Power of 2 Stroke v 4 Stroke?

93evinrude · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

There are so many variables that determine the performance of a 2 stroke vs 4 that the results can reverse themselves from from boat to boat and motor to motor and the needs of the owner would also effect the best choice.ie for slow speed and trolling,I believe most would be best fitted with a 4 stroke while a high performance bass boat a 2 stroke.That's today,tomorrow could be different..... I subscribe to a great magazine(bass & walleye boats) that does many tech articles,boat comparisons,manufacturer comparisons and in this case a comparison of motors and here is an example that you may find interesting: May 2002 issue boat -skeeter zx2200 bay boat Motors yam 200 hpdi and yam 200 4 stroke results top speed 47.1 for the 2 stroke vs 46.8 0 to 30 6.6 sec. for 2 stroke vs 7.2 fuel at optimum cruise speed 4.0 mpg for 4 stroke at 30.7 mph vs 4.3 mpg at 30.3 mph for 2 stroke surprise; in this case on a tank of fuel at optimum cruise speed the 2 stroke had a range of 232 miles vs the 4 stroke at 216 miles. in addition -at 1000 rpm the 4 stroke had a distinct advantage -from 3000 to 3500 rpm the 2 stroke had the distinct advantage -at wide open there was a slight advantage for the 4 stroke with a range of 146 vs 135 miles As some have mentioned already,this topic could go on forever with all the variables involved.This is just an single example on a particular boat and I'm sure the results would vary using a different boat or horsepower or manufacturer,etc....... Thank you to B&W Boats for their great articles.

DJ · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Forktail, I wasn't going to comment any more, but I couldn't stand it.

Your comments: "Torque and HP will never cross at different places on a graph or dyno chart. This holds true whether it is an outboard engine, a lawnmower, or a race car. Torque and HP will always cross each other on a graph (be equal) at or very near 5252 RPM. Physics doesn't lie. If you experience a dyno chart where Torque and HP do not cross at 5252 RPM or begin crossing at different places, the information is a fraud." I disagree with the 5252 rpm premise. How do explain an engine in which the top rpm. is, say 3600 rpm? Or, a 14 liter diesel with a top rpm (max hp at 1600) of 1800 and peak torque at; 1100? The torque and horsepower lines are going in different directions, they'll never cross. Even if they did, it would be nothing but academic since the engine operates at 20% of 5200 rpm. The two lines (hp and torque) may cross at say; 1250 rpm. It's amazing to look at 14 liter diesels from the big three, CAT, Cummins and Detroit (12.7 liter-Series 60) and compare the "maps". They're really not even close. The 14's have a distinct torque advantage, mainly due to displacement advantage and a longer stroke.

Forktail · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Djohns, The 5252 RPM rule always holds true. 5252 is the only RPM at which the Torque (ftlbs) can equal the HP, or in other words, where they cross or connect on the HP/Torque/RPM curve. HP = Torque X RPM / 5252. Plugging in any HP value at 5252 RPM will get you the same exact Torque value. Plugging in any Torque value at 5252 RPM will yeild you the same exact HP value. This is where they cross...5252 RPM. This RPM cancels out the 5252 constant derived by physics (which I won't get into here). For example: 200HP = Torque X 5252 RPM/5252. So Torque = 200 ftlbs. They cross at 200. For example: HP = 60ftlbs Torque X 5252 RPM/5252. So HP = 60. They cross at 60. For example: 150 HP = 150ftlbs Torque X RPM/5252. RPM = 5252. They cross at 150. Remember, you said, "The differences are where these numbers cross each other, on a graph...." Again, they always cross at the same RPM. You kind of answered your own question. Low RPM, high Torque engines like the diesels you mentioned do not cross. They don't create enough speed for the HP and Torque values to be equal. Therefore these values never cross on the chart. However, if one did extrapolate the values out, knowing that the large Torque would eventually fall off quickly as RPM increases, they would find that they do cross at 5252 RPM. But of course our simplified HP formula already told us they would. Your example that the HP and Torque "may" cross at 1250 RPM is incorrect.

Forktail · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

vinney, THANK YOU FOR YOUR CONTRIBUTIONS TO THIS DISCUSSION. PLEASE SEE THREAD NOTHING TO DO WITH BOATS. (By the way, there's a correct way of doing things, and an incorrect way of doing things. You don't have to be an engineer to know that. But it helps when talking HP, Torque, and dynos.)

