Some updated information was brought to my attention concerning one of the remaining candidates, the Project B5 RS6-X turbo.

I had thought the compressor wheel on this turbo was 45mm at the inducer and 56mm at the exducer.  I learned that it is 42.6/56 which is the same as the RS4-X (and FrankenTurbo F21), but the turbine side is the larger RS6 size.  What this means is that the Project B5 RS6-X turbo is dimensionally the same as the BW RS6, but at a fraction of the cost.  That makes it an interesting option in the mix.

Next: F21 vs K04

Next up in the decision process is an acknowledgment that whatever turbo system goes on the car is not going to be the last.  As I mentioned previously, at some point I expect to have the car built to a state that supporting the TiAL R770’s is a realistic option.  The time frame for that configuration is still undetermined, but it is probably at least two years out.  Knowing that the choice of turbochargers I make now is temporary I’m not looking to pay a premium for performance that can be matched by a similar product.

Taking a look over the candidate list I don’t see any point in paying for the Borg-Warner RS6-K04 setup, nor is there much justification for paying the added cost that comes with the JHM RS6-R turbo’s.

• Borg-Warner K03
• Borg-Warner K04
• Project B5: RS4-X
• FrankenTurbo F4H-BT
• FrankenTurbo F21
• Borg-Warner RS6-K04
• Project B5: RS6-X
• JHM RS6-R
• LOBA Motorsport LO530P
• Silly Rabbit Motorsport RS6 K24
• TiAL Sport s605.2
• Garrett GT28RS
• TiAL Sport R770
• Single Turbo Kit

The remaining candidates span the performance range for a stock motor B5 S4 and will require some detailed analysis to determine which product will be best suited to my evolving goals.

Even though I’ve managed to whittle the field down pretty quickly there is always the chance that a wild card will pop up – somebody selling a new/used set at a tremendous discount that piques my interest.

Next: Turbo Selection

The list of candidates from the last round includes a couple of options that are fairly easy to eliminate from the list just because they are on the edge of what I’m looking to install right now.

• Borg-Warner K03
• Borg-Warner K04
• Project B5: RS4-X
• FrankenTurbo F4H-BT
• FrankenTurbo F21
• Borg-Warner RS6-K04
• Project B5: RS6-X
• JHM RS6-R
• LOBA Motorsport LO530P
• Silly Rabbit Motorsport RS6 K24
• TiAL Sport s605.2
• Garrett GT28RS

• TiAL R770

• Single Turbo Kit

The Garrett GT28RS is to my understanding more ‘work’ to get installed than what I want to undertake, I’m interested in a bolt-on solution.

The TiAL 605.2 is closely related to the TiAL R770, aside from being a journal bearing it is essentially functionally the same, at least for my intended use, and is more turbocharger than what I am looking to install right now.

The SRM RS6 K24 is also a bit bigger than what I am interested in.  It’s a turbocharger that would best be paired with an upgraded motor, one that has aftermarket rods installed, an undertaking that I’m not planning on starting at this stage.

The LOBA LO530P is similar to the RS6 hybrids with an eye-popping price tag just over $4,000 USD, the price premium is too high for me to consider spending for what I expect this turbo to be capable of.

The FrankenTurbo F4H-BT is no longer available from FrankenTurbo but occasionally a used set comes up for sale.  The product is essentially a lower cost alternative to the Borg-Warner K04 with slightly better top end and no discernible drop in spool up rate.  While I enjoyed operating with the F4H-BT it is time to move on to something different.

The Project B5 RS4-X is spec’d like the FrankenTurbo F21 but slightly less expensive.  It’s unclear what the warranty policy is for the product, but the fact that the FT21 has been more widely used is comforting when trying to assess product reliability.  Given the similar cost and size spec’s I consider it a toss up between these two and with my prior positive experience with FrankenTurbo I give the edge to the F21.

The revised list is now:

• Borg-Warner K03
• Borg-Warner K04
• Project B5: RS4-X
• FrankenTurbo F4H-BT
• FrankenTurbo F21
• Borg-Warner RS6-K04
• Project B5: RS6-X
• JHM RS6-R
• LOBA Motorsport LO530P
• Silly Rabbit Motorsport RS6 K24
• TiAL Sport s605.2
• Garrett GT28RS
• TiAL Sport R770
• Single Turbo Kit

Next: Turbo Selection Path Consideration