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 06-17-2003, 17:14 Post: 57863
johnfundy



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 horsepower vs displacement

I just read a reply on this board that stated the horsepower rating of an engine is less relevant than it's displacement. In essence, the comment suggested less horsepower from a large cubic inch displacement engine is better than more horsepower from a smaller CID engine. I disagree, but maybe there is some truth here. I understand "torque" as the act of doing work, frequently expressed in ft.-lbs. ( or newton-meters for Metric fans). I understand the rate of doing work per unit time, ft.-lbs. per second, is "horsepower". So where does engine displacement come into play?



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 06-17-2003, 20:38 Post: 57881
TomG

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 horsepower vs displacement

There are quite a few discussions on this subject in the archives. I don't recall this one but I probably did read it. I can only guess the reasons behind the comment but there can be various valid ones. For example, other things being equal a relatively low HP engine/displacement should last longer that a high HP smaller one. High HP/displacement often is produced by tuning so peak torque occurs at higher rpm, which generally means more engine travel per hour of operation. Other means of increasing HP such as higher compression places greater stress on components like engine bearings which tend to be smaller in smaller engines.

The reasons may not have had anything to do with engine life but could simply be a preference for how low rpm engines feel when they're working. Big engines with low HP engines often produce peak torque in a narrow band at low rpm's. The torque may be greater than in higher rpm engines. They also tend to have more rotating mass that gives them better resistance to load increases. There's a bunch of things that can be said for old iron and many people prefer working with such engines.






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 06-18-2003, 11:13 Post: 57927
AC5ZO

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 horsepower vs displacement

Tom pretty much covered it, but let me add a couple of things. Larger displacement engines deliver more torque. (Longer stroke is more important than bore size in an engine from the torque standpoint.)

Power or HP is the rate of delivery of torque to produce useful work and as such is essentially the engine torque multiplied by the RPMs and a factor to get the right units.

I have seen many tractor and engine specs where they will nudge out an extra HP or two by turning up the rated RPMs on the same basic engine by 10% or so. A 10% increase in RPMs increases the wear rate on the engine by 33%. (Wear is a factor of the cube of speed in RPMs) This is the wear rate during running, but in my experience running wear rate is less than the wear that occurs during starting.

Allowing the top engine speed to increase to provide a couple more horsepower does nothing to improve torque in the low end or mid range of the engine power. I don't know about you, but I spend much more time at mid-range of the throttle than I do at rated speed or full RPMs.

So, what does this all mean? A 28HP and 30HP engine might perform almost identically in the low and mid ranges if the engines are derived from the same basic mechanical block and have identical bore and stroke. The extra 2 HP may come from allowing the engine to rev out a little further.

One other example that you may recall relates to racing. You may have heard the term "qualifying engine." A qualifying engine is a very "hot" version of the standard racing engine. It will produce more peak HP with the same bore and stroke, but its life will be measured in minutes not hours. Its purpose is to put your car above the cutoff for the qualifing cars on raceday. You have to also remember that racing engines will get close to 1000 HP out of 2 liters of displacement by turning well over 10,000 RPMs, but they only last one race.

If a tractor manufacturer gets your business by specifying 30 HP over comparable models rated at 28 HP, then I suggest that you may want to look into this a bit further.






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 06-18-2003, 11:35 Post: 57928
buzst1



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 horsepower vs displacement

Hi John,
I'm guessing your refering to my comment. The issue is that there is no easy way for a buyer to compare tractors based upon HP rating because there is no standard means for expressing HP. ie: would anyone believe a 24HP Sears lawn tractor as powerfull as a 24HP Kubota? The Mfg's have the ability to publish a HP rating based upon any rpm they desire, regardless of the fact that it may be on the down side of the torque curve. An example of this is a comparison of a kubota BX2200 and B7500. Because the published HP on the BX2200 is 22HP and that of the b7500 is 21HP many people actually believe the bx is the more powerful of the two tractors. If you examine the engine specs of the two machines you quickly find that the B7500 is a far more powerfull machine. Given that the technology of most of todays diesel engines is so similar, when comparing machines it really easiest to look at displacement to see which has the larger and "most likely" most powerfull engine. after potential choices have been made then you can compare the power curves of the engine to make a final selection.