DJ · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Forktail, What I missed in your post was the word "equal"-(hp and torque) Sorry. Frequently heavy duty engine charts show the lines crossing to show peak operating range. They are playing with the scales to show that relationship. I still do not see how 70 hp. is NOT 70 hp?

Granted, there is the 10% allowance issue.

SCO · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Forktail, I think you have a mistaken notion here. Power is energy( or work) consumed for some time increment like foot pounds per second. Jb is correct about that except that the max hp might not be at the max rpm, but we know what he means there and it is correct. What you say, that hp is torque times rpm/5252 is also correct. You also say that torque is not work. This is also correct. However, when you rotate a propeller, and are applying a force against the water with it, you are consuming energy per time...power. Torque units are force times distance. Energy units are force times distance, but the rub occurs when you consider the revolutions. The revolutions are expressed in radians, a diminsionless quantity, and torque times radians is indeed work or energy with the units force times distance. For example, for 1 revolution of a prop under a torque load of 1 foot pound, the energy consumed is 2 pi ft lbs. If that propeller is rotating at ten revolutions per minute, then the amount of power being output is 2(pi)(10)/(60) ft lbs/sec. That is an expression of energy per second, not torque per second. Power is not torque per second because you could hold a propeller to stop it from moving, with torque applied, for as many seconds as you like and have zero work and power.

Forktail · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Djohns, the "peak operating range" is sometimes called the "powerband", or where the most efficient and most usable power for that engine exists. An engine is usually geared to operate in this range. This power band is the RPM range between the maximum torque and the maximum HP values. This leads into your question about 70 HP is 70 HP, assuming we are comparing two different engines both making 70 HP. Although the HP is the same, the Torque and RPM may be totally different, and give the owner a totally different use. For example, we know that HP = Torque X RPM/5252. So we could have a 70HP engine that makes 200 ftlbs of Torque at 1838 RPM. Because 70 = 200 X 1838/5252. A pretty torquey low RPM operating engine. It probably would have a small power band and would need many gears and a transmission to be functional. Much like a big diesel in an over-the-road truck. Or we could have a 70 HP engine that makes 65 ftlbs of Torque at 5656 RPM. Because 70 = 65 X 5656/5252. Not a lot of torque here, but higher operating RPM and probably a wide power band. Much like our outboards. So you can see that although both these engines are rated for 70 HP, they are very different. They both will do very different amounts of Work. If you get stuck on the fact that HP is HP, remember that HP is not measured. It is only a calculation made from measurements of Torque and RPM. SCO, I'm not sure what your point is, but when calculating HP in rotating engines, Work is not used....Torque is. And it must be used with RPM. Yes, you can calculate Power in just about anything including a light bulb, because Power equals a certain amount of work done over a certain time frame. But when dealing with engines, the HP (power) is related to Torque (Work) and RPMinute (Time). I did not say that Torque was not Work. After all, Work equals Force times Distances and Torque equals Force times Distance. But as you said, the "Distance" in Torque is in radians, which is what we must use in rotating engines. Again, Torque is the correct propery when measuring HP in engines. Work is best understood as the area under the Torque/RPM curve.

SCO · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

My point is that you are taking issue with JB's description "The horsepower rating of an engine is the maximum amount of work it can do at it's top rpm" and he is not wrong there in my view . This is not precise language( work as a rate is implied on my reading) nor meant to be I expect, but a generic description of power in terms of work. That max work output rate occurs near max rpm. Did you think JB was confusing torque and work? I think work is a good way to describe power. You also said "false hp" somewhere. What do you mean there?

ob · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

ChrisE, You still with us buddy or did you already buy a motor.

Forktail · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

I think Chris is "torqued" off and at the dyno doing some "work" to see which 70 hp outboard can catch fish while trolling 5252 RPM.

