Scenario 1: Stock turbo maxed out.
Airflow range: up to 230g/s (31lb/min) (442cfm)
Typical power outputs: 200 ~ 270whp
This is typical Stage II country for LGT / FXT / WRX models with the small stock turbos VF-40, VF-46 or TD04L-13T.
Shortblock: OK assuming healthy tune
Clutch: LGT models may see clutch slip at these figures
Intake: Stock intake ok
Intercooler: Stock TMIC may leak at or around 18psi boost
Injectors: Stock injectors should be running at more than 90%
Pump: Stock pump within safety limits if in good condition
EGBP turbine: Choke point
EGBP exhaust: High-flow catted or catless downpipe mandatory
Scenario 2: Turbo upgraded, stock injectors / TMIC / clutch maxed out
Airflow range: 270 ~ 290g/s (36 ~ 39lb/min) (513 ~ 556cfm)
Typical power outputs: 270 ~ 320whp
This is the typical range for EJ255s with moderate turbo upgrades such as the following, listed in rough order of potential. There is very little to choose between some of them:
VF-39 / VF-43 / VF-48*
TD05H-Evo16G (BNR and Kinugawa offer direct bolt-on fitments for '05~'09 LGT models)
Shortblock: OK assuming healthy tune.
Clutch: LGT models will most likely have clutch slip at these figures. WRX models should hold.
Intake: Stock intake ok
Intercooler: Stock TMIC may leak at or around 18psi boost and these turbos could produce higher peak torque if run up to higher PRs, even on pump gas. Evidence suggests the stock intercooler will see 2 to 3psi pressure drop over the core at high boost / high rpm states and an aftermarket TMIC or FMIC will be of benefit, if not strictly necessary.
Injectors: Stock injectors should be running at around 95% so this could be said to be the critical limiting factor. These turbos do run out of breath at high rpm and high boost, and in most cases more power and reliability can be had by tapering boost off and running more ignition timing. Therefore adding larger injectors to this range of turbos will not usually improve power potential on pump gas.
Pump: Stock pump may limit AFR available even if in good condition. Expect stock pump life span to be shortened considerably by being pushed this hard. Recommend upgraded aftermarket pump or stock GRB item (which flows 20l/hr more than LGT/FXT/WRX unit).
EGBP turbine: Good.
EGBP exhaust: High-flow catted or catless downpipe mandatory. Headers will add power and torque to this set-up but will almost always slow spool and response to some degree. Catback exhausts will add something at this point but still optional.
Other: Some stock BPVs may leak at these pressures. 3-port EBCS will add control authority and help smooth boost response. Stage III tune required at this point, pro-tune, dyno-tune or e-tune, they all cost $ unless you have the chops.
Scenario 3: Sensible limit for stock block?
Airflow range: 290 ~ 320g/s (39 ~ 44lb/min) (556 ~ 627cfm)
Typical power outputs: 300 ~ 340whp
This is the typical range for EJ255s with turbo upgrades like the following, listed in rough order of potential:
TD05H-68HTA on pump gas (this turbo really meant for E85)
Blouch Dom 0.5R
Shortblock: Marginal, reliability only possible on a healthy tune, expect shorter life.
Clutch: All models require a clutch upgrade at this point, but disk-type WRX clutches with streetable spring pressures are fine. 6MT cars with stock clutches will be fine.
Intake: Stock MAF tube close to being maxed out. Consider aftermarket intake / big MAF etc.
Intercooler: Stock TMIC is well past its choke point now and would be a serious restriction. Aftermarket TMIC is minimum requirement and FMIC will be of benefit.
Injectors: Recommend 740cc or larger.
Pump: Aftermarket pump mandatory.
EGBP turbine: TD05H-20G will be held back by the small turbine. 18G compressor can use TD05H turbine for quicker spool or TD06SL2 turbine for better top-end. Recommend 8cm2 housing at minimum for turbos in this class running on gasoline.
EGBP exhaust: High-flow catted or catless downpipe mandatory. Headers may not be necessary on the smaller TD05H-18G setup but will add solid gains, at the expense of slightly slower spool / response. Catback exhaust also helpful at this point.
Other: Some stock BPVs may leak at these pressures. 3-port EBCS will add control authority and help smooth boost response. Stage III tune required at this point, pro-tune, dyno-tune or e-tune, with injector and / or MAF scaling to do they all cost $ unless you have the chops.
* Stock-location STi fitment turbo, doesn’t fit the stock TMIC, the typical AVO / Perrin aftermarket TMICs or straight bolt-on FMIC kits. Check fitment.