The Challenges of Modifying a Toyota C150 Transmission.

Toyota C150 Transaxle

The C150 is one of the first 5-speed transaxles made by Toyota, here in the US this gearbox would be used in the Echo, Tercel, Paseo, and 80's Corolla. These cars were smaller and made a lot less power than they do now, so it makes sense that the C150 is more compact than the modern units. This was my first time building one of these for an Echo track car, so I logged all of my findings:

Case

C150 Bellhousing

The bell housing is the foundation of these C transmissions, since this is where the differential and main bearings reside. After inspecting and measuring, the C150 and Yaris C50 bell housings are very similar. Both have the 4.5in distance from the Output shaft to the differential, and all of the internal features are identical. The main differences are the threaded holes on the C150 for a rear mount(C50 uses a pitch stop mount at the bottom), and the Yaris C50 has threaded holes for the shift cable mount(C150 does not). So use the bell housing for your cars engine and mounts, then build upon that.

Case Comparison

The first thing I noticed was the compact design. The whole C150 is almost the same length(5/16ths shorter) as a C5x, but the middle case and end caps are completely different. The C150 center case that houses the shafts and gears 1-4 is a half inch shorter than the C5x case, this also means the shift rails are different since the detents are a half inch closer to the shift selector. The C150 end cap is a 1/2 longer, because it houses the same size 5th gear components but has to reach that extra 1/2 in due to the shorter center case. This is where I started comparing the internal parts.

Gears

C150 Internals

Because this is an older economy transmission, a lot of what's inside is low budget. What I mean is that the 3-4 shift hub uses the C-clip style dog spring, 2nd gear has a 1 piece syncro, and it uses the most common ratios. 

The differences are obvious here, aside from having a different synchronizer system and thickness the tooth count and journal size are the same!

Same thing here with 2nd gear, the only difference is gear thickness.

Third gear is where they start to look similar, only a 1mm difference and the synchros aren't that different either.

I also measured 4th and 5th gears from the 2 transmissions, and they seem to be the same so we'll skip over them.

Shafts

You can see here that the C150 shafts are much shorter as well. If you look closely you can see that only the clutch splines, pinion, and 1st-3rd sections of the shafts is where the differences are. All of the journal diameters are the same size.

Bearings

The C150 bearings are essentially identical copies of the C5x, just 2mm smaller.

Differential

The differentials are very different in the two transmissions. Although they both have a 67 tooth ring gear, they are not interchangeable with other pinions. This is because the Echo has a 3.5FD compared to the C50's 3.7FD, and that means an extra tooth on the C150 output shaft. The extra tooth adds an extra 2mm to the pinion diameter and that leaves no room for any backlash(ZERO). If thats not enough, the C50 ring gear is 2mm thicker so it wont clear 1st gear on the C150 output shaft. 

The other areas the differential is different is in the bearings. The taper on the C50 and C150 bearings is different, AND the size of the bearing. The ring gear attachment is also different, the C150 has threads in the differential where the C50 has threads in the ring gear.

What now?

Now that we know what the parts look like, let's discuss upgrade options.

RATIOS:
 Because gears 1-3 are different from the C5x/C6x, you cannot swap in any other ratios except those from the C160. That requires importing a whole C160 from Canada or Japan, since many of these parts were discontinued. Alternatively, you can retrofit to a C5x/C6x, but you need to make room for fitment and sometimes new axles as well. Honestly though, the 3.5/1.9/1.3 1-3 ratio spread is not that bad. You may be able to fit a C56 1.03 4th gear and instantly have a close ratio box. 

FINAL DRIVE:
Again, since the ring gear and pinion are proprietary in the C150, there are no simple options. A C5x/C6x retrofit is the only option that I know of, that takes you from the 3.5 to either a 3.7, 3.9, 4.3, or 4.5(6-speed).

DIFFERENTIAL:
I believe there are only 2 options here for the C150, Quaife or Cusco. Many companies advertise they have the correct LSD, but usually it's really an 8-bolt for the Yaris...

I feel like the C56 retrofit is still the best option for every 5-speed, and this will remain my recommendation. You can install the Cusco C110 LSD and do the WPC treatment to the C150 and have a nice build, but the 3.5FD means the gears will feel super long. Check out the chart below: