2 or 3 cylinders, does it really matter?

[Forktail]

Re: 2 or 3 cylinders, does it really matter?

Declared power is what the manufacture "calls" their motor, such as a "150" and the rated rpm has to be in the middle of the full throttle operating range. Most engines have a 1000 rpm operating range, such as 5000 to 6000 rpm, with the max horsepower at 5500 rpm. Some engines only have a 500 rpm operating range such as 5500-6000 with max power at 5750 rpm.

You're just repeating what I've already said here... <br /><br />

By Forktail - Their rated (for sale) power must be the full throttle power at the declared speed (at prop shaft), based on the corrected power on one or more engines. For example 150 HP at 5500 rpm.

And here... <br /><br />

By Forktail - That declared speed must be the mid-point of the full throttle speed range recommended by the manufacturer for prop selection. For example 5500 rpm is the mid-point between a recommended 5000-6000 operating rpm.

Corrected power (on outboards) cannot deviate more that 10% each way from the declared power. (ICOMIA Section 7)

Your statement that a 150 hp outboard can have an "output of as high as 165hp or as little as 135 hp and still meet the industry standards" is wrong. It's wrong because you are deviating the declared (prop shaft) power 10%, not the corrected power (engine dyno).<br /><br />Outboard manufacturers declare their power at the prop shaft per Section 6. Thus, ICOMIA Section 6.2 clearly states, "...the highest power within this range does not exceed declared power by more than 6%. If the highest power exceeds declared power by more than 6%, both powers shall be stated for that model"<br /><br />So in your example, the 150 HP outboard would also have to be declared a 165 HP model (dual rated), since the declared power deviated more than 6%. Clearly, the maximum HP a 150 HP outboard could have would be 159 HP (additional 6% of 150 at propshaft, assuming corrected deviation was not exceeded to do it). It's not that tough seahorse.<br /><br />BTW 10% deviation only applies to a certain HP class of outboards. And the Coast Guard wouldn't be to happy if your 150 HP rated boat actually had 165 HP on it.<br /><br />

High performance models such as Yamaha's Vmax and Evinrude's HO series have horsepower outputs at the very top of the 10% limit.

A repeated spin of what I've already said here... <br /><br />

By Forktail - Now you know, in part, why manufacturers can use nearly the exact same outboard for different HP classes. And now you know why some of us like to prop for the top of the RPM range.

Most techs know this and the factories put out this information.

Most techs have no clue how outboards are rated. Try asking one next time. It's laughable. And most techs don't bother reading squat.<br /><br />Even you, the master of all, seem to be learning as you go.

 

[seahorse5]

Re: 2 or 3 cylinders, does it really matter?

Perhaps a discussion with the different outboard manufacturers about how they rate their outboards and what limits they have would be beneficial to you.

 

[Forktail]

Re: 2 or 3 cylinders, does it really matter?

Seahorse, you don't have a clue. You just like to argue with me. <br /><br />Contact ICOMIA. They set the standard, not the manufacturers. Maybe then you'll see the light.

 

[rickdb1boat]

Re: 2 or 3 cylinders, does it really matter?

I beleive the original question had to do with torque, not HP. Two different things. Maybe you guys could discuss that, instead of arguing about HP and how it's rated. Where does this end, guys?

 

[Forktail]

Re: 2 or 3 cylinders, does it really matter?

RB, the original post concerned outboards that were the same power, which in turn brought up a discussion about HP rating tolerances (Chinewalker). <br /><br />I tried to help with that discussion, describing the standards for rating HP. I know many here appreciate that.<br /><br />Unfortunately, we have a member here (seahorse) who shadows my comments, trying to make himself appear as the master of all masters. I'm not sure why. IMO, he is only the master of seaching google on the internet.<br /><br />If you would like to discuss torque more completely than what's already been said here, please feel free. I'm a mechanical engineer, and will try to help you the best I can.

 

[rickdb1boat]

Re: 2 or 3 cylinders, does it really matter?

