Horsepower question, 2 vs 4 stroke

[hornblower]

Ok, I'm convinced that horsepower must be measured or computed differently for 2 stroke and 4 stroke motors but am unable to find anything to support this. My reasoning...

For example, I have two 8 HP motors. An Evinrude 2 stroke and a Honda 4 stroke. Both motors a running perfectly.( proper propeller, rpms checked etc.) Now I KNOW that the 4 stroke is heavier but, the 2 stroke is able to get a 14' jon boat on plane with 3 adults. The 4 stroke can plane with 1 person but not 2 on the same boat and that more than makes up for the difference in motor weight.

This is just 1 example of what I have noticed so my question,

What is the dirty little secret about how HP is measured on 2 and 4 stroke motors?

 

[speed_demon]

Re: Horsepower question, 2 vs 4 stroke

They are measured the same but manufactures are allowed to fudge 10% up or down. Because 2-strokes have much more power to weight they tend to be on the underrated side of the spectrum. 4-strokes are usually on the other end.

 

[hornblower]

Re: Horsepower question, 2 vs 4 stroke

Hmmmm, I guess if you say so but it just seems there's more to it. The difference between the 2 motors is more like the difference between a 5 hp and a 15 hp. I've noticed this on other motors as well as hearing it from other boaters. Thanks, are you sure it's not some sorta conspiracy?

 

[trendsetter240]

Re: Horsepower question, 2 vs 4 stroke

The difference is torque. Both motors are putting out 8hp but the two stroke has much more torque at lower RPMs allowing it to push the heavy loads up onto a plane. Once on a plane they should be near equal.

 

[JB]

Re: Horsepower question, 2 vs 4 stroke

HP is measured based on torque at a specific rpm. It is a peak value and has little or nothing to do with torque at other rpm.

Two stroke engineers have had about a hundred years to figure out how to make torque at the rpm where it is most usable by an outboard, off-peak rpm. Two engines, one 2 stroke and one 4 stroke, can have the same peak HP and different torque curves.

In your case your low HP engines are both underpowered for your boat. The 2 stroke barely planes it and the 4 stroke won't with a difference of a couple of passengers and different weight distribution. You just found a case that illustrates a not very important difference.

If it bugs you try comparing two 15HP outboards and see if you can measure the difference.

 

[speed_demon]

Re: Horsepower question, 2 vs 4 stroke

You've got two hands mount them both.

 

[seahorse5]

Re: Horsepower question, 2 vs 4 stroke

Torque is a major factor and many 4 strokes have much less at lower rpms that a 2-stroke does. The biggest reason is that each cylinder in a 4-stroke fires ONLY every other revolution. In a 2-stroke, it fires EVERY revolution.

With your outboards, the 2 cylinder Honda only fires once per revolution as the 2 cylinders alternate firing each time the flywheel turns once. The Evinrude, being 2-stroke has TWO cylinders firing for every revolution. The TWO power pulses at each turn of the flywheel generate much more torque at the same rpm as the Honda does.

In a nutshell, torque is what gets you up to speed, and hp is what keeps you there.

 

[steelespike]

Re: Horsepower question, 2 vs 4 stroke

The 4 stroke is more sensitive to proper setup it needs to be propped at or near its max rated rpm.
with a typical load.The four stroke has a power stroke every other rpm a 2 stroke every rpm.
I think you can see the torque curve is going to be different.
The general consensus is that the 4 stroke may get slightly better gph ratings but wot is slightly slower.
If propped for the best top speed it becomes sensitive to added weight.
True for a 2 stroke as well but less sensitive.

 

[hornblower]

Re: Horsepower question, 2 vs 4 stroke

Well there ya go. Torque huh? Figures. Thanks for the explanation. No wonder 2 strokes are still popular.

By the way JB, the 2 stroke jumps right up on plane with 1 or 2 people and can still get up with a third (over 500 lbs. passengers) Doesn't seem underpowered under most conditions.