93evinrude · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Ob. I think that Chris has made up his mind to go with the 4 stroke for low speed economy reasons based on his post at 5:30 am on the 2nd.

vinney · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

CANADA I have never seen a performance boat with a four stroke outboard. None of my fish guys thats a Fountains,Regulators,Contenders,Donzis have the clean four strokes. They all want one thing performance. Most of them are self employed,own there own business and can afford what they want. They all drive them hard and fast. As for a four strokes the last time I looked in the service manual for a four stroke yamaha they required a valve adjustment. Two strokes dont require valve lash. More rotating part increases probility. As four the goverment. More 426 and 526 hemi engines have been sold last year than the last ten years Why people want performance. The numbers done lie. Men want one thing the badest and the best. I dont here my customers say it runs so clean they say that ***** ran like a scalded dog. We were running the best its ever run. Real men want perfofmance. V

Ficht Guy · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Wow, I think I need to go back to school. I knew I slept through too many of those science classes

vinney · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

EVERBODY WINS. I KNOW I WILL BE THE LAST MAN STANDING WITH MY TWO-STROKES IN HAND. BOATS OR SNOWMOBILES. IF THEY EVER OUTLAW THEM I WOULD SET THE ON FIRE BEFOR ANY ONE WOULD TAKE THEM AWAY. I DONT SEE TO MANY SNOWMOBILE THAT ARE FOUR STROKES IN CANADA. I RUN A T CAT WITH A PREDITOR BIG BORE KIT DYNO AT 240 HP FOR THE TRAILS IT DOESNT CLAIM TO BE CLEANEST GUST THE BADEST TRAIL SLED ON THE PLANET. ITS A TWO-STROKE V

Forktail · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

SCO, what I said was that the HP of engines is a "false measurement". HP can not actually be measured. It can only be "calculated" based on actual measurements of Torque and RPM. In order to get HP, the twisting force of Torque (something we can actually measure on a dyno) must be converted into these units of HP. This is why Torque is the correct property instead of Work. Torque is the only property to take into account rotational forces. I didn't want to get into it this deep in fear of losing others, but there is linear physics with Power and Work, and there are rotational physics with Power and Work. When we deal with HP in engines we are dealing with rotational physics. In fact, a dynomometer which measures Torque (so we can get HP) uses rotational inertia and angular accelleration (in radians) as key components. In linear physics Force = mass X accelleration, in rotational physics Torque = rotational inertia X angular accelleration in radians. So Power actually becomes Torque by angular velocity. Yes, I do believe JB was confusing Torque with Work. I too realize what he was trying to say and I realize he's not an engineer, but IMO his description was somewhat misleading and too generalized. You'll get a funny look asking your dealer how much "Work" your new outboard does at 5500 RPM, but you might get a smile if you ask him how much "Torque" your new outboard makes at 5500 RPM. Again, Torque is what is measured on the dyno and charted on the graph, not work. Work an engine does is more commonly referred to as the area under the Torque curve between the starting and ending speed of the crankshaft (RPM range). Torque is a property that can occur without factoring in time or movement. It is only until that Torque is actually moved that Work occurs. And although Work might seem more important to us as outboard operators, the reality is that outboards are not rated or sold by how much Work they can do. They are sold by how much HP they produce. That HP can only be determined by measuring the Torque, not the Work. To put this to rest, I will agree with you in the most general sense that "Work" is a way to describe "Power". Mechanical devices incorporate the unit of "Work" into HP. And this HP or unit of Work has a time element associated with it. Torque does not have a time element. This is why when JB mentions "Work" at a "RPM" he is almost, shall we say, double dipping the time element. Again, this is mostly due to the fact that we must consider and understand rotational physics. The best way to describe HP would be Torque at a RPM. From there we can determine HP as a unit of Work. As you say JB did describe Power in terms of Work, but HP in engines must be calculated in terms of Torque.

JB · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Wont let it go, huh, Forktail? I did not say that work is power, or vice versa. I said power, horsepower, is the potential to do work and it is rated at a particular rpm. It is disappointing that an engineer who is so quick to lecture on mechanics doesn't seem to think it has anything to do with physics. That is like saying algebra has nothing to do with math. You seem to have forgotten the basics of physics from which your shortcut equations are derived. My undergrad degree is in engineering, my doctorate in adult education. I only taught physics to engineering students for twenty or so years. Nothing moves, changes temperature or changes state unless work is done. Power, be it horsepower or kilowatts is merely the potential to do work. Mechanical work is the consequence of force over distance for a time. Torque is force and a revolution is distance, or have you forgotten geometry too?

