Subaru WRX/STI turbo charger basics

What turbo construction method is best? There is no irrefutable evidence that one construction method is better than the other. The real difference in turbo construction is the bearings. There are two main types: floating bearings and ball bearings. Ball bearing turbos were designed for increased reliability and decreased lag. Though both of these elements are true, neither advantage is especially prevalent, so the construction method should not be the main consideration when choosing an aftermarket turbo.

What is a twin-scroll turbo? Generally speaking, it is a turbo with a divided turbo inlet to isolate the pulses coming from each exhaust port to maintain more of the pulse energy from each cylinder all the way down to the turbine wheel. A twin scroll setup will respond faster and produce boost quicker than a equally CFM sized regular turbo.

Twin scroll setups are generally costlier and require more components than the average turbo upgrade to work efficiently though due to their requiring a true twin scroll header to operate correctly. Fitting a regular header to a twin scroll turbo basically negates the pros of this type of turbo.

So a twin-scroll turbo spools faster? When properly set-up and compared to an equal CFM regular turbo, yes. However there are many regular turbos that will out spool twin-scroll units. Generally speaking, buy a turbo based on HP/CFM and consider a twin-scroll if there is one that fits your goals and you are looking for decreased spool. Never buy a twin-scroll for the sake of saying you have a twin-scroll. Play into the benefits of a properly thought out twin-scroll set-up, not the marketing hype.

What is a rotated mount turbo? Any turbo that’s physical size prevents mounting in the stock location. They also generally have larger than an OEM inlet size which necessitates TGV deletes and a larger turbo inlet pipe. They also require custom exhaust components and generally use an external wastegate set-up.

How can I decrease turbo lag? There are a number of steps that you can perform to decrease the lag:
a. A silicone Y-pipe IC hose can decrease lag
b. An aftermarket intercooler with decreased pressure drop can decrease lag, though its physical design may negate the benefit
c. An aftermarket uppipe can decrease lag
d. Port and polish turbo services can decrease lag
e. Tuning. Through the tuning of EGTs, wastegate duty cycles, and gains, spool can be accelerated. Properly tuned cars create full boost ~500 rpm sooner through these techniques.
f. Large diameter downpipes and exhaust decrease lag. If the downpipe is catless, lag with be further reduced over a high flow cat model. You also want the cat to be as far from the turbo as possible to promote quick spool.

Can I upgrade my existing turbo? Yes. Most turbo upgrade facilities offer improvement services. State your turbo, its current disappointments, and improvements you would like to see. They might offer services that can save you from buying a new turbo. Though not an all inclusive list, possible services include:
Port and polish: Includes heavy porting of exhaust housing, removal of flow obstructions, smoothing of the factory material, and reduction of internal angles to alter flow and evacuation. Entrance to turbo housing (where the exhaust enters the unit) is also heavily ported, removing flow obstructions and smoothing exhaust path. Wastegate pass-through from exhaust housing is also ported, increased slightly in size, and any flow obstructions removed. The most significant improvement with this service is the decreased spool time. Lack of low end torque is a common complaint with 2.0L motors.
Internal ported and polish: This opens up the area where the air exits the compressor housing. The turbo has between 1 & 2 mm of material removed, then the rough casting edges are smoothed out. You will pick up a fraction of a HP but get better response.
Light clip: Clipping removes part of the turbine wheel. This allows air to pass over it faster. It will help out with high boost (16.5 PSI +) situations but will slow down spoolup.
Coating: Ceramic coatings keep heat inside the turbo for increased thermal efficiency.
Larger waste gate flapper: New technique where a large wastegate flapper is installed in place of the stock unit. The large piece covers a specially ported, larger waste gate hole in an effort to combat boost creep.
Changing internal turbo components: This service depends on the turbo. Generally speaking it means things such as replacing the compressor wheel with a larger unit for increased performance, changing the wastegate spring, and other internal improvements.What questions should I ask of the Vendor and my tuner before purchasing? Once you perform your research, you should have your choices narrowed down. Before making your final decision, it is always wise to ask these questions:
1. What fuel injectors should I run?
2. What fuel pump should I run?
3. Is this turbo bolt on or does it need any special fabrication?
4. I plan on _______ racing. Is this a good turbo for my plans?
5. I plan on running the quarter mile in XX.XX or making XXXWHP on XXX dyno. Is this a good turbo for my plans?
6. Other than a fuel pump, bigger injectors, and proper tuning, is there anything else you would recommend for increased safety and reliability?
7. What about a larger intercooler?