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  1. #11
    Epic Contributor MossRoad's Avatar
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    Power Trac PT425 2001 Model Year

    Default Re: PT425 vs. PT1430 Mystery?

    BTW...

    Here's a link to this same discussion that took place around March of 2004. [img]/forums/images/graemlins/wink.gif[/img]

  2. #12
    Veteran Member Charlie_Iliff's Avatar
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    Default Re: PT425 vs. PT1430 Mystery?

    <font color="red"> If we hopped up the Kohler to get 30HP out of it, the 30HP Deutz would still pull it all over the place if they were hooked to the same driveline....I think , because it has much more torque. </font>

    Sorry to disagree, but just ain't so unless the driveline itself won't let the Kohler get to its power peak with equal efficiency.
    Horsepower = torque*rpm/5252 (The constant assumes torque is stated as ft lb) Often horsepower is measured by a brake holding the full throttle engine at the specified rpm, with the dyno measuring the torque applied to the brake. The ones I've used have been water pumps, rather than friction brakes, with a lever arm to a strain gauge. Simple ft lb measurement.
    If you gear the machines to turn each 30 hp engine at its horsepower peak at the same wheel speed, you will get the same pulling force. Said differently, when two engines deliver the same horsepower, the torque at the wheels, and thus pulling force, will be the same if the gearing that changes the rpm is of equal efficiency. Gearing - either with hardware or hydraulic controls - increases or reduces output torque by reducing or increasing output RPM.
    But, of course, diesels will <font color="red"> seem </font> to have more pulling force because their torque is at a lower rpm so you have more part throttle/low rpm pulling power.

    End of rant. [img]/forums/images/graemlins/grin.gif[/img]

  3. #13
    Epic Contributor MossRoad's Avatar
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    Default Re: PT425 vs. PT1430 Mystery?

    It's not a rant... I'm still learning here and appreciate the discussion without the yellin' [img]/forums/images/graemlins/tongue.gif[/img] [img]/forums/images/graemlins/grin.gif[/img]

    OK, I see your point about them not working the same if they are on the same driveline due to the engines producing peak torque at different RPMs.

    So, I'm looking at it this way...

    Let's take the driveline out of the equation for now.

    If I take a 30HP gas engine similar to my Kohler and run it at it's peak torque RPM and I take a 30HP diesel engine and run it at it's peak torque RPM, which one is going to produce more torque at the driveshaft? [img]/forums/images/graemlins/confused.gif[/img]


  4. #14
    Veteran Member Charlie_Iliff's Avatar
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    Default Re: PT425 vs. PT1430 Mystery?

    Got all the way through typing this and my connection timed out. Maybe the internet spirits are trying to tell me something.
    <font color="red">
    If I take a 30HP gas engine similar to my Kohler and run it at it's peak torque RPM and I take a 30HP diesel engine and run it at it's peak torque RPM, which one is going to produce more torque at the driveshaft? </font>
    OK - looking at the crankshaft or crank pulley, with no gears or pumps:
    The diesel will measure more, because you asked about peak torque, not peak horsepower. In fact, the diesel will probably make more torque at the crankshaft at every RPM in its - the diesel's - range. Without looking up real torque or horsepower curves, the following may not be far off: Assume both engines have peak torque at 2500, which is possible. The diesel may have 60 ft lb or torque for 28.56 hp, but the gasoline only 40 ft lb for 19 hp. at the 2500 torque peak of each. The diesel may then have max horsepower, 30 at 2750 rpm, but with only 57.29 ft lb of torque. The diesel won't turn much more than the 2750, but if it does, both torque and power will fall off.
    The gasoline engine at its power peak may be 4500 rpm, where the 30 hp means it is only putting out 35 ft. lb of torque, down from the 40 at 2500, but developing 30 hp instead of 19.
    What ultimately makes pulling force is the torque at the wheels, of course. If the gears reduce the diesel's 2750 rpm to 42 rpm (about 3 mph with a 24" wheel) at the wheels (65:1 ratio), and the gears on the gas carriage reduce the rpm from 4500 to 42 rpm (107:1 ratio) , they will have identical 3751 ft lb of torque, and thus 3751 lb of pulling force if the tires don't slide. This assumes, of course, the impossible 100% efficiency of the gears. [Please anyone chime in if I've screwed up the math. I'm working only with the HP=Torque X RPM / 5252 formula, 1 foot wheel radius and direct rpm relationships.]
    Maximum torque at the wheels is delivered by maximum horsepower at the crankshaft, and gears (or hydraulic torque conversion). In order to get maximum torque at the wheels for drag racers, they configure the engines to turn as high as they can get them to go without exploding or running out of air, and gear them to turn the wheels from a stop. Not uncommonly, they turn big V8s at 9000 rpm, where horsepower is massive but torque fairly low. The torque at the rear axles, however, may wring them off.
    It isn't just a diesel v. gasoline comparison, either, since some newer diesels are lighter and turning faster, and some workhorse gasoline engines actually have torque and power with peaks at low rpms. You could probably find a much larger and heavier gasoline engine than the Kohler, with large displacement, short cam duration, long intake and exhaust, etc. with torque and power curves like the Deutz diesel, and the same 30 hp maximum output, at 1500 or 2000 rpm. It would be even bigger and heavier than the diesel, however. One of the advantages of the Kohler, Robin and car engines, etc. is that you can get power out of a small light engine by turning it up.
    And, the maximum power or maximum torque figures do not tell you much about the real working capability of the machine. If it has a steep and high power peak, it will deliver high power only at or near the best-horsepower rpm, and at any other range it will be deficient. The great advantage of the hydrostatic transmission is its continuous variability, so that you can pin the engine at its horsepower peak and vary the wheel or implement speed over a wide range with the hydraulics. Although the hydrostatic systems are a lot less efficient than hard gears, they allow you to hold the engine near high power so average power to the work is greater.
    And boy can you find a lot of threads on TBN strenuously debating that point. [img]/forums/images/graemlins/smile.gif[/img]

