BW K03 vs BW K04

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.

Picture of BorgWarner K03 and RS4 K04 turbochargers
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.

k03_vs_k04_pre-turbo_pressure

k03_k04_ratio

k03_vs_k04_closed_flow
Flowbench comparison

 

k03_vs_k04_maf

k03_vs_k04_n75dc

Chart of K03 vs K04 EGT readings on dyno
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 Torque

 

K03 (NVR & Nef tunes) vs K04 (Daz tune) Wheel Horsepower
K03 (NVR & Nef tunes) vs K04 (Daz tune) Wheel Horsepower

BorgWarner K03

Functional Requirements Verification Matrix
NumberFunctional Requirement DescriptionVerification MethodPass / Fail
1Rapid acceleration at 2000-3000 rpm.DemonstrationPass
2Maintain strong acceleration from 5500-6500 rpm.DemonstrationFail
3Biturbo configuration.InspectionPass
4Does not require unique parts.InspectionPass
5Equal or exceed RS4 K04 performance.TestFail
6N75 DC <80% at 6800 rpmTestFail
7Wastegate modulation of boost at part throttle.TestPass

BorgWarner K03

Technical Requirements Verification Matrix
NumberTechnical RequirementVerification MethodPass / Fail
8Charge air temperature < 300 degFTestPass
9Exhaust back pressure:Manifold pressure ratio <= 1.5TestFail

BorgWarner K03

Supportability Requirements Verification Matrix
NumberSupportability RequirementVerification MethodPass / Fail
10Reputable vendor with good product supportDemonstrationPass
1112 month warrantyInspectionPass
12Cost competitive with alternativesInspectionPass
13Price not greater than 20% over RS4 K04 InspectionPass

BorgWarner K03

Performance Specifications
NumberSpecification DescriptionVerification MethodPass / Fail
14Develop 20 psi of manifold pressure by 3100 rpmTestPass
152-11 psi time should be <= K04 time at all rpmTestPass
16Maintain >= 25 psi to 7000 rpmAnalysisFail
17<= 45 psi of exhaust back pressure at 6500 rpm and 25+ psi of manifold pressure.AnalysisFail
18Mass Airflow Rates >= 375 g/s at 6000 rpmTestFail
19FATS time <= 3.5 sec (Sea level, 60 degF)TestFail
20Torque >= 400 ft-lbsTestFail
21Wheel power >= 400 whpTestFail
22MTBF >= 48 mo. or 50,000 milesAnalysisPass

BorgWarner RS4 K04

Functional Requirements Verification Matrix
NumberFunctional Requirement DescriptionVerification MethodPass / Fail
1Rapid acceleration at 2000-3000 rpm.DemonstrationPass
2Maintain strong acceleration from 5500-6500 rpm.DemonstrationPass
3Biturbo configuration.InspectionPass
4Does not require unique parts.InspectionPass
5Equal or exceed RS4 K04 performance.TestPass
6N75 DC <80% at 6800 rpmTestPass
7Wastegate modulation of boost at part throttle.TestPass

BorgWarner RS4 K04

Technical Requirements Verification Matrix
NUMBERTECHNICAL REQUIREMENT DESCRIPTIONVERIFICATION METHODPASS / FAIL
8Charge air temperature < 300 degFTestPass
9Exhaust back pressure:Manifold pressure ratio <= 1.5TestPass

BorgWarner RS4 K04

Supportability Requirements Verification Matrix
NumberSupportability RequirementVerification MethodPass / Fail
10Reputable vendor with good product supportDemonstrationPass
1112 month warrantyInspectionPass
12Cost competitive with alternativesInspectionPass
13Price not greater than 20% over RS4 K04 InspectionPass

BorgWarner RS4 K04

Performance Specifications
NumberSpecification DescriptionVerification MethodPass / Fail
14Develop 20 psi of manifold pressure by 3100 rpmTestPass
152-11 psi time should be <= K04 time at all rpmTestPass
16Maintain >= 25 psi to 7000 rpmTestFail
17<= 45 psi of exhaust back pressure at 6500 rpm and 25+ psi of manifold pressure.AnalysisFail
18Mass Airflow Rates >= 375 g/s at 6000 rpmAnalysisPass
19FATS time <= 3.5 sec (Sea level, 60 degF)TestPass
20Torque >= 400 ft-lbsTestPass
21Wheel power >= 400 whpTestFail
22MTBF >= 48 mo. or 50,000 milesAnalysisPass

Requirements Summary

REQ. #BW K03BW K04FT21SRM RS4+TTE 550
1PassPass
2FailPass
3PassPass
4PassPass
5FailPass
6FailPass
7PassPass
8PassPass
9FailPass
10PassPass
11PassPass
12PassPass
13PassPass
14PassPass
15PassPass
16FailFail
17FailFail
18FailPass
19FailPass
20FailPass
21FailFail
22PassPass
The BorgWarner RS4 K04 turbochargers meet more requirements than the BorgWarner K03 turbochargers making them the preferred candidate from this comparison.

Audi B5 S4