What turbo? wiki... with a twist
 

Every day there are new threads asking for help choosing a turbo. Most of the time they start out either with no power targets or targets that are unrealistic. Then the questions come up on what mods are needed to support a given turbo, and a lot of turbo choices are made and then abandoned when the person realizes the extent of supported mods needed to run said turbo and the targets are revised or abandoned when the parts bill is added up. Most of us now realize that buying a bigger turbo is not the only expense in a project and that the parts list needed gets longer the bigger the blower we bought, and that's before anything goes pop.

A lot of discussions on which turbo to buy for which power level and so on end up in disarray because of differing supporting mods applied. You can’t easily compare two turbos if they are supported differently.


Here I want to try a different approach and explore some of the limits of the stock systems in our cars, and how they should affect our purchasing decisions. Call it a turbo chooser matrix if you like but it’s more about entire systems. Any of the knowledgeable members will tell you it's not individual parts that make a strong car, but parts that work well together.


I’m going to pick some points which may seem arbitrary but I think many people shop system setups in this way. A lot of people have some kind of deal-breaker that sets a threshold that they won’t cross, or won’t cross until that part has failed or worn out already.

Laid out below are some scenarios which to me represent the limit of some fairly important or hard to replace part which represents a threshold of complexity or expense. From this would-be builders can hopefully plan their setups more easily / realistically. Note that power and torque figures are going to fall into a wide range because of varying factors like supporting mods, atmospheric conditions, fuel quality used, dyno correction factors, heartbreaker dynos, the time of the month and phase of the moon. Please use your discretion and by all means look for plots which support the numbers you think are reasonable. I suggest we level the playing field here by assuming we are using pump gas of 93MON, reasonable temps and pressure as per sea level.
Of course tuning is a big factor in final output so I will again assume that timing is within a degree or two of knock, and AFRs are in the 11:1 range since this is what the vast majority of us are doing.

The scenarios laid out so far are:

1. Stock turbo is maxed out
2. Turbo upgraded, stock injectors / TMIC / clutch maxed out
3. Sensible limit for stock block?
4. Time to go built short-block?
5. I just got divorced and I want to go crazy

For purpose of comparison I’m using the following factors:

1lb/min = 14.27cfm
1liter air = 1.0914g
1ft3 air = 0.0682lb
1cfm = 30.90g/s
1lb/min = 7.35g/s

Since the best way to get good information around here is not by searching or asking, but by posting wrong or incomplete info and then waiting for those who know to come out of the woodwork, I'll start with a few scenarios for you guys to critique / fix.

What I'd like to happen is for the mistakes to get corrected, more turbos to be suggested (along with data to support their categorization). Unique turbos can be added to the scenarios which best describe the mods required to support them. More scenarios can be added to expand into the bigger setups. Hopefully like this people can get a ballpark idea of what is going to be required to reach a given power range beyond an actual turbo.

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.

Limits

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*
VF-52
TD05H-Evo16G (BNR and Kinugawa offer direct bolt-on fitments for '05~'09 LGT models)

Limits

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 95 ~ 100% so this could be said to be the critical limiting factor. Some people will want to upgrade injectors just to get that last few whp out of the turbo.
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.
EGBP turbine: Good.
EGBP exhaust: High-flow catted or catless downpipe mandatory. Headers may not add very much to this set-up but will certainly slow spool and response. Catback exhaust superfluous 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, 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-18G
TD05H-68HTA on pump gas (this turbo really meant for E85)
Blouch Dom 0.5R*
Blouch 380XT
TD05H-20G
ATP GT28RS*
ATP GT2871R*
TD06SL2-18G
TD06SL2-20G

Limits

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 on the larger turbos, 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.


Notes:
* 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.

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 440XT
MD321T*
Blouch Dom 1.0XT
EFR6758**
ATP GT3071R*
ATP GT3076R*
GTX3071R**
GTX3076R**

Limits

Shortblock: This is really well beyond what the stock pistons are going to deal with. A health stock bottom-end should be okay at this point but the pistons are really out of their depth.
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

Suggestions please, in the range of MD321V, Dom 1.5R, 2.0XT etc. 400 ~ 450whp range.

ATP GT3576R*
ATP GT3582R*

Limits:
Shortblock: Upgrade pistons are mandatory, forged rods probably are too. Head studs a good idea. Other suggestions welcome.
Heads: Stock ports are not yet the choke point but you would gain with a TGV-delete. It might be time to consider cams.
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.


Notes:
* 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.

Mod matrix, based on text above...

Attachment 139578

x = No, unnecessary, perhaps even counterproductive.
m = Maybe. Might help, might be a good investment for the future, might give negligible gains, might even hurt.
y = Yes. Will most likely be worth doing, maybe essential.

great info, very helpful to this noob. thanks

Very helpful to this Connecticut noob as well. Subsribed AND bookmarked... so you know its good! :)

Cant wait to be able to sit down and read it all!

Thank you!

In the absence of a "thanks" button, I'll chime in with... "Looking great, fahr_side!" ;)

Thanks Legend. I do hope you'll propose some scenarios or turbos to grow this.

I'm looking forward to seeing this thread grow. A definitive resource like this will help everyone who is wandering the forum thinking they are going to make 400hp with xxx modifications.

Jeff

Great stuff indeed. Thanks for taking the time to put thoughts to keyboard!

Thanks. If you can pull together peak boost and timing at peak boost, those numbers could help too. Also, I wouldn't say you 'need' a catback for any turbo. I'm on the stock CBE on a BNR68 with e85, and my car is great. I know I'm leaving some on the table, but it is quick enough, and I haven't been able to find a wife-approved CBE.

If you're going to continue scenarios on the stock block and 93 up to 400, I would be interested to see where meth is required.

The spt 1.5 is a quality catback, and FWIT, my wife says its a little noisy (its really not), but she's ok with it! She's sensitive to loud noises, and she deals with it just fine.

I wish you had this together when I was still asking you stupid questions and being a smartass. Thanks button

subi scribed ..[thanks]button.

You're still a smartass, and your current questions are just kinda meh.


Semi-lazy subscribe!(thanks button)