K04 vs FT21

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

FrankenTurbo F21

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

FrankenTurbo F21

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

FrankenTurbo F21

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 rpmTestFail
16Maintain >= 25 psi to 7000 rpmTestPass
17<= 45 psi of exhaust back pressure at 6500 rpm and 25+ psi of manifold pressure.TestFail
18Mass Airflow Rates >= 375 g/s at 6000 rpmTestPass
19FATS time <= 3.5 sec (Sea level, 60 degF)TestPass
20Torque >= 400 ft-lbsTestPass
21Wheel power >= 400 whpAnalysisPass
22MTBF >= 48 mo. or 50,000 milesAnalysisPass

FrankenTurbo Requirements Summary

REQ. #BW K03BW K04FT21SRM RS4+TTE 550
1PassPassFail
2FailPassPass
3PassPassPass
4PassPassPass
5FailPassFail
6FailPassPass
7PassPassPass
8PassPassPass
9FailPassFail
10PassPassPass
11PassPassPass
12PassPassPass
13PassPassPass
14PassPassPass
15PassPassFail
16FailFailPass
17FailFailFail
18FailPassPass
19FailPassPass
20FailPassPass
21FailFailPass
22PassPassPass

Audi B5 S4