"from a guy at work that used to work at MHI turbo":
The letting system are generations of blade design for at the time the wheel was designed. the A series went out of production in the 80s and were replaced by the B and C series. The B/C series eventually gave way to the G series in the early 90s. the T series were designed in the late 90s and early 2000s. Generally, the later wheels are structurally stronger, and typically have improvements to increase map size and maximum wheel speed. You can see this in the blade design itself, as the older turbos generally have evenly sized and spaced fins while later turbos have more aggressive appearing fins with mismatched blades and uneven spacing.
The number rating (15T, 9B, etc) is historically used to designate the wheel's maximum airflow in the peak efficiency island (in m^3/sec), at a PR of 2. So based on that, a 15T turbo should be better than 15G. Maximum efficiency performance should be slightly improved, as well as the map becoming wider at the surge and choke lines, as well as support for higher boost with higher maximum wheel speeds. So although the 15G and 15T may have similar number designations, the 15T will likely support more power when pushed to the limit. This is why the 13T has similar power numbers to the 15G.
The "E" designation, such as in TE04-13C (the gtpro 347 turbo), stands for "exchange". This is used when an MHI turbo is actually designed by another manufacturer. For example, Garrett designed the OEM turbo for the chrysler 2.5L engine. When MHI took over production, the adopted this turbo as a "TE04".
Firstly turbines and I go by inducer diameters with them .
1) TD04L ..... 46.8 x 41.2mm 7.7mm tip width .
2) TD04 std ... 47.2 x 40.0mm 6.8mm tip .
3) TD04H ...... 52.0 x 44.0mm 7.7mm tip .
4) TD04HL ..... 52.0 x 45.6mm 8.9mm tip .
13T-6 ... 40.64 x 56.0 4.98 tip . 52.6 trim .
15T ...... 42.00 x 56.0 5.38 tip . 56.2 trim .
16T ...... 43.40 x 56.0 5.70 tip . 60.0 trim .
Compressors , MHI is all over the place with B/C/G/T series wheels . Most people seem to think that the T spec wheels are the go , from 13T to 18 they are 56mm major dia wheels with the inducer diameter increasing with the "t" number ie
13T is 40.64 mm ~ 52.5 trim
15T is 42.00 mm ~ 56.2 trim
16T is 43.40 mm ~ 60.0 trim
The 19T is 58mm major diameter and it's inducer is 45.9mm so 63 trim .
