BW K03 Vs BW K04 - My Audi S4

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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
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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 = 48 mo. or 50,000 milesAnalysisPass
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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 = 48 mo. or 50,000 milesAnalysisPass
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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.

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