This page provides specific information resulting from the effort documented on the best stock motor turbo page.
The turbocharger upgrade consideration any original Audi B5 S4 owner faces is whether to keep their stock K03’s, or swap out for something else. Limiting my consideration to the biturbo configuration, I am unaware of any option that is smaller than the K03’s. Therefore consideration number one is if it is desirable to install a larger turbocharger.
The first upgrade option that became available was to replace the BorgWarner K03 turbochargers with BorgWarner K04 turbochargers that were original factory equipment on the Audi B5 RS4.
Comparing these two commonly used turbochargers gave me an opportunity to exercise the requirement set I had created for the best stock motor turbocharger selection effort.
BorgWarner K03
Functional Requirements Verification Matrix| Number | Functional Requirement Description | Verification Method | Pass / Fail |
|---|---|---|---|
| 1 | Rapid acceleration at 2000-3000 rpm. | Demonstration | Pass |
| 2 | Maintain strong acceleration from 5500-6500 rpm. | Demonstration | Fail |
| 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 | Fail |
| 7 | Wastegate modulation of boost at part throttle. | Test | Pass |
BorgWarner K03
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 |
BorgWarner K03
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 |
BorgWarner K03
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 | Pass |
| 16 | Maintain >= 25 psi to 7000 rpm | Analysis | Fail |
| 17 | <= 45 psi of exhaust back pressure at 6500 rpm and 25+ psi of manifold pressure. | Analysis | Fail |
| 18 | Mass Airflow Rates >= 375 g/s at 6000 rpm | Test | Fail |
| 19 | FATS time <= 3.5 sec (Sea level, 60 degF) | Test | Fail |
| 20 | Torque >= 400 ft-lbs | Test | Fail |
| 21 | Wheel power >= 400 whp | Test | Fail |
| 22 | MTBF >= 48 mo. or 50,000 miles | Analysis | Pass |
BorgWarner RS4 K04
Functional Requirements Verification Matrix| Number | Functional Requirement Description | Verification Method | Pass / Fail |
|---|---|---|---|
| 1 | Rapid acceleration at 2000-3000 rpm. | Demonstration | Pass |
| 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 | Pass |
| 6 | N75 DC <80% at 6800 rpm | Test | Pass |
| 7 | Wastegate modulation of boost at part throttle. | Test | Pass |
BorgWarner RS4 K04
Technical Requirements Verification Matrix| NUMBER | TECHNICAL REQUIREMENT DESCRIPTION | VERIFICATION METHOD | PASS / FAIL |
|---|---|---|---|
| 8 | Charge air temperature < 300 degF | Test | Pass |
| 9 | Exhaust back pressure:Manifold pressure ratio <= 1.5 | Test | Pass |
BorgWarner RS4 K04
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 |
BorgWarner RS4 K04
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 | Pass |
| 16 | Maintain >= 25 psi to 7000 rpm | Test | Fail |
| 17 | <= 45 psi of exhaust back pressure at 6500 rpm and 25+ psi of manifold pressure. | Analysis | Fail |
| 18 | Mass Airflow Rates >= 375 g/s at 6000 rpm | Analysis | 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 | Test | Fail |
| 22 | MTBF >= 48 mo. or 50,000 miles | Analysis | Pass |
Requirements Summary
| REQ. # | BW K03 | BW K04 | FT21 | SRM RS4+ | TTE 550 |
|---|---|---|---|---|---|
| 1 | Pass | Pass | |||
| 2 | Fail | Pass | |||
| 3 | Pass | Pass | |||
| 4 | Pass | Pass | |||
| 5 | Fail | Pass | |||
| 6 | Fail | Pass | |||
| 7 | Pass | Pass | |||
| 8 | Pass | Pass | |||
| 9 | Fail | Pass | |||
| 10 | Pass | Pass | |||
| 11 | Pass | Pass | |||
| 12 | Pass | Pass | |||
| 13 | Pass | Pass | |||
| 14 | Pass | Pass | |||
| 15 | Pass | Pass | |||
| 16 | Fail | Fail | |||
| 17 | Fail | Fail | |||
| 18 | Fail | Pass | |||
| 19 | Fail | Pass | |||
| 20 | Fail | Pass | |||
| 21 | Fail | Fail | |||
| 22 | Pass | Pass |
The BorgWarner RS4 K04 turbochargers meet more requirements than the BorgWarner K03 turbochargers making them the preferred candidate from this comparison.