IW STi Forum banner
1 - 20 of 25 Posts

· Registered
9,435 Posts
Discussion Starter · #1 · (Edited)
This is a guide for modding a Subaru beyond Stage 2 – from 300whp to 1000whp. Read my other guides in my signature for knock monitoring and taking care of a built motor.

An engine is just a glorified air pump, the power ceiling is absurd if you can devise ways of forcing MORE AIR into the engine, which is where turbos come into play. You can strap a humungous turbo to any engine but you have to make sure the engine internals and supporting modifications are there to handle the extra compressed air or else KABOOM.

I feel like the STi is probably the best and most cost-effective setup for making such power (GTR transmission upgrade 20-30k!). The symmetrical AWD/differentials prevent it from getting squirrely on you and apply the power to the ground effectively so you won’t be smoking your tires. The STi transmission can handle a hefty amount of power before breaking. The stock STi brembos are very good. The short gear ratios of the 6spd are actually a blessing since it makes it easy to be at an optimum rpm and lessen turbo lag.

I like to keep compression and stroke stock for reliability reasons – let the turbo do the work.

NOTE:The recommendations listed are for the smoothest progression and least amount of backtracking (avoiding selling your aftermarket TMIC for a FMIC later on) to really high horsepower starting from 350whp. Some of the initial recommendations are total overkill but lay the groundwork and headroom for growth later :tup: There are a million and one ways to hit 350-450whp and my recommendations are not absolute and by no means cost-effective for that starting power level ;)

Firstly the main prerequisite is going rotated for these reasons:
  • The sheer size of the turbo (above a 35R frame) cannot fit in the stock location.
  • You will have the ability to run a huge 4” diameter intake directly routed into the turbo. Since you will definitely be on speed density, the intake diameter is a non-issue.
  • Make sure you get V-banded turbo kit which obviates the need for gaskets. V-bands provide leak-free spool for many many miles versus having to deal with especially pesky blow uppipe gaskets.
  • All rotated kits will come with intake, downpipe, uppipe with Tial EWG so you can sell a bunch of your stock location components. Additionally factor in not needing a turbo inlet and having to upgrade those parts anyway for a stock location turbo, starting with rotated is not astronomically more expensive than stock location.
  • Going rotated in the beginning is DEFINITELY cheaper than switching to rotated later.
  • Rotated allows unobstructed accessibility to the turbo for quick turbo swaps.

Other bare bones essentials you need to have are
  1. Grimmspeed EBCS
  2. TGV deletes
  3. FULL 3” catback exhaust (I emphasize FULL due to Poiseuille’s law in relation to radius and flow)
  4. Speed density since you will max out the MAF sensor
  5. You most certainly need equal length headers which not only give you gobs of horsepower and torque over unequal length but ensure a safer running engine from lowering and equalizing cylinder temperatures.
  6. Definitely need a FMIC for the higher cooling capacity.
  7. High quality blow off valve that can hold a ton of psi (Tial)
  8. I don’t know the power contribution of doing the heads piecemeal. So it is either Stage V heads (Pnp, +1mm valves, Manley valvetrain) with 272 cams or stock (***FAQ***How much would upgraded cams/Pnp heads/bigger...). Nothing in between :lol:
  9. I don’t support meth if you have E85 readily available :)
  10. FIND A REPUTABLE TUNER. Knock will destroy not only the pistons but also the bearings and spikes in cylinder pressures will crack the liners.
  11. Highly recommend OEM headgaskets or JE Pro Seal

The brands listed are the brands I'm familiar with, you can find something of the equivalent otherwise. I've logged 60-100k miles on these parts so I can personally attest to their reliability :tup:

Turbo: GTX3071 or GTX3076 on 93 octane
Shortblock: Stage 1 (Manley platinum Series pistons with stock forged STi rods), or potentially stock motor
Heads: stock
Fueling: ID1000-1300cc
Clutch: ACT HDSS
Comments: Rotated is complete overkill for this power level as you can hit this easily with stock location, so lets move on :lol:

Turbo: GTX3576 or GTX3582 on 93 octane/GTX3076 on E85
Shortblock: Stage 2 (Manley Platinum Series pistons + Manley H-tuffs)
Heads: Stock if you using E85. Stage V if you are sticking with 93 octane, ARP625 head studs
Fueling: ID1300cc-1700cc, Aeromotive FPR, upgraded fuel rails, Aeromotive 340lph pump
Clutch: ACT XTSS
Comments: Now we are talking :devil: This is definitely the sweet spot power wise for the street and road course. You will be trapping around 120-125mph.

