Originally Posted by some place on the internet

Mitsubishi does not seem to use seperate designations for different size turbine wheels, other than the TD04, TD04H, TE04, TD05H, etc., designation. The "6cm2" in the model name is similar to the A/R ratio used by other manufacturers. The "A" in an A/R ratio is the cross-sectional area of the smallest intake passage in the turbine housing before the passage spreads around the circumferential volute that leads to the turbine wheel. The "R" in the ratio is the distance from the center of the "A" to the center of the turbine wheel. The MHI "6cm2" designation is just the "A" in the A/R ratio, that is, it is just the cross-sectional area. Like A/R, the smaller the size of the "cm2" number, the faster the exhaust gases will discharge onto the turbine wheel, and so the faster the spool up will be (less "lag"). The size of the "cm2" number or the A/R ratio also determines the amount of exhaust gas backpressure and, thus, reversion into the combustion chamber. A larger "cm2" number (or larger A/R) means less backpressure at high exhaust flow. Extreme Turbo says that the TD05H-7cm2 housing is equivalent to a 0.50 A/R. The Rocky Mountain DSM turbo guide presents the following conversion between Mitsubishi's "cm2" number and the standard A/R.

6 cm2 = 0.41 A/R
7 cm2 = 0.49 A/R
8 cm2 = 0.57 A/R
9 cm2 = 0.65 A/R
10 cm2 = 0.73 A/R
11 cm2 = 0.81 A/R
12 cm2 = 0.89 A/R

Originally Posted by Slo_crx1

It could to some degree, but not in large enough amounts. The biggest problem would be the thickness of the casting after you port it to achieve any major a/r changes as these turbine housings aren't especially thick to begin with in some spots. You would risk cracking the housing if the walls became too thin

Originally Posted by COL Knock

You can alter it by doing that for sure, by making it worse.

Originally Posted by JohnBradley

Oh well its Kinugawa so it must be good