The second direct comparison of the search to find the best stock motor turbo involves the BorgWarner K04 and FrankenTurbo F21.
The information shown below was recorded using my Audi B5 S4 with each turbocharger option. The only change in vehicle hardware, aside from the turbochargers, was a change from using Bosch EV14 72# injectors with the K04’s to Bosch EV14 60# injectors with the FT21’s.
FrankenTurbo F21
Functional Requirements Verification Matrix
| Number | Functional Requirement Description | Verification Method | Pass / Fail |
| 1 | Rapid acceleration at 2000-3000 rpm. | Demonstration | Fail |
| 2 | Maintain strong acceleration from 5500-6500 rpm. | Demonstration | Pass |
| 3 | Biturbo configuration. | Inspection | Pass |
| 4 | Does not require unique parts. | Inspection | Pass |
| 5 | Equal or exceed RS4 K04 performance. | Test | Fail |
| 6 | N75 DC <80% at 6800 rpm | Test | Pass |
| 7 | Wastegate modulation of boost at part throttle. | Test | Pass |
FrankenTurbo F21
Technical Requirements Verification Matrix
| Number | Technical Requirement | Verification Method | Pass / Fail |
| 8 | Charge air temperature < 300 degF | Test | Pass |
| 9 | Exhaust back pressure:Manifold pressure ratio <= 1.5 | Test | Fail |
FrankenTurbo F21
Supportability Requirements Verification Matrix
| Number | Supportability Requirement | Verification Method | Pass / Fail |
| 10 | Reputable vendor with good product support | Demonstration | Pass |
| 11 | 12 month warranty | Inspection | Pass |
| 12 | Cost competitive with alternatives | Inspection | Pass |
| 13 | Price not greater than 20% over RS4 K04 | Inspection | Pass |
FrankenTurbo F21
Performance Specifications
| Number | Specification Description | Verification Method | Pass / Fail |
| 14 | Develop 20 psi of manifold pressure by 3100 rpm | Test | Pass |
| 15 | 2-11 psi time should be <= K04 time at all rpm | Test | Fail |
| 16 | Maintain >= 25 psi to 7000 rpm | Test | Pass |
| 17 | <= 45 psi of exhaust back pressure at 6500 rpm and 25+ psi of manifold pressure. | Test | Fail |
| 18 | Mass Airflow Rates >= 375 g/s at 6000 rpm | Test | Pass |
| 19 | FATS time <= 3.5 sec (Sea level, 60 degF) | Test | Pass |
| 20 | Torque >= 400 ft-lbs | Test | Pass |
| 21 | Wheel power >= 400 whp | Analysis | Pass |
| 22 | MTBF >= 48 mo. or 50,000 miles | Analysis | Pass |
FrankenTurbo Requirements Summary
| REQ. # | BW K03 | BW K04 | FT21 | SRM RS4+ | TTE 550 |
| 1 | Pass | Pass | Fail | | |
| 2 | Fail | Pass | Pass | | |
| 3 | Pass | Pass | Pass | | |
| 4 | Pass | Pass | Pass | | |
| 5 | Fail | Pass | Fail | | |
| 6 | Fail | Pass | Pass | | |
| 7 | Pass | Pass | Pass | | |
| 8 | Pass | Pass | Pass | | |
| 9 | Fail | Pass | Fail | | |
| 10 | Pass | Pass | Pass | | |
| 11 | Pass | Pass | Pass | | |
| 12 | Pass | Pass | Pass | | |
| 13 | Pass | Pass | Pass | | |
| 14 | Pass | Pass | Pass | | |
| 15 | Pass | Pass | Fail | | |
| 16 | Fail | Fail | Pass | | |
| 17 | Fail | Fail | Fail | | |
| 18 | Fail | Pass | Pass | | |
| 19 | Fail | Pass | Pass | | |
| 20 | Fail | Pass | Pass | | |
| 21 | Fail | Fail | Pass | | |
| 22 | Pass | Pass | Pass | | |