Forktail, since you are a "Mechanical Engineer", then answer the question:<br /><br />

I've heard that 3 gives more torque, but that 2 might give better fuel economy.<br /><br />This, of course, is regarding 2 engines of the same horsepower.<br /><br />Any comments or ideas?

So is it true or not???? You're the one with the answers. I never claimed to know. You did...

 

[Forktail]

Re: 2 or 3 cylinders, does it really matter?

No thanks RB. <br /><br />There's no reason for me to subject myself to hostility and argument. I only offered to help you because I thought you were sincere.

 

[rickdb1boat]

Re: 2 or 3 cylinders, does it really matter?

I am sincere, forktail! I would like to know from an engineers standpoint the correlation between HP and torque. I really don't know. Other than some 6 cylinder automotive engines put out more torque than 8 cylinder engines with more HP. I would like to know why!! Is it gearing, how they set up the motor for the power band? Just curious. Sorry if I seemed hostile...<br /><br />If you want to explain it to me without the hostile replys, feel free to E-Mail me and I will be more the cordial to you.<br /><br />rickdb1@earthlink.net

 

[seahorse5]

Re: 2 or 3 cylinders, does it really matter?

The thing to remember is that horsepower is only a formula based on torque. A motor's torque times its RPM divided by 5252 = horsepower. A high torque low rpm engine can have the same horsepower rating as a high revving low torque motor. <br /><br />A graph showing both torque and horsepower curves will always intersect at the 5252 rpm point, as long as both scales are the same. <br /><br />This and other info is found in almost any physics book or engineering manual.<br /><br />For a more in depth lesson, I'm sure others here would be glad to chime in.

 

[kd6nem]

Re: 2 or 3 cylinders, does it really matter?

Water,<br />All the above discussions are very interesting (and often heated), but I would suggest the bottom line to your question that started this thread is that it depends on WHICH 2 cylinder or 3 cylinder motor of a given horsepower. I'm not too experienced in this matter at all, but I can't imagine that (most generally speaking)the two banger would often win in either category of torque or fuel efficiency. But bottom line remains which two you are comparing. <br />Right guys?

 

[rickdb1boat]

Re: 2 or 3 cylinders, does it really matter?

but I can't imagine that (most generally speaking)the two banger would often win in either category of torque or fuel efficiency.

But my question was and still is why? It's my understanding that the 3 cylinder loopers produce gobs of torque. Not trying to argue, just want to know why a 2 cyl, as some say, produces more torque than a 3,4 or 6 Cylinder? I just can't comprehend it. Just a SIMPLE reason why would be fine. And I'm not talking HP. Seems irrelevent...

 

[catfish1]

Re: 2 or 3 cylinders, does it really matter?

i'm just guessing here! but maybe its the stroke? does a 2cyl have a shorter stroke than the 2cyl?

 

[jim dozier]

Re: 2 or 3 cylinders, does it really matter?

As I said in a post at the top of this discussion, this question has less to do with the number of cylinders on a motor, and more to do with the total engine displacement, loop charging vs. crossflow porting, port design and timing, and a myriad other engine parameters.<br /><br />Also, often when someone says one engine is more torquey than another they may mean it has torque at a lower rpm which may help in the hole shot and may feel more powerful, OR they may mean that the engine develops more maximum torque OR it may have to do with the prop selection (a car starting in first will apply more torque to the wheels than a car starting in second gear).<br /><br />I have a small Acura Integra GSR with a 1800cc engine. It has a very wide torque band partly because of DOHC VTEC. But even with its wide torque band the power still comes on at a relatively high rpm when compared to most American OHV V8s. Its maximum torque comes on at 6000 rpm and max horsepower comes at about 7700 rpm. If someone wants to beat me in a drag race all they have to do is catch me in a crusing gear and go, because I have to downshift 2 or maybe 3 gears to get on top of my power curve. Most minivans will beat me to 25 mph, but if I am in the right gear it will spin the tires at max hp and I will give you a run for your money (big blocks excluded please). <br /><br />My point is that you need to be careful about comparing apples to apples. There is a lot more to engine performance than some general comparison as stated in the original question and the least (in my opinion) important aspect of this whole discussion is the number of cylinders.

