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.
BorgWarner K03 (Left) and K04 (Right) Turbochargers
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.
Flowbench comparison
K03 vs K04 EGT readings on dyno
K03 (NVR & Nef tunes) vs K04 (Daz tune) Wheel Torque
K03 (NVR & Nef tunes) vs K04 (Daz tune) Wheel Horsepower
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.