 

[Silvertip]

Re: Horsepower question, 2 vs 4 stroke

While much of the torque vs HP discussion thus far is true, the end result in the performance discussion is not so much about torque and HP as it is about WHERE it is on the curve and WHAT must be done to compensate for the difference. Look at the lower unit gearing for two engines of the same HP -- one being a 2 and the other a 4 stroke. GENERALLY, the 4-stroke will run a higher numerical ratio gear set than the two stroke. This is done to compensate for the low end hole shot problem so typical of four strokes. What that also means is that a four stroke needs to run at higher rpm at the same speeds across the board than the two stroke does. Which brings up another "economy" discussion. People tend to look at fuel economy charts and compare fuel consumption numbers at a given RPM. A 4 stroke will always win in that situation. But since a four stroke needs to run higher rpm at any speed, one should be looking at fuel consumption at SPEED rather than rpm. I've been down this road before and if one does a "Two-Stroke Four Stroke Fuel Economy Study" search in this forum you would find my personal research done about three years ago using my personal boat and using data from Yamahas web site. As for top speeds -- again because fo the gearing difference, the two stroke will generally take those honors as well. However, with a higher numerical ratio gear set (lower gear) the 4-stroke could potentially give up even more hole shot in favor of higher top end by running a prop with more pitch provided the RPM stays in check. I have both so I'm not biased one way or the other but my nod still goes marginally to the two-stroke.

 

[Charlie in TX]

Re: Horsepower question, 2 vs 4 stroke

It is horse power that is a measure of work. The problem with engines (and this goes for lawn mowers, truck motors, etc as well as outboards) is the marketing department get to push the engineering department for a high peak number. This peak number is what is reported by the manufacture and it has added sizzle from marketing. As you have found, peak horse power is not that important. Horse power were you need it is important. In the case of a planing hull boat, the important horse power number is in the rpm range between displacement and planing. The power in this range is not the sizzle that sells but it is the sizzle that maters.

 

[tomhath]

Re: Horsepower question, 2 vs 4 stroke

Hmmmm, I guess if you say so but it just seems there's more to it. The difference between the 2 motors is more like the difference between a 5 hp and a 15 hp. I've noticed this on other motors as well as hearing it from other boaters. Thanks, are you sure it's not some sorta conspiracy?

HP = Torque x RPM ? 5252, so at any given RPM the motor with more torque delivers more HP. Getting the boat up on plane happens at lower RPMs and that's where you are seeing a big difference in horsepower between the two motors. Once they reach max RPM you will be getting somewhere around 8hp from both motors.

 

[pootnic]

Re: Horsepower question, 2 vs 4 stroke

You say the motors are both running at the right rpm....if your not getting on plane I doubt your getting the proper rpm.In other words,if you have a load with the 4 stroke,you will probably have to down size in prop.Small outboards(both 2 and 4 stroke) sometimes have different props,with different loads.That being said,in small motors I'd take a 2 stroke over a 4 anytime...it seems the 4 stroke has to be geared differently to preform close to a 2 stroke.

 

[hornblower]

Re: Horsepower question, 2 vs 4 stroke

You say the motors are both running at the right rpm....if your not getting on plane I doubt your getting the proper rpm..

I stand corrected, the motors both run at proper rpm in static tests or light loads. Yes, you are correct the 4 stroke is under rpm at WOT when under heavy load (2 people)

 

[Texasmark]

Re: Horsepower question, 2 vs 4 stroke

But you can cheat! You can get a good hole shot and still have the top end you seek.........punch holes in the barrel of the prop, called porting. Check out most of the high perf. SS props made for bass boats. The holes unload the prop in the hole (worst of all conditions) so that the engine can develop what hp it has to offer and once out of harms way (on plane) they seal off and you can go about your business with your top end rated prop. BTDT

Mark

 

[tomhath]

Re: Horsepower question, 2 vs 4 stroke

But you can cheat! You can get a good hole shot and still have the top end you seek...

Well...maybe. Remember, he's talking about 8 hp motors here. The hole shot isn't going to get your sweetie fired up even with a ported prop. :grumpy: