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alright well lets figure this out...this is LONG as shit, but im bored...so bear with me. now, this is mostly approximate, but something to go off.
to choose a turbo, you need to first know how much cfm's of air it digests at redline. formula for a 4 stroke engine:
(CID x RPM)÷3456 = CFM
lets input some 2gnt data, 121.8 cu in at 7250rpm
(121.8 x 7250)÷3456 = ~255CFM
the engine will flow ~255CFM assuming 100% VE, lets assume a 85% VE for a 420a engine, so the formula now looks like this:
(CID x RPM x VE%)÷3456 = CFM
or for our application:
(121.8 x 7250 x 0.85)÷2456 = ~217CFM
so a 420a 2 liter at 7250 rpm sees about ~217CFM assuming a 85%VE, this is the first thing we need to know when choosing a turbo. heres some other factors
pressure ratio (on a compressor map)-
(desired boost psi +14.7 psi) ÷ 14.7 = pressure ratio
temperature rise-
compressor will obviously raise the temp of the incoming air as it becomes compressed, as temp increases so does the volume of air
t2 = t1 (p2÷p1)^0.283
t2 = outlet temp *R
t1 = inlet temp *R
P1 = inlet pressure, absolute
p2 = outlet pressure, absolute
*R = *F + 460
so i assume intake air about say, 65* and you said you want like 20-21psi boost, so here we go:
t1 = 65 + 460 = 525 *R
p1 is obviously atmospheric (or whatever elevation you are) p2 will be our boost pressure, 21psi. to figure outlet pressure, add boost to your atmospheric pressure, so:
p2 = 14.7 + 21 = 35.7 psi
now we have everything we need to figure ideal outlet temp, now we plug it all in to find the last factor, outlet temp...or t2, here we go:
t1 = 525 *R
p1 = 14.7
p2 = 35.7
so:
t2 = 525 (35.7÷14.7)^0.283 = 675 *R or 214 *F outlet temp, this is an ideal temp rise of 149*F, if the compressor had 100% adiabatic (sp?) efficiency (no gain or loss of heat) actual temp rise will def be higher.
usual compressor adiabatic efficiency is somewhere between 60-75%, so heres another formula:
(ideal outlet temp rise) ÷ (adiabatic efficiency) = (actual outlet temperature rise)
lets assume a 70% efficiency:
149 ÷ 0.7 = 212 *F actual outlet temp
desnsity ratio-
to compare inlet to outlet airflow we gotta know tyhe density ratio too (as air is heated becomes less dense, this increases volume) heres another formula
(inlet *R ÷ outlet *R) x (outlet pressure ÷ inlet pressure = density ratio
so here we go:
(525 ÷ 675) x (35.7 ÷ 14.7) = 1.89 density ratio
compressor inlet airflow-
using ALL ^^^^ info you can figure out the actual inlet flow in CFM, another formula:
outlet CFM x density ratio = inlet CFM
so ours would look like this:
217 CFM x 1.37 = 297.29 CFM inlet air flow
thats about a 37% increase in airflow, so about 37% increase in power. compressor maps are in lb/min, multiply CFM by .069 to convert CFM to lbs/min
297.3 CFM x 0.069 = 20.5 lb/min (that is NOTHING for a turbo)
now you can use these formulas with flow maps to select a compressor to match what you want.
now im not asure if you can do it this way or not, but i'll see if this makes sense...
140CHP = ~217CFM and you want to make about say, 450CHP...thats a 215% increase in airflow at 7250, approximately of course. 683.55 cfm or about 47 lb/min
hopefully i helped you learn a little more about turbo sizing and  someone, anyone, correct me if i messed up or if im confused on anything, i might have messed up somewhere i dunno but is seemd about right
well looky what we have here  big t04
looking at this compressor map, 16psi (or 2.0 pressure ratio) will flow about 47lb/min of air
hope i maybe helped some 
_________
96 talon esi-t
san clemente, ca
as needles of ice
are the ill winds' talons
the coldest of shadows
they seep unto the bone
silent souls leave .308 holes
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