Best Stock Motor Turbo


This page is a documentation of my efforts to find the best turbocharger for my stock motor Audi B5 S4.

For about two years now I have been considering various turbocharger upgrade options for my S4 and have identified several good candidates.  I’ve been weighing the tradeoff between top end power versus low end grunt and was curious to learn what appealed to others so I conducted a two week poll asking which would be preferred if upgrading from K04’s, more top end with some noticeable ‘turbo lag’, or a smaller bump in power but similar to K04 boost onset.  The results with 150 people voting were a little surprising to me:

Chart showing Power vs Spool Poll Results
Power vs Spool Poll Results

Interest in keeping quick spool up was valued by more people, which I find interesting since it seems that most of the K04 hybrid turbochargers are going for the opposite; more power with a penalty in the spool up department.

The ideal situation would be to increase power AND keep quick spool up – a product to meet the goals of both groups.  That is the objective of this search, to identify the turbocharger system that can best remain close to K04 spool up while maximizing the power limitation of a stock S4 motor.



Operate a turbocharger system on my Audi B5 S4 that maximizes responsiveness and power within the limitations of a stock motor.

 Note: Many of these requirements will be further refined after establishing baseline performance of the BorgWarner RS4 K04’s. 


 Note: “Should” defines a desirable trait. “Shall” defines a required trait. Inability of a product to achieve a “Shall” requirement will eliminate it from consideration.  

Functional Requirements

  1. The turbocharger shall be capable of rapidly accelerating the car when operated at lower engine speeds, approximately between 2000 and 3000 rpm.
  2. The turbocharger should be capable of maintaining strong acceleration at higher engine speeds, from approximately 5500 to 6500 rpm.
  3. The turbocharger shall fit on the Audi B5 S4 motor in a twin turbocharger configuration.
  4. The turbocharger shall not require unique parts, i.e. exhaust manifolds or downpipes/flanges, be purchased in order to work with the existing components on the vehicle.
  5. The turbocharger shall equal or exceed the performance of the BorgWarner RS4 K04.
  6. The turbocharger wastegate actuator should be capable of maintaining maximum boost pressure at redline without exceeding 80% duty cycle of the N75 boost control valve.
  7. The turbocharger wastegate actuator should modulate boost during partial throttle driving conditions.

Technical Requirements

  1. The turbocharger should not generate unusually high, exceeding 300 degrees fahrenheit, charge air temperatures.
  2. The turbocharger should not cause excessive back pressure, defined as being greater than 1.5 times the manifold air pressure at a given engine speed, in the exhaust manifold.

Supportability Requirements

  1. The turbocharger shall be sourced from a reputable vendor with good product support.
  2. The turbocharger should be delivered with a product warranty that covers the product for at least 12 months.
  3. The turbocharger cost should be competitive with similar alternative products.
  4. The turbocharger price shall not exceed that of the BorgWarner RS4 K04, plus 20%.


Performance Specifications:

  1. The turbocharger should develop 20 psi of manifold pressure by 3100 rpm.
  2. The 2-11 psi time should be equal to or less than the BW K04 time at all engine speeds.
  3. The turbocharger should be able to maintain at least 25 psi to 7000 rpm.
  4. The turbocharger should not exceed 45 psi of exhaust back pressure at 6500 rpm and 25+ psi of manifold pressure.
  5. The turbocharger should support Mass Airflow Rates in excess of 375 g/s at 6000 rpm.
  6. When equipped with the turbocharger the vehicle FATS time shall be no more than 3.5 seconds under standard environmental conditions (approximately sea level, temperature around 60 degrees Fahrenheit).
  7. The vehicle shall generate a maximum torque of at least 400 ft-lbs.
  8. The vehicle should generate a maximum wheel power of at least 400 whp.
  9. The turbocharger mean time between failure should be at least 48 months or 50,000 miles when operated in accordance with supplier instructions. (Note: Because it will be impossible to obtain this measurement I will be assigning Pass/Fail based on subjective observations.)

Selection Process:

Selection of the turbocharger that meets the requirements will be made in four phases.

Phase 1: Generation of Candidate List

Information will be collected for all turbocharger systems that could possibly meet the system requirements.

Phase 2: Technical Data Review

Vendor supplied data and user results will be evaluated for candidate systems to determine which are eliminated and which undergo further review.

Phase 3: Testing

A few of the best qualified systems, approximately 2-3 turbochargers, will be installed on the same Audi B5 S4 and their performance measured and recorded.

Phase 4: Analysis and Selection

Data from the qualified systems will be compared and a determination made as to which turbocharger system best meets the system requirements.

Phase 1: Generating a Candidate List

I’ve discussed the considerations I made when generating and then narrowing a list of candidate turbochargers.  The series of posts begins here:

Turbocharger Selection

Phase 2: Data Review

From the search that I have been on for the past couple of years I have compiled this short list of turbochargers that may potentially meet the requirements I have established.

  1. FrankenTurbo F21 Mixed Flow
  2. The Turbo Engineers TTE 550
  3. Silly Rabbit Motorsport K04 RS4+
  4. Borg Warner RS4 K04 (retain baseline system)
  5. Turbo Concepts (late entrant)

Phase 3: Testing:

Initial performance data was collected on the B5 S4 when equipped with BorgWarner K03 turbochargers and documented via the following posts:

Next I moved on to collecting data with the S4 equipped with BorgWarner K04’s as documented in these posts:

FrankenTurbo provided a set of their mixed flow F21 turbochargers for evaluation.  Some infomration I have collected on F21’s can be found in these posts:

The Turbo Engineers provide a number of turbochargers options for the B5 S4.  The TTE550 is the product that has marketing claims most closely aligned with the requirements I have established.

Turbo Concepts provides a range of four turbochargers for the B5 S4.  Based upon the advertised performance of the Stage 1 product I chose that as a good candidate for this comparison.

Phase 4: Selection

Round 1: BorgWarner K03 vs BorgWarner K04

Round 2: BorgWarner K04 vs FrankenTurbo F21

Round 3: FrankenTurbo F21 vs TTE 550

Round 4: Turbo Concepts Stg1 vs BorgWarner K04

The table below summarizes the evaluated turbochargers against the requirements.

The Silly Rabbit Motorsport RS4+ has been dropped from consideration based upon the similarity in size to the other K04 hybrid turbochargers and an absence of any reliable information to show that it would perform significantly different than the products already evaluated.

K04/Hybrid Requirements Summary

REQ. #BW K03BW K04FT21TC Stg1TTE 550

Audi B5 S4 Information and Testing