A long time friend and I have a dinner bet. I say that four GP-7s can pull a heavier train than two GP-40s due to them having 16 traction motors as opposed to just eight. Who is right? How about two SD40s? I have submitted this question to “Ask Trains”
Depends…
Generic GP40s, at 3000 hp each, or GP40-2?
Ed
Let’s say 4 rebuilt GP-7s still at 1500HP ea and 2 rebuilt GP40s at 3000 hp ea. How about 2 rebuilt SD40s at 3000 hp?
So which one is it?
You make no mention of speed. With 4KHP, you can pull a light train fast or a much heavier train if you’re in no rush (Hence 6 motor units in coal service & 4 motor units in psgr) With 16 motors you can appl;y twice as much amperage than w/8 so you can move more tonnage but the 4KHP puts a limit on your speed.
You make no mention of speed. With 4KHP, you can pull a light train fast or a much heavier train if you’re in no rush (Hence 6 motor units in coal service & 4 motor units in psgr) With 16 motors you can appl;y twice as much amperage than w/8 so you can move more tonnage but the 4KHP puts a limit on your speed.
Thankyou, jimrice4449.
You make no mention of speed. With 4KHP, you can pull a light train fast or a much heavier train if you’re in no rush (Hence 6 motor units in coal service & 4 motor units in psgr) With 16 motors you can appl;y twice as much amperage than w/8 so you can move more tonnage but the 4KHP puts a limit on your speed.
It is not just the power of the locomotives you have to consider. The tractive effort will probably be more important here, if the train will not be going fast. Unless GP40s has significantly higher coefficients of friction between them and the rail than GP7s do, then four GP7s should be able to pull more weight than two GP40s. Also, since four 4-axle locomotives have twice as many traction motors as two 4-axle locomotives, assuming that the GP7s and the GP40s have the same traction motors, the GP40s will reach their current limit before the GP7s reach it. I would say that the 4 GP7s could pull a heavier train than 2 GP40s.
I agree, definitely. Speed is a different matter. But starting the train from a full stop, the four GP-7’s will pull a heavier train.
GREAT IMPUTS!!! Thank you jimrice4449, ericsp,and dave klepper!! You guys have given us some interesting points to consider. For the sake of argument, we are going to assume that it’s a general manifest train on straight and level roadway. The speed say, is 20 mph. So far it looks like I get dinner.
Is that 20 MPH without any consideration for what it takes to start your train?
Your are not asking the question very well. Part of the answer has been given above in that your 4 GP7’s (6,000 BHP) will start a heavier train than 2 GP40’s of the same HP. The reason is the tractive effort of each locomotive set. You will have more tractive effort from the 7’s than the 40’s starting from a dead stop.
The same holds true of 2 SD40’s (6,000 BHP) and 2 GP40’s. the SD’s have more tractive effort than GP’s of the same model.
That is, until you reach 13 MPH. At this speed, the GP 40 is putting full HP to the rails ( below 13 MPH, the GP40 reduces HP to about 2200 to protect the traction motors). This speed has a name - minimum continuous speed. Below this speed, you are into your short time rating.
Each locomotive model has a different MCS. For your GP7/9 it is 11 MPH. The SD 7/9 it is at 7.5 MPH. SD40 is 11.5 MPH. GP 40 is 13 MPH.
So, your locomotive, standard HP for any combination to be 6,000, the SD 7 will start the heaviest train and also pull the heaviest train up to 7.5 MPH. After 7.5 MPH, the train must be lighter to get any more speed. Plugging the appropriate numbers into the equation for each model type, STARTING the heaviest train would be - in order - the SD7, GP7, SD40, GP40.
To OPERATE a train at 20 MPH with consideration to forward movement only, the GP 40 would be the locomotive, followed by the SD40 (almost equal, actually), followed by the GP 7 and lastly the SD 7. The BASIC difference in tonnage at 20 MPH is the number of traction motors in each locomotive. Your GP 7 would have 16, your SD 7 would have would have 24, your SD 40 would have 12 and your gp 40 would have 8. The difference here is in friction and rotary inertia. Simply stated, it takes more HP to turn 12 traction motors than it does to turn 8.
You have many more factors involved, such as the co-effecient of traction, moving tractiv
Getting back to the original question before everyone started muddyin’ it up.
Back when we ran GP9’s on a grand scale, their tonnage rating was set at 1200 tons each. That would be 4800 tons for four units. SD40’s were rated at 2000 tons each or 4000 tons for two. 4800 beats 4000 any day you want to pull tonnage. Now how much simpler can that be???
Which was a big selling point, the railroads could replace 3 GP-9s with 2 Sd40s. Good economics in my eyes.
Randy
Short answer to the question you asked is that the GP 9 set will start a heavier train than the GP 40 set for reasons Kenno explained very well.
Top speed is function of horsepower, so in theory the GP 9 set should do slightly better than the GP 40 set. In fact, the GP 9’s today are probably not producing their rated horsepower so I would expect top speed to be about a push. It would depend on which set was producing the most horsepower on the day you tested.
Mac
Mac
Thanks everyone!!! Really interesting to me!! We hadn’t even considered how drawbar pull tails off with increase in speed. You guys are GOOD!!!
We also have to figure in the changes in wheel slip technology between the GP7 era and the GP40 era. These are not inconsequential and the difference beginning with the -2 era became even greater. Big Jim’s comparison of 4 GP’s @ 7000hp for 4 units with 2 GP 40’s @ 6000hp for the set fails to allow for the additional hp of the GP9 hp advantage. Probably the best match for wheel slip systems and hp would be comparison of 4 GP15’s with 2 GP40 or SD40 units. That might be hard to find tonnage ratings since the units were designed for dissimilar services. The SLSF (BN) or Chessie (CSX) timetables might give such data, though since those roads had all the proper units on their rosters at the same time.
ARBFBE’s points are well taken and can be added to my list above of all the other factors that get mixed in. I just listed a few to provide an idea of the complexityof the issue.
To make comparisons as equal as possible in answering ZEPUDA’s original question, GP 15’s need to be compaired with GP 40’s, GP 38’s with GP 40’s, SD 38’s with SD 40’s and GP 39’s with SD45’s. Although the specific number in the end will differ somewhat, the formula remains the same. 4 GP 15’s (6.000 HP) will start a heavier train than 2 GP40’s (6,000 HP) but the '40’s will move it faster once above 13 MPH.
ZEPUDA — in case you don’t know, the following power sets all equal 6,000 BHP –
4 Axel trucks
4 GP 15
3 GP 38
2 GP 40
6 Axel trucks
3 SD 38
2 SD 40
Unequal comparison == 2300 HP GP 39 (3 units = 6,900 BHP and 12 Axels) and the 3600 HP SD45 (2 units = 7,200 BHP and 12 Axels). Look carefully - there is a “trick” here.
axel=axle[;)]
A GP40 and a pair of double axels - now there’s a picture…