Prius C electric motor swap possible?

There are "generations", and then there are other "generations".

The first Prius came with a "generation 1" HSD, better known as a THS, or, unambiguously, as a THS I.

The later Priuses came with a "generation 2" HSD, better known as a THS II.

In these transaxles, the MG2 has a 1:1 ratio with the ring gear, hence the MG2 must be comparatively large, some might say very large, compared to the MG1. Also, the MG1, under some conditions, must operate close to its maximum speed, or the ICE must be activated to cause a reduction in the MG1's speed. Not an ideal choice, but a necessary one.

Still later Priuses came with a "generation 3" HSD, sometimes known as a THS III. This includes the P4xx transaxle in the Prius v and Prius Plug-in, and possibly others (Camry, Highlander, ?), and the somewhat smaller P5xx transaxle in the Prius c.

In these transaxles, the ring gear is really a compound gear, with separate reductions from the ICE/MG1 and the MG2. The ICE/MG1 path remains known as the "power split device" path. The MG2 path has been named the "motor speed reduction device" path.

Depending upon the specific implementation, the MG1 ratio could be 0.9 to 1.1, whereas the MG2 ratio is generally around 2.5:1.

A direct consequence of this 2.5:1 reduction is the MG2 turns about 2.5 times as fast as the compound gear, thereby providing a torque multiplication equal to the reduction ratio. As torque and horsepower are directly related, a much smaller MG2 is capable of delivering equal or greater power than the earlier, much larger MG2.

In some THS III examples, the cross-section of the MG2 is about equal to the cross-section of the MG1. This facilitates a size and weight reduction of the entire THS.

 

The THS-III has been employed on Toyota, Lexus (and even on competitive) products since 2006 (possibly the P310 transaxle, but I am not certain about that). Since 2010 for the full-sized Prius (Gen 3; P410 transaxle). Since 2012 for the mini-sized Prius (c; P510 transaxle).

Depending upon the specific implementation, the MG1 ratio ("Power Split Device") may be 0.9 or 1.0 or even 1.1, with an average of 1.0.

Again, depending upon the specific implementation, the MG2 ratio ("Motor Speed Reduction Device") may be 2.4, 2.5 or even 2.6, with an average of 2.5.

Final drive ratios are tailored to the weight of the vehicle and the size of its engine, and those have a number of options as well.