughout the RPM range in comparison to 2 strokes. In general, a 4 stroke can spin a higher pitched prop.

 

[what200]

Re: 2-Stroke vs. 4-Stroke Torque Output

4 strokes have a broader torque profile throughout the RPM range in comparison to 2 strokes. In general, a 4 stroke can spin a higher pitched prop. <br />

Do you work for Honda?<br /><br />Whatever. My 370 lbs Mercury will out spin, out torque, and out perform any 4 stroke out there that weighs the same unless it is turbo'd or on N20. And I could get a Nitrox kit too. <br />$4 strokes are good for cruising and putting around. Try going to a OB drag race show and blowing that smoke over there. Should be good for a laugh.

 

[JB]

Re: 2-Stroke vs. 4-Stroke Torque Output

Well, good for you, What200.<br /><br />I'm sure that SRF will be interested if he wants to go drag racing.

 

[what200]

Re: 2-Stroke vs. 4-Stroke Torque Output

JBB - I'm not trying to say that 2 strokes suck. I would recommend one in certain applications, possibly SRF's.<br />But he asked if 4 strokes put out more torque than 2 strokes of equivalent HP. That is actually a trick question but anyway he got the WRONG answer. <br /><br />I assumed he meant which one has more torque down low in the rpm range. The biggest determining factor of this is the displacement of the motor. Per cubic inch a 2 stroke will make more torque than 4 stroke. 3 ways a 4 stroke can overcome this:<br />1. More displacement - like all the big v6's<br />2. Spin faster<br />3. Be more efficient - They a little bit but it's not that big of a difference.<br />To me the comparison should be based on weight rather than horsepower ratings.<br /><br />I think SRF is not comparing the same boat so you can't draw a conclusion about the props used but I don't think you can answer that question without specific models.<br /><br />BUT - they main reason I posted is:<br /><br />I think it's great that iboats provides this board free of charge to help boaters which may or may not benefit them. However, for whatever reason, this place has a problem of people giving advice as though they are experts when in fact they are not sure or maybe even have no idea at all. <br />JB - I think you are good example to follow but I'm sure you can think of a few problem posters in this regard. I don't regularily follow this board but when I see something that I think is false then I'm gonna jump on it.<br /><br />I'd rather get no advice that bad advice. If I want guesses then I'd rather rely on my own.

 

[jim dozier]

Re: 2-Stroke vs. 4-Stroke Torque Output

Here we go again

 

[SRFARRAR]

Re: 2-Stroke vs. 4-Stroke Torque Output

Appreciate all of the feedback! Just couldn't see how the 4-stroke could turn a 26in without having some serious low-end torque and don't see how any 115 could reach 67 MPH. Maybe that was a souped-up 115 because they pushed that 18-ft boat (bigger & heavier than my KW) to 67 MPH & had good hole shot too. The gear ratio on the 4-stroke is the same as the 2-stroke, so how could a 115 turn a 26 fast enough to get 67 MPH. The rev limit is 6000 on the 4-stroke vs. 5250 on the 2-stroke, but I'd think the 4-stroke would not have the horses to turn the prop much beyond 4800-5000 RPMs. I asked Merc if they would give me torque numbers for their 115 4-stroke, carburated 2-stroke, and Optimax. If they give those numbers, I'll post them. I know the tourque band is important too, but we'll see if they pony up any numbers--might be proprietary info. Props are funny too, how do you explain my engine running to 5450 with a stainless 20-in Laser 2 with vents, but only 5250 with the 19-in stainless without vents? I have read a lot about vent holes and based on that the vents should have no effect on top end, only hole shot. The hole-shot with the 20 in was terrible. The RPMs got up super fast, but the prop wasn't biting until the boat got to about 20 MPH, then the prop got a good bite and the boat took off. I know it's off the original topic, but thought it was pretty interesting. The boat "jumps" out of the water with the 19 in. Jerked two large slolam skiers out of the water--they both jumped right up. If my 2-stroke was as reliable as other Mercs I've been associated with, I'd stay with it.<br />What's the word on the street on the 115 Optimax? I see they changed from a V-6 to a 3 cylinder. Thanks again, Scott