twin-screw vs roots vs centrifugal vs turbo

Early WRXes were notorious for having really laggy turbo systems, primarily due to restrictive exhausts featuring multiple catalytic converters, and even a cat positioned before the turbo—definitely a poor choice for performance responsiveness. While I've noticed improvements in later models, the extent of this enhancement remains uncertain.

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Most original equipment manufacturer (OEM) turbos tend to be relatively small, specifically designed to produce significant torque at lower RPMs while minimizing lag. For instance, my previous B5 S4, equipped with a 2.7L engine and twin KKK K03 turbochargers, achieved peak torque at around a certain RPM, which noticeably dwindled above a specific threshold (with a redline of too high). The subtle whooshing sound made it almost imperceptible that the car was turbocharged, and it notably outperformed the naturally aspirated 4.2L V8 in the B6 S4 that later replaced it, particularly in low-end torque delivery.

Aftermarket turbo kits often feature larger sizes compared to OEM versions. The K03 turbos max out around 180-200 crank horsepower each, whereas the GT model mentioned by Keith—one of the smallest options in the aftermarket—can support approximately 250 horsepower. In my Miata, the GT turbo is rated at 300 and can transition from vacuum to full boost in less than half a second while driving on the freeway. During autox events, in 2nd or 3rd gear, it reaches full boost almost instantaneously as your foot hits the accelerator.

Another critical distinction among forced induction systems is their altitude response. Positive displacement superchargers (and most traditional centrifugal models) behave similarly to naturally aspirated engines; as altitude increases and ambient pressure decreases, the torque curve proportionally drops but maintains its original shape. On the other hand, turbocharged vehicles tend to spin their turbos faster to compensate for altitude losses. Elevating altitude typically raises the boost threshold (the minimum RPM required for full boost) while introducing increased lag; however, peak power loss is significantly less than in supercharged or naturally aspirated arrangements. Depending on the vehicle's MAP-referenced electric boost control and turboheadroom, it may not experience a decline in peak power at all.

A notable challenge I've faced with turbos during autox events is the shifting torque at certain RPM/TPS points as boost levels rise. This dynamic can complicate corner exit performance compared to naturally aspirated or supercharged vehicles, yet it is not an insurmountable issue.

Twin Screw Vs Roots Vs Centrifugal Supercharger