 

[kd6nem]

Re: 2 or 3 cylinders, does it really matter?

Jimd is quite right. Depends on so many things and is difficult to make an accurate generalization. If, however, I were to go way out on a limb I would question whether the twos tend to have any particular advantage at all except perhaps cost to purchase and maybe overhaul. Not that those might not be valid, but I prefer more cylinders for mid size and larger motors. Smaller pistons/cylinders may or may not tend to be more thermally efficient, but it is possible to get more power (and ultimately torque) out of a given displacement which is derived from more (but smaller) cylinders without stressing the motor quite as much. Possible does not necessarily mean that is how manufacturers are tuning their products, just that it certainly can work that way. True of import autos, motorcycles, snow mobiles, PWC, etc. Just depends on a given manufacturer's approach. I would note that outboard motors tend to operate at a somewhat reduced RPM compared to some of the other categories. May allow for longer life of the motor compared to some over-tuned race engine. <br />If nothing else the reduced vibration alone is worth it to have at least three cylinders on mid sized and larger motors.

 

[cobra 3.0]

Re: 2 or 3 cylinders, does it really matter?

I'd say both are similar if all things are equal. The advantage of the three cylinder is that it runs and idles much smoother.

 

[Tinkerer2]

Re: 2 or 3 cylinders, does it really matter?

At the risk of introducing ignorance and stupidity into a highly technical argument (which is my specialty as I know nothing), why does torque matter that much?<br /><br />A given motor of any type can be made to produce a range of torque curves by fiddling with the settings of various stock accessories on it (e.g. ignition timing) or by fitting different accessories (dizzy, breakerless ignition, electronic ignition, single coil, multi-coils, carb, single point EFI, multi point EFI, different air filters, various chips, various sensors, and so on). Or you can alter bore and stroke and combustion chamber shape and so on. Not to mention matching / altering the output through the drive train and various gear ratios.<br /><br />Torque is not fixed but is something that can be made to be high or low through most of the rev range for the purposes necessary for the motor's application. Of course, if you make it high at a given point you lose the ability to make it low at just any old other point, and vice versa.<br /><br />Shouldn't the question really be: Is the torque produced by a given motor suitable for my application? <br /><br />Which really is just another way of saying: Is the motor suitable for my application? <br /><br />In which case it might be one, two, three, four or six cylinder.<br /><br />And then if the fuel economy is unacceptable, look for something else, in which case torque clearly ain't a significant factor.

 

[phatmanmike]

Re: 2 or 3 cylinders, does it really matter?

SOMEBODY PLEASE LOCK THIS SO NOBODY ELSE WILL FIGHT OVER WHO KNOWS MORE...... ADMIN.. ARE YOU OUT THERE

 

[Tinkerer2]

Re: 2 or 3 cylinders, does it really matter?

phatmanmike <br /><br />That's a bit unfair. I'm trying to prove I know less!

 

[seahorse5]

Re: 2 or 3 cylinders, does it really matter?

Just an update on horsepower ratings and allowable tolerances in production outboards, after doing some research. <br /><br />According to ICOMIA, the "ISO DIS 8665 - Small craft - Marine propulsion reciprocating internal combustion engines" is still under development at this time.<br /><br />According to Bombardier Engineering, they use + or - 10% as that is the SAE 1228 standard that is mandated by Title 40 of the Code of Federal Regulations part 91.115 except for some of the small motors. They do not use ICOMIA except for the duty cycles.<br /><br />The ISO and SAE standards are similar.

 

[cobra 3.0]

Re: 2 or 3 cylinders, does it really matter?

....wake me when this post dies.....snore.