Comparing outboards, either HP vs. HP or 2-stroke vs. 4 stroke, is almost impossible. No matter what type of comparisons are made, it's never apples to apples.First of all, outboards are sold based on a rated maximum power output. Not power output at idle, troll, or 3/4 throttle. Not power under the load of your particular boat. Not power trimmed, jack plated, or proped. Just max power.More importantly, HP is really a "false" measurement. HP is only a true function of torque and rpm, and it can only be measured by that torque and rpm. When torque and rpm are plotted on a curve, the area under the curve is called "work". So although some outboards may have equal HP in comparison, they will do different work. An outboard with a broad torque ranging over a large rpm band will do the most work. In fact a lower HP outboard may do more work than a higher HP outboard becasue it makes more torque over the entire rpm range.With that said, outboards of exact HP can have totally different torque outputs and operating rpm. For example a 225 HP 2-stroke may make its maximum torque in a very narrow band of rpm. Sure, it has the 225 HP, but only in a specialized area that we might want to use for racing, and not cruising or trolling. A 225 HP 4-stroke may make its maximum torque at a lower and wider band of rpm. This may not be a racing motor, but a very efficient cruiser or troller.Many other apples to oranges factors are involved in comparing outboards....Engine design. V-designs operate much different than I-designs. Although they may put out the same HP, I-designs usually run greater low end torque and lower operating rpm.Compression. Smaller displacement, higher compression engines may make similar HP to larger displacement and lower compression outboards.Operating RPM.Displacement.Fuel intake induction.Weight.Propping.And a biggy that everyone seems to ignore.....gear ratios!So identical HP ratings can mean totally different outboards. Lets not forget that inboards are 4-strokes. There is no way a 250 HP 2-stroke outboard will push a heavy 26' fiberglass V-hull as well as a 250 HP 4-stroke diesel. Why? Because the diesel makes almost 3 times as much torque.If I needed max speed and light weight, and want to save a few $$ the 2-strokes are awesome. But I don't need those things as much as I need maximum, overall enjoyment from my outboards. That's why I have 4-strokes. My own experience with 4-strokes vs. 2-strokes leaves no questions asked.
JB,It will be a discussion that will NEVER end. (Re; Yamaha price-US vs. Australia.)I've got too many irons in the fire.To each, his own-"may the buyer beware".See ya, you won't hear until we solve the 320 electrical problem.
Interesting information here about power differences. Thanks all.JB, exactly what didn't you understand? And who is the they you are referring to everyone but you and Djohns?Djohns, sorry for your attitude and the comparison between this thread and another. I don't see that.Id like to know more about power of these engines.
Hi, Runner.I didn't understand what points Sloopy and Forktail were trying to make. I thought maybe DJohns did.I wasn't flaming anyone. If anyone took it as that, I apologize. Thanks for the heads-up Vinney. Got it.
JB, my points were right along the same line as yours.....comparing power ratings, horsepower, torque, work, and all the variables to consider when making side to side comparisons between different outboards. Given what you had to say, I didn't think it was that hard to understand.I'll try again, clearing up any confusion. I just don't like to get too technical here, but at the same time I don't want others to be mislead.
JB said, "The horsepower rating of an engine is the maximum amount of work it can do at it's top rpm."Not really JB. The horsepower rating is not based on "work" at rpm. HP is based on Torque at RPM, where HP = Torque X RPM / 5252 (a constant).Torque and Work are not the same. 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. For example, you can apply a Torque to a lug nut wrench, but there is no Work done until the lug nut begins to move. Work an engine can perform is better understood as the area under the Torque vs. RPM curve." So, a 70hp 4 stroke and a 70hp 2 stroke, each running at it's top rpm, are pretty much equal."This is only true in a sense of maximum measured HP, or one point of Torque at one point of speed (RPM). More realistically overall performance of an outboard is measured by the ability of the engine to do Work. This Work includes the entire RPM operating range and how Torque is managed throughout that range."Dealer's wrenches are in the habit of telling you what the dealer wants you to hear. I seriously doubt most of what you were told, particularly his tale about Yamaha warning that the engine would be damaged."It's a known fact that outboard engine damage can occur if the engine is being operated and lugged continually below its functioning RPM operating range. If you are running twins and one engine fails, it is highly unlikely the remaining single outboard can function at its normal operating range without lugging. Thus the suggestion to run throttled back. I would heed to the dealer's suggestion and the owner's manual." Horsepopwer vs. torque curves vary greatly from engine to engine. In a 2 stroke, port timing and exhaust tuning make a big difference, as do cam profiles in a 4 stroke."This is why it is virtually impossible to compare engines. The variables are too many, right on down to propping and gear ratios. But this does bring out a good point in that 2-strokes do generally produce a more erratic and peaky Torque/HP curve. In order to obtain a flatter and more desirable curve on a 2-stroke, things like intakes, tuned exhausts, timing, and carburetion are carefully calibrated to fill in these dips in the curve. But there is generally a drawback on performance somewhere else since the 2-stroke is more dependent on these factors. Djohns19 said, " Horsepower is horsepower, and torque is torque. The differences are where these numbers cross each other, on a graph, and how quickly each one of these build and remain constant."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.Also, peak RPM is always shown later than peak Torque on the curve. The usable power band is found between this peak Torque and peak RPM. This is why a broad Torque curve over a wide range of RPM will provide the most Work, and the best overall performance, efficiency, and economy." Their reasoning, as I was told, is because they had a displacement advantage over their competition and it helped them achieve a better holeshot "feel". This was particularly true of the crossflow V-4's over the comp's. in line 4's."Displacement has little to do with Torque and RPM. It is how that displacement is used and designed. Many smaller displacement engines create more torque and power. This can be due to things like higher compression, longer strokes, and intake/exhaust/timing differences. Inline blocks almost always have longer strokes and many have higher compression than the V's. Thus the good Torque. The 4-strokes turn me on because they run higher compression, have longer strokes, and run at higher RPM. All things that make the "Work" and performance of the outboard more pleasurable.Here's a fun example:Suzuki's 50 hp vs. 60 hp.50 hp = T X 6500 RPM/5252 = 40.4 ftlb Torque60 hp = T X 5300 RPM/5252 = 59.4 ftlb Torque16.6% HP increase, but an amazing 32% Torque increase at max RPM. Guess that's why the gear ratio's are 2.27:1 for the 50 hp and 2.42:1 for the 60.There's a lot involved in comparing outboardsÂ…Â…
Hmmmm.Let us revisit 8th grade physics.Work: The application of force over distance.Horsepower. 550 pounds moved 1 foot in one second. (Force, distance and time.)Torque: twisting force. Measured as an amount of force applied to a radius. ex: pound feet or Newton Meters.Both your definitions and mine express these values accurately but in different terms, they do not contradict one another.The statement I challenged was that the Yamaha manual advised against running in on one engine. I said nothing about whether an engine could be injured by abuse.You are looking too hard for "yeah, but" (YABUT), Forktail. You, DJohns and I have said essentially the same thing: Any generalization about types and classes of engines will be incorrect most of the time. We have each explained why, but from different angles.I was confused by the tone of your post, which seemed argumentive. I didn't understand what you were arguing about.Are you comfortable now?
No, not comfortable JB. I've always respected you here JB, but there was no argument in my response to ChrisE's original question, and I'm offended that you posted that there was. I tried to make my post as informative and unbiased as I could, without stepping on toes. I fail to see where it was argumentative or where I argue. I didn't mean for anyone to interpret it that way, at least until you brought it up. At least until you and Djohn responded with sarcasm.Coming from an Engineer and someone who has dynoed many engines, my second post clarifies and better states any misconceptions about some of the information given here. I realize no member likes their information corrected, etc. But that is the beauty of many opinions and a public forum. I guess it's bothering me why at first you were confused and didn't understand my post, yet now you say we said basically the same thing from different angles. Seems contradictory. It bothers me that you deleted a post and threatened to close a thread for insults and lack of discussion (whatever that post said), but yet allow Djohns and yourself to do the same. I thought it was inappropriate to bring up the Aussie thread, etc. It also bothers me that you need to ask me if I'm comfortable. What does that mean? And whats up with "8th grade physics" and "YABUT's"?How about we all get along and quit the childish games and repeated apologies (damage control)? This is a good thread with good information.With that out of the way, please simply understand the following...."Torque" is used when measuring HP from a rotating object, such as a crankshaft or propshaft on an outboard. "Work" is not. Torque is the force that produces rotation. Work is not. Work can not take into consideration a force over a radius, only Torque can. Torque, unlike Work, may exist even though no movement (distance and time) occurs. And finally, Torque is the correct and accepted property used when measuring HP in engines. We commonly call it "full load motor Torque", or "breaking Torque". Again, HP in rotating objects equals Torque times RPM divided by a constant 5252. Work can be used in finding the HP in non-rotating objects or non-outboard applications. Work is represented by the area under the Torque/RPM curve, from zero RPM and zero Torque to throughout the entire operating range. Hope this helps people here understand HP in their outboard a little better. It would be great if manufacturers would give us the Torque/RPM curve for our outboards. We could taylor fit our purchase for our particular need or use. Maximum HP ratings mean little other than how much power the engine puts out at a single point in the RPM range. I run my outboards throughout the entire RPM range, sometimes needing the most power down low, if I'm in a rough sea.
IM AN ENGINEER BY TRADE BUT MAKE MY LIVING IN THE BOAT BUSINESS. I LOOK AT EVERYTHING FROM AN ENGINEERING POINT OF VIEW. I WONDER WHY EVERYONE DOESNT LOOK AT THING THAT WAY. LOOK FOR THE CLUES. AS FOR JB WHY IS HE IN THE MIDDLE OF EVERYONE THOUGHTS. SIT BACK AND CHILL.WE DONT NEED ANY HAND HOLDING HERE. THERE OTHER FORUMS FOR THAT. SEMPER-FI
Hmmm. It seems we both feel we have been contradicted, yet neither of us seems to believe we have contradicted the other.I don't think either of us intended to contradict, merely to have our point of view respected.The post I deleted said nothing about the subject of this thread, but included name calling. That sort of post will not be of any value to anyone and will be deleted.Now. This post, and your latest, do not deal with Chris' question, but our egos. If you feel it is necessary to continue, please don't clutter up the thread. Email me. email@example.com.
Vinney, I disagree that 4 strokes need more maintenance. Prove it ?I think the general consensus is that the 2 strokes DFI offer slightly better hole shot,but regardless the strange thing is people are still lining up to buy 4 strokes for all sorts of good reasons.More moving parts doesn't automatically mean more maintenance, I think that's irrelevant.And most of us don't work for the DEA or Immigration or drive expensive fountain/Baja boats so that's why most people are adopting the clean, reliable quiet 4-stroke. In fact consumers are demanding them based on their great reputation.
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 issueboat -skeeter zx2200 bay boatMotors yam 200 hpdi and yam 200 4 strokeresultstop speed 47.1 for the 2 stroke vs 46.80 to 30 6.6 sec. for 2 stroke vs 7.2fuel at optimum cruise speed 4.0 mpg for 4 stroke at 30.7 mph vs 4.3 mpg at 30.3 mph for 2 strokesurprise; 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 milesAs 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.
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.
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.
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.)
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.
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.
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.
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?