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 06-18-2003, 14:29 Post: 57937
Chief



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 horsepower vs displacement

I realize this is an orange conversation but here is some info. that applies to what you are talking about. Same thing applies to Kubota. I posted this in another forum on a conversation pertaining to the difference in power between the John Deere 4310 and 4410:



The 4310 engine (Yanmar 3TNE84) displaces 91.3 cubic inches. The 4410 engine (Yanmar 3TNE88) displaces 100.2 cubic inches. 8.9 cubic inches displacement is not a world of difference but certianly does add something. That something is a 9.1% increase in displacement and a corresponding increase in hp & torque as well as a 4mm longer stroke. This comes to 73.1 ft./lbs torque for the 4310's (Yanmar 3TNE84) and 81 ft./lbs. torque for the 4410's (Yanmar 3TNE88). That comes to about 9% increase in torques as well. Most would consider this a noticable and sizeable increase. Question is.......... is it worth the extra money to you?






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 06-18-2003, 15:15 Post: 57939
AC5ZO

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Even though the first question that a tractor dealer is going to ask is "What HP?; it is probably better to consider frame size and what you are going to do with the tractor. This puts you into a class that can use the right Backhoe, plow, FEL and so forth. The companies have done a pretty good job of matching the engines to the proper frame sizes. Competition selects the leaders. After you have selected a basic frame size you ask the question about whether more power is worth more to you.

With respect to the final selection of HP, you may want to also consider the standard hydraulic flow capacity and other things that might guide you. If you have a choice of engines, I personally would choose the slow, long stroke version. In my case, I live at over a mile above sea level and engine HP is lost. That can be made up for with more displacement.

BTW, I was not referring to anyone's comments outside of this immediate thread. I do not have a Kubota but this question came into my area of interest so I responded.






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 06-18-2003, 18:03 Post: 57950
DRankin



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 horsepower vs displacement

Ok. I am NOT trying to stir the pot here, I just want a bigger perspective.

I know that a well made, well maintained diesel pick-up engine can go 250,000 miles...... what is the RPM range for these engines?

And, does anyone know at what RPM range diesel car engines such as VW's and Audi's run?

I am thinking that my BX engine running at 3200 RPM (max) can't be all that unusual or all that fast.






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 06-18-2003, 19:50 Post: 57959
Chief



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My Cummins is set to limit fuel at 3250 rpm. I rarely take it above 2800. Never need to. I makes 505 ft./lbs. torque at 1800 rpm. With a reflash or a chip/box it can go to 660 ft./lbs. and much more. 75 mph = about 2000 rpm.






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 06-19-2003, 07:19 Post: 57993
Art White



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 horsepower vs displacement

Horsepower and displacement has little to do with each other as far as a relationship. When the word torque enters in then it changes. Normally bigger is more at this point. There are so many variables here, my sled has 800cc and produces 160 horsepower. Thats about 50 cubic inches, that's good horsepower and virtually no torque.






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 06-19-2003, 10:25 Post: 58002
AC5ZO

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Diesel engines and other engines for that matter will last for a very long time with good lubricant and long running. In my experience it is starting the engine that has the worst effect on wear. This is especially true of engines that are not run for quite a while. I have even used engines where you can pressurize the oil externally before starting it.

The oil film on cylinder walls will thin out after a time of draining. The oil film in the journal bearings will also thin out. When you start the engine, it has little lubricant until the oil pump gets it flowing and the crank gets some oil into the bottoms of the dry cylinders. This is why I personally use the best synthetic oil that I can find. I am not trying to start an oil debate here.

What I said about wear rate being the cube of engine speed is true, but if the wear rate is very small to begin with a 33% increase may not show up in practical terms. But, when selecting the engine for my tractor, I want the torque. That means more cubic inches.

Mark, you live at about the same altitude as I do, I think. At 5300 ft, my engine only produces about 85% of what the published specs say. I still vote for more cubic inches.

Changing the chip or programming of an engine affects the fuel usage primarily. But, there is a limit to how much fuel you can burn in a given engine cylinder and how many RPMs you can safely run.






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