Forktail · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

Vinney, performance is a relative term, whether you're a real man or not. I love 2-strokes myself. But for example, your big bore Thunder Cat would be the worst performing sled I could imagine. I mountain ride, as do most people here. Sometimes in order to mountain ride we must run trails 50-100 miles. Not only would your sled perform like a heavy slug in the deep snow, but it would be a pig to maneuver in the trees and virtually impossible to climb sidehills with. To top it off, you would need to sled in 20 gallons of fuel to make the trip. Riding that sled with us would be the worst day in hell you could imagine. In fact, you'd probably have to find your own way home. Not exactly performance. Performance here would be a 151" paddle long-tracked, light weight, narrow stance, high handle bar, fuel efficient jumper. Another one. Not all boat owners run their boats "hard and fast" like the DEA or Police. Many of us fish. I can't imagine trolling for 10 hours at 2mph with performance 2-strokes when I can do it with a 4-stroke. Been there done that. I can't imagine beating up my fishers in the common 5-foot chop running WOT. Performance to me is a dependable, powerful, quiet, fuel and oil efficient, mid-range cruise. My Yamaha 4-strokes don't require a valve adjustment. Yamaha recommends a valve clearance inspection every 2 years. They can be adjusted if they need it. Mine never have. It probalby takes less time to do this than the time a 2-stroke owner has taken over the 2 years adding oil to their injection tank. My 2-strokes recommend an ignition timing inspection and adjustment after the first 10 hours, and every 100 hours after. I'd like to know how you determined that more rotating parts relate to more failure probability. I've never seen this data? I always thought the pressurized oiling system of the 4-strokes made them more failure resistant, than the gas/oil mix 2's. If people want performance, why are they buying 4-stroke Hemi's? Where's those 2-stroke Hemi's? The diesel market is growing way faster than the Hemi market. Performance? You bet. Not performance in a way the diesels are kick'n the Hemi's, but in an overall performance experience. People buy 4-stroke outboards for more than the fact they run clean. They buy them for their overall performance. And performance is a relative term.

Forktail · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

"Wont let it go, huh, Forktail?" JB, what exactly are you implying? I didn't realize these threads were limited in what we had to discuss, or that we should feel guilty for continuing a discussion just because you are having trouble agreeing with it. This is a very interesting and informative discussion here relating to the HP of our outboards. SCO and Dj have some good knowledge. Please either join in or quit the childish antics and snide comments. "I said power, horsepower, is the potential to do work and it is rated at a particular rpm." Oh. I must have missed that part.

Relax JB. We know what you meant. Hopefully with your education, you will know what I mean. "It is disappointing that an engineer who is so quick to lecture on mechanics doesn't seem to think it has anything to do with physics." Uhh, you lost me there. I was the one who brought up physics, the equations relating Torque and Work to physics, and how these issues applied to physics. I even provided examples. "You seem to have forgotten the basics of physics from which your shortcut equations are derived." No, sorry. You, as a physics teacher and engineer should recognize that the equations I posted are derived from Newtons basic physic idealologies. Force = mass X accelleration, and Torque = rotational inertia X angular accelleration. I do not feel this forum is the correct place to go into how HP = Torque X RPM/5252 is derived. The constant 5252 alone would take a page. "Mechanical work is the consequence of force over distance for a time." Yep. In engines we call it HP. And that HP is calculated by measuring a "force" called "Torque". Be happy JB.

SCO · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

This stuff is painful to read. Gotta call you this thread's villain forktail based on your style of argument. Instead of trying to read and get what people try to say, you are pouncing on any imprecision. I do enjoy reading what you have to say though, you obviously have a lot of specialized experience and I for one hope you continue to throw it out. I thought you were saying hp is a false measurement because it is based on torque, not work and therefore not really a power (a false power) or something like that. You have set that straight and I accept it. Can any power be measured directly? It all must be determined by false measurement then. I point this out to illustrate that it is tough to convey ideas here. I still don't know where you were going w that line of thought. Most power measurements don't involve torque, so some of us who want to store the fundamentals remember that power is the rate of energy consumption.

vinney · Nov 4, 2002

Re: Power of 2 Stroke v 4 Stroke?

THANK YOU ALL

Power of 2 Stroke v 4 Stroke?

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