1. #1
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    Default New Question on Torque Rating

    Thanks to all of you for your informative input on my prior thread on 'truck torque ratio'. However, a certain individual in our dept. is now being told by used fire truck dealers that horsepower is not as important as the torque.They suggested that a 300 hp with a minimum of 860 ft lbs of torque could fit the bill for a 2000 gal tanker pumper. Examples: Cummins 300hp @ 950 ft lbs. torque, Cattapiler 300hp@ 860, Freightliner 250hp @ 860. To be honest, I'm starting to get confused.

  2. #2
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    Default Horsepower and torque

    Horsepower and torque go hand in hand. Torque, or twisting effect, is what gets you going and up to speed. It's what overcomes that massive amount of inertia of a vehicle at rest, or when it's in motion and you want to increase its speed. Once the vehicle is up to speed, horsepower keeps you going and allows you to maintain speed.

    Horsepower also is what pumps water. Think of it this way. In the pump, there isn't a resistance to motion like there is with a vehicle at rest. If you got under a vehicle and tried to move it by grabbing ahold of its driveshaft and twisting, it wouldn't go anywhere. But if you shifted to pump gear and twisted the shaft, you could turn the pump. Not very effectively, but with some effort, you could get the pump to turn.

    There's a formula used to calculate how much horsepower is needed to pump x gallons per minute of water. I don't recall it and my notes on it are buried somewhere. I do remember that it takes about 235 horsepower AT THE PUMP SHAFT to move 1500 gpm at 150 psi NET PUMP PRESSURE. When you factor in all of the parasitic losses in the engine and drivetrain (fan, alternator, engine water pump, air compressor, power steering pump, transmission, transfer case and on and on, you are approaching 350 engine hp to get what you need at the pump. And that's when the engine is new (broken in) and producing its full rated power.

    Another example: our new engine that we just ordered (see Toyne wins!), will be equipped with a Cat C13, 430 hp., 1550 lbs./ft. torque, the same as Sdors122 is using in his rescue pumper. We were considering dual rating the pump as 1500/2250. But the most we can do is 1500/2000, because 430 hp isn't enough for 2250. We would have to be at about 460 hp to do that.

    Truck dealers used to have a program called SCAAN that was used to determine horsepower/torque requirements for trucks. It gave you acceleration, startablity, gradability and the like. Trouble is, it was based on highway vehicles, not fire apparatus.

    Unless you only go uphill empty, I can't believe that you'd be happy with what you've listed. And as others have commented, inadequate drivetrains always cost more to maintain, but never give satisfactory performance. Just as a for instance, that 250/860 is less than an old 6-71T Detroit and hardly any more than a 6-71N.

    Stay safe out there, everyone goes home!
    Last edited by chiefengineer11; 06-08-2006 at 02:08 AM.

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    Default Torque, HP, Acceleration, Hill Climbing

    The SCAAN program has been replaced by an updated version called I-SCAAN.
    It is an Allison transmission software. As long as the chassis has an Allison transmission, any truck chassis dealer or quality fire apparatus manufactuer can run estimated performance print outs with various engine models, horsepower, torque, and weight ratings.

    It allows you to compare acceleration times to say 50 mph. and the gradeability capability of various horsepower engines.

    It will show the % grade a vehilcle can climb in what gear and at what mph.

    You'll be able to compare how fast it takes the various hp. engines to get to a certain speed at whatever the estimated weight of the unit may be.

    It's a great tool. You can compare apples to apples. Obviously a higher torque and horsepower engine will out accelerate a lower horsepower engine. But you can make a cost/ benefit ratio evaluation.

    Is the cost of a 400 hp. engine worth the extra cost verses a 330 hp. engine if one has a 0 -50 mph time of 35 seconds and the lower horsepower engine is 0 - 50 mph in 38 seconds for a HYPOTHETICAL example.

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    Buy as much HP and torque as you can afford, the more the better. You will never read or here anyone say that their truck has too much power.

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    Quote Originally Posted by Ledebuhr1
    Buy as much HP and torque as you can afford, the more the better. You will never read or here anyone say that their truck has too much power.
    This cuts it down to the simplest terms. Ledebuhr1 is on the mark.

    Our engine that is on order has a 370 with 1200 ft/lbs, if I remember correctly. It is what we could afford with the rest of the truck.

    Get a demo or two up to drive in your area. See what it does....They will tell you what it has for an engine and judge for yourself.

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    If you get a demo to test in your area make sure your comparing same truck same weight. Oh yea and make sure the water tank is full. 1000 or 2000 gal of water is a lot of weight

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    Default

    One thing to keep in mind when talking about engines is what size chassis you want is going to limit your choices. If you are just moving water and you don't go into a lot of tight situations a heavy tandem axle chassis with a large block (Pre 04 12.7L 04 and later 14.0L) motor can make sense. If you want to put 2000 gallons and a pump and full engine company equipment on a single axle chassis you are probably going to have to downsize the motor and chassis a bit to make the thing safe and legal. If you are looking at used tanker apparatus be careful about weights and maneuverability.

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    For adequate performance in a tender (tanker) you need at least an 11 liter engine. 14 is better. Anything smaller will be straining its guts out and probably cost you in the long run.

    Birken

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