I've just about had it...I've searched high and low, close and far. It's time to TURBO COMPARE on a new level. I have got to get the kinks (pros and cons of each turbo) out of my system (actually...into my system because I still don't understand them fully). So far, this is what I have---
K03 - Stock.
K04
Nice OEM replacement. Spools insanely fast and the torque makes you feel
like your riding a high hp car. Also, you get the all-beautiful turbo spike that some of us seek. However, a lot of hp is not made, making the existing kits a bit expensive for the relatively small power gains. However, the kits do allow you to run only a turbo and few supporting mods successfully/safely. DV is necessary, while an upgraded exhaust and fmic is recommended, but not needed. Further, a common claim is that, while the K04 produces much more power than Stage 1, and a bit more than Stage 1+ (in the form of smoother torque, though still with a lot of kick), people are left desiring more power after they've had it for a few months. This turbo can be tweaked a bit, and if the driver has the money to spend, tuners like MTM have produced K04's with supporting mods that are good for as much power as a BT setup. However...$$$$$$$$$.
K04 Dyno
MTM K04 Dyno
GT2X (BT)
It seems like a wolf in sheep's clothing. It is not quite as big as a GTRS/GT28rs, which means a spool time almost as quick as a K04. And on top of that, it's good for almost 300 crank hp. Power comes on near 3000 rpm, and lasts up until 6000rpm. Finally, as its torque comes on in full by 3,000 rpm, you get an intensified turbo spike of those in the K03 and K04s. It's perfect, right? WRONG - Evidently, it's only oil-cooled (not also water-cooled, like the GTRS), which means that it has a tendency to have a shortened life. This makes a turbo timer almost necessary. Somebody also has gone as far as to say that it is better for the turbo if you don't rev high for the last few miles before the car reaches its destination (I don't understand this at all). On top of that, it's not a ball-bearing turbo, but rather a thrust-bearing turbo (I don't understand what difference that makes [not from a hardware perspective, but from a symptom point of view]). Next, to my knowledge (I'd love someone to prove me wrong), there exists no solid software tuning for it.
ATP Dyno (a bit ambitious)
GT28r (BT)
This is the turbo provided by AWE Tuning. Again, to my knowledge, this differs from the GTRS/GT28rs in that the turbine is of smaller diameter than those present in the GTRS/GT28rs. Unlike the GT2X, it is both oil- and water-cooled, making use of newer technology and prolonging life. Now, while the housing is quite a bit larger than the GT2X, this turbo takes quite a bit more time to spool. Now, while it produces similar hp/tq numbers as the GT2X (maybe a little more), significant hp does not kick in until ~3700 rpm. Torque does not kick in until 3500 to 3700 rpm, as well. However, the power/tq (especially tq) deliverance is much smoother, lasting until 5500-6000 rpm. While this corrects the lack of high-end power inherent in the K03 and K04s, it requires you to rev your car much higher...which is flat out not good for your engine (unless it has been upgraded with stronger internals). Also, with this turbo, it now becomes absolutely necessary to run an upgraded catback exhaust and highly-recommended fmic if you want to see the numbers previously-described...safely. A HFC/Test pipe is not required, but will help gain some horsepower. Keep in mind, however, that with a HFC or test pipe, more pollutants will enter the environment and your car will not pass smog tests. Also, some people have claimed that, despite the $3,695 price tag, the kit is not well-engineered (I don't understand why).
Dyno
GTRS/GT28rs
Highly-revered, and supported by great software. The full kit, with supporting modifications (sold by EuroCode Tuning), produces octane-dependent power of up to 400 crank hp. However, with premium pump gas and a slew of supporting mods, you will most likely see ~350 hp range. Torque exists at ~320-340 lb/ft based on dynos I've seen recently. While this kit contains the same sized housing as the GT28r, it has a larger turbine wheel, which allows for more flow, which leads more efficiency, and therefore, more hp. However, as it is larger, it takes even more time to spool, resulting in useable power only after 3700 to 4000 rpm. While I've never ridden in/driven a BT A4, higher-revving in this kit is required to utilize the power available. Understandably, this puts a lot of pressure on the engine, creating the potential for more problems. A DV, FMIC, and upgraded exhaust system become a necessity by this point. While a GTRS eliminator kit is another popular option, low boost and relatively no timing is required to keep EGT's down. So, don't expect to see more than 230 hp/260 tq (I assume this is crank hp/tq, but am not entirely clear. This info was gleaned from a dyno performed by EcodeTuning here - http://www.ecodetuning.com/gallery/D...GTRS_91oct.jpg). However, this kit is incredibly "tweakable," and many people see many different numbers based on what supporting modifications they use.
Dyno
GT2871r
This turbo kit is yet another variation of the GT28 series. While the housing is still the same size, the turbine wheel is a bit larger than the GTRS/GT28rs, allowing for yet more flow, more efficiency, and therefore more power. If the entire kit is purchased, you are privy to an octane dependent 450hp at the crank. However, the cons of this turbo, aside from the added stress on engine internals, is that it takes even longer to spool, keeping significant power from hitting until 4000-4200 rpm. This power holds (with only slight decreases in hp and tq) until 7000 - 7500 rpm. Similar to the GTRS/GT28rs, a dv, upgraded exhaust, exhaust manifold, fmic, and turbo inlet pipe are needed to run this kit safely, and actually achieve the power numbers quoted by the kit. Again, similar to the GTRS/GT28rs, there is an eliminator kit available for this setup. And, as for the same reasons stated above for the GTRS/GT28rs eliminator setup, you will only see 300 crank hp / 305 tq.
Dyno
Notes - Now, for my questions - I wanted to go with the GT2X, but after doing research, it became clear that it was not reliable. You mean to tell me that there is no turbocharger that spools fast, has usable power down low (aka between 2000 and 6000 rpm), and is both oil- and water-cooled for reliability. Then I'd say...GT28r - However, it seems to take a bit long to spool, and I'm afraid it takes away my beloved turbo spike (I'm not a fan of overly smooth power in a turbo car). Still, for some reason, people seem to say that the GT28r kit is not the best. Why is this?
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