So, the motors are to small, draw to many amps, and the gearing is wrong. That I understand. Would it be almost impossible to modify one and get it right???
Unfortunately whenever the Bachmann K-27 is discussed a lot of emotions seem to be put forward. We all have differences of opinion but hopefully it is possible to have civil discussions.
Dave has posted some useful information so lets continue the thread by filling in some of the blanks omitted.
The current production Bachmann K-27 uses a Pittmann 19 volt 9234 motor with ball bearings. This is one of the most powerful motor used in any large scale locomotive.
It has a max torque rating of over 41 oz-in and is an excellent choice for such a large model.
I am a modeler and not an expert in motor selection or gear ratio so when Dave first raised the question of gear ratio I asked several manufacturers who are known for excellent motor/gear selection.
What I learned is that the motor gear ratio is not as simple as a set number, there are many factors that go into the optimal selection.
Dave has a lot of experience with the Pittmann 8000 series motor and Dave has observed that for the 8000 series motor in a typical large scale locomotive that 30 to 1 is optimum for his usage. That of course assumes a similar driver size and similar desired top speed.
But there are differences. For one the 8000 series motor is a 10,000 RMP motor while the 9000 series motor is a 6,000 RPM motor, The same gear ratio for both motors is going to result in a much different speed curve as well as a much different top end speed.
Other factors are driver size weight, trying to stay at the high efficiency side of the motor curve and ensuring that the locomotive slips well before motor stall.
When all this is factored into a locomotive the final and key issue is both slow speed performance and top speed. And it is the top speed that is the hardest for modelers to agree on.
As has been pointed out on a few other threads there is a great difference in what modelers desire in their large scale models. We are very fortunate that multiple manufacturers are building some beautiful locomotives in a variety of large scales. But we as a community do face an interesting issue. It takes a lot of money to produce a mass produced model so the manufacturers have to support a large variety of modelers.
On the high end we are very concerned about fidelity to scale and gauge and also prototypical speed. The mass market however tends to operate trains faster and is less concerned with the details and specific prototype.
What the manufacturers are trying to do is develop models that can appeal to both groups of modelers. This can be a win win for all concerned.
The K-27 prototype has a max allowed speed of 30mph. This speed limit is posted in the cab. A typical RC user operated his locomotive at 14.4 volts and the Bachman K27 will have a top end speed in the 30mh range at that voltage.
Some time back when this issue first came up I actually measured the speed of the Bachmann K-27 at a scale of 1:20.3. Unless I made a mistake in the calculations the unloaded speed on DC was 41 scale MPH at 18 volts DC. At 23 VDC 69 scale MPH and at 13.8 VDC about 33 scale MPH. Personally I think the 24 volt speed is a little high. I personally prefer a mid 50s speed at 24 volts as this allows headroom at the slightly lower voltages but can live with the slightly faster top speed. At the low end I measured 0.2 scale miles per hour which is well below what the prototype is capable of.
If I do the math right, for you 1:29 folks for comparison that would be a top speed at 24 volts of under 50mph which is slower than the max speed of most of your equipment and well in your preferred operating range.
If we as a community could agree on a desired top speed then I am sure we could get the manufacturers to implement that desire. But as it is now a lot of us like to run our locomotives much faster than prototype narrow gauge speeds and reducing the 24 volt DC top speed of the Bachmann K-27 by half as Dave suggests would I fear only appeal to a very few.
I am confident that if there are enough models that prefer a lower speed that after market folks will develop a new set of gearing. For me the current gearing is fine as most of the electronics I use has the ability to customize the speed range and it is trivial to set the top end speed to be what I desire. For DC users that want a slower top end speed simply do not crank the throttle up to full voltage.
Hopefully the above provides a balanced view of gearing issue and perhaps will lead to a positive and productive discussion on desired top end speed for the community.