  5. #15
    J_J
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    Default Re: PT425 vs. PT1430 Mystery?

    And I thought shaft HP was just shaft HP. [img]/forums/images/graemlins/grin.gif[/img]

  6. #16
    Epic Contributor MossRoad's Avatar
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    Default Re: PT425 vs. PT1430 Mystery?

    OK.

    Lets talk about the PTO circuit on the Power Trac.

    Lets use the same pump for comparison. To get 8GPM out of the pump attached to the gas engine, you have to run it at 3600 RPM. Since the deisel will not run 3600 RPM, we have to use some gears to get it there. So now we have both pumps turning 3600 RPM. Both engines are putting out 30HP. Both pumps are flowing 8GPM. We flip the switch and start up the mower and head into thick wet grass. The gas engine goes rrrrrrrBLAAAAAA and croaks and the Deutz goes rrrrrrrRRRRRRRR and keeps going. How come? [img]/forums/images/graemlins/confused.gif[/img]

  7. #17
    Epic Contributor MossRoad's Avatar
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    Default Re: PT425 vs. PT1430 Mystery?

    <font color="blue">And I thought shaft HP was just shaft HP. </font>

    [img]/forums/images/graemlins/grin.gif[/img] [img]/forums/images/graemlins/laugh.gif[/img] [img]/forums/images/graemlins/cool.gif[/img] [img]/forums/images/graemlins/smile.gif[/img]

    I need analogies that I can relate to...

    Anybody got one to compare beer and alcohol content to torque and horse power??? [img]/forums/images/graemlins/tongue.gif[/img]

  8. #18
    J_J
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    Default Re: PT425 vs. PT1430 Mystery?

    I think that is called low end torque.

  9. #19
    Veteran Member Charlie_Iliff's Avatar
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    Default Re: PT425 vs. PT1430 Mystery?

    <font color="red">Both engines are putting out 30HP. Both pumps are flowing 8GPM. We flip the switch and start up the mower and head into thick wet grass. The gas engine goes rrrrrrrBLAAAAAA and croaks and the Deutz goes rrrrrrrRRRRRRRR and keeps going. How come? </font>
    If the first part of the assumption is right, and both are putting out 30 HP, and both are geared to turn the same pump at the same speed, then the second part of your assumption is wrong. Either neither will bog or both will. But, if both bog, the gasoline engine is peakier, so its horsepower will fall off faster as rpm decrease, and the loss will compound. That's why you have to feather the go pedal to keep the engine in its high output range, and probably have to do that more with the gas engine. If you let it drop to the 2500 in my earlier example, you're down from 30 to 19 hp and maybe not enough left to go back up. If 30 hp is enough to charge into the grass with mower singing, it's enough whether the rpm of the engine is 2500 or 17,000 (formula one), but if you knock the engine off its power peak, the steeper the curve, the more you'll lose. Typically, gas curves are steeper than diesel.
    (Of course, if the assumed numbers in my prior calculation were right, the gas engine would have a very flat curve and not fall off so much -- the problem with assumptions for discussion examples.)

  10. #20
    Epic Contributor MossRoad's Avatar
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    Default Re: PT425 vs. PT1430 Mystery?

    <font color="blue"> If the first part of the assumption is right, and both are putting out 30 HP, and both are geared to turn the same pump at the same speed, then the second part of your assumption is wrong. </font>

    Oh, well, uh.... that could explain things. I've been wrong before. [img]/forums/images/graemlins/tongue.gif[/img] [img]/forums/images/graemlins/grin.gif[/img]

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