Turbo: GTX3582 on E85
Shortblock: Stage 3 (Manley TurboTuff pistons + Manley TurboTuff I-beams), Stage 4 regular (just closed deck)
Heads: Stage V, ARP625, consider 2.5i intake manifold (Outfront motorsports has a ready to install one for $330)
Fueling: ID1700cc-2000cc, Aeromotive FPR, upgraded fuel rails, Aeromotive 340lph pump, potentially need -8AN feed lines
Clutch: ACT XT6puck
Comments: This is where it gets crazy if you drive it on public roads. Simply put, you will get thrown in jail if you get caught during a pull due to how obscenely fast the acceleration is. You’ll be trapping around 130-135mph, upgrade axles for sure if you are aggressively launching the car on sticky tires.

Turbo: GTX3586, PTE6266 on E85
Shortblock: Stage 4 Tuff/extreme (Closed deck, Manley Turbotuff pistons, Manley TurboTuff I-beams, pinned mains)
Heads: Stage V with 14mm IAG headstuds, IAG Firelock O-ring, consider 2.5i intake manifold
Fueling: ID2000cc, -8AN feed lines/-6AN return lines, Radium dual pump with surge tank, upgraded fuel rails, Aeromotive FPR
Clutch: ACT XT6puck, Exedy triple disc
Comments: You’ll need to run R compounds for traction at this point which isn’t practical if you drive this car in any weather. You are definitely closing on the limits of your average max performance summer street tire. You’ll be trapping around 135-140mph.

750whp-1000whp (and beyond)
Turbo: GTX3584RS, PTE6466 and bigger on E85
Shortblock: Stage 4 Tuff/extreme, additionally you should consider the Manley billet crank if you are thinking more than 800whp, you will need thicker Manley wrist pins
Heads: Stage V with 14mm IAG headstuds, IAG fire-lock Oring, Larger intake manifold (AMS, Cosworth, Process West, 2.5i IM)
Fueling: ID2000cc, -8AN feed lines/-6AN return lines, Radium dual pump with surge tank, upgraded fuel rails, Aeromotive FPR
Clutch: ACT XT6puck (barely), Exedy triple disc (definitely)
Comments: This is not streetable because you will need to be driving on drag radials at this point or you will be doing 4WD burnouts. Get a parachute and cage if you are doing the quarter mile. This is way too much power even for a road course.

· Registered
9,435 Posts

· Registered
9,435 Posts
Discussion Starter · #7 · (Edited)
Yup I agree that there are lots of ways to get 350-400whp. This guide was intended to make the smoothest progression to really high horsepower starting from 350whp-400whp so some of the supporting mods (rotated, FMIC) are definitely overkill at the start but they are really intended to lay the framework for later. For instance I recommend starting with a FMIC instead of recommending an aftermarket TMIC and then backtracking/selling it once you outgrow at the next stage :)

I wanted to give people perspective on building for headroom by laying out what is needed at the next stages, how far they want to reach is up to them :)

· Registered
9,435 Posts
Discussion Starter · #9 ·
Also general note, there are certain breakpoints in the progression and certain limits you hit before others. For example, it wouldn't be wise dropping the extra money on a closed deck/o-ring/pinned main setup if you are still on stock heads, running ID1000cc injectors, with H-beams or spending extra dough on a Manley billet crank when you just have upgraded pistons :lol:

· Registered
1,549 Posts
There are a lot more areas as you surpass the 550-650 whp level than what you have shown though. The one that jumps out to me is axels. 600-650 is where you start playing with breaking the stock axels. Overall though there are many ways as you have said to build for these ranges. I agree anything over 750 becomes unstreetable. That's why my goals are just below said level ��

· Premium Member
See about
6,562 Posts
Sure Yahmaha is correct that there are items in the needs section that someone like me who never plans to exceed 450 dont need. In fact some of those those things are why I don't plan to exceed that power level. Rotated/FMIC and speed density are all in that category. But User is clear that this list is for those who do plan on acquiring more power. In that respect the list is dead on.

What I'd like to add is that this list is just a basic engine list. It doesn't detail all the little things that go into making a great car. Further I wouldn't want even my 350+ whp with stock handling. Even my moderate car can get squirrelly running through the lower three or four gears. I'm not asking User to do them, but a sheet for up to 450 AWH on meth or E85 - essentially below rotated and head work would be good, as would handling and drive train mods to go with them. I do understand that handling is a larger subject too, since there are differences in our chassis generations.
  • Like
Reactions: user1029

· Registered
9,435 Posts
Discussion Starter · #15 ·
Still on Stage 4 closed deck, Junior's car :lol:

I think they added O-rings to the head as well to clamp on both sides

· Registered
9,435 Posts
Discussion Starter · #17 · (Edited)
FMIC just as safe with an aftermarket front crash bar :) I think the FMIC itself also adds a layer of crumbling

1 - 20 of 25 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.