Scenario 4: Time to go built shortblock?
Airflow range: 320 ~ 360g/s (44 ~ 49lb/min) (627 ~ 700cfm)
Typical power outputs: 340 ~ 400whp
This is the typical range for EJ255s with the following turbos, listed roughly in order of potential:
Blouch Dom 1.0XT
Blouch Dom 1.5XT
Shortblock: This is really well beyond what the stock pistons are going to deal with. A healthy stock bottom-end should be okay at this point but the pistons are really out of their depth. It would be very difficult to crack 400whp without exceeding 18psi boost, which is all the stock head bolts and thus gaskets will withstand. Even on a perfectly clean tune you are pushing the pistons very hard (like 100whp each) and you can expect them to fail at any time. Stock rods and rod bolts are okay to 7k0rpm but some of these turbos will leave with quite a narrow usable power band with that rev limit.
Heads: Stock ports and cams are not yet the choke point but you would gain with a TGV-delete.
Clutch: All models require a clutch upgrade at this point, but disk-type clutches with streetable spring pressures are fine.
Transmission: Subaru 5MT is a question here. Many fail, some don't. If you're careful how you drive you may get away with this. If you're the launch-control kind of driver you may break things.
Intake: Stock MAF tube critical at this point as it can read up to about 380g/s with typical scaling. While there are turbos in this size range that can fit the stock intake it's probably the time to go aftermarket intake and big MAF.
Intercooler: Most people running in this power bracket have a FMIC fitted. While it's possible to run a very good TMIC like the Process West item you will probably do better with a FMIC. Note there are very few flange outlet turbos in this size range.
Injectors: Recommend 850cc or larger.
Pump: Aftermarket pump mandatory.
EGBP turbine: MHI-based turbos need an 8cm2 housing minimum, and consider 10cm2. With the Garretts stick with the .70 A/R housings and all will be well.
EGBP exhaust: High-flow catted or catless downpipe mandatory. Headers will add solid gains on these larger turbos, and the stock manifolds would probably be a serious choke point. Catback exhaust also really needed at this point unless you want to leave significant power on the table.
Other: 3-port EBCS will add control authority and help smooth boost response. You may even consider a hybrid system with both an EBCS and MBC. 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. This is also the point where many people will choose to go to a rotated setup to avoid stuffing a big intake under the intake manifold, fouling the TGVs etc. There are still good choices in the stock-location sector however.
A lot of people are fitting catch cans or air-oil separators along with mods like these as blow-by can and does become an issue.
Scenario 5: I just got divorced and I want to go crazy
MD321V, Dom 1.5R, 2.0XT etc. 400 ~ 450whp range.
Shortblock: Upgrade pistons are mandatory, forged rods are too because you will not be happy having to shift at 7k0rpm when the turbo is telling you to keep going. Head studs mandatory. Larger bearing clearances advised.
Heads: Stock ports are not yet the choke point but you would gain with a TGV-delete. It's now time to consider which cams to buy because the stock ones are definitely holding top-end power down.
Clutch: Shop 6MT clutches to match your new...
Transmission: This is really 6MT territory now unless you like the sound of straight-cut gears and insist the 5MT has better ratios for drag racing.
Injectors: Just get the ID1000s, you will need them.
Pump: Available drop-in pump? Think booster pump, surge tank, parallel pump set-up etc.
Other: 3-port EBCS mandatory. You may even consider a hybrid system with both an EBCS and MBC. 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. This is also the point where many people will choose to go to a rotated setup to avoid stuffing a big intake under the intake manifold, fouling the TGVs etc. There are still good choices in the stock-location sector but you're being stubborn now.
Catch cans or air-oil separators are probably a must-have as blow-by can and does become an issue.
* STi fitment turbo, doesn’t fit the stock TMIC, the typical AVO / Perrin aftermarket TMICs or straight bolt-on FMIC kits. Check fitment.
** Non-standard uppipe and discharge fittings. You will need exhaust components to match the turbo, might be available as a kit, might not. Quite likely that the turbo does not fit into the stock location and/or the intake pipe cannot be fitted under the intake manifold.