This chart of torque from a number of dyno’s illustrates how different size turbochargers will delivery a different driving experience.
The car setups can differ greatly, having a significant affect on the outcome, but the general idea is to find a median value from a number of cars to help illustrate how a particular type of turbocharger might perform on a typical S4.
Next: New option from SRM
Dyno charts for F21 equipped cars are hard to come by but I did find one tuned by EPL that has some usefulness in comparing the F21 to K04’s. The comparison is limited in the upper region due to the F21 car using K04 wastegates, but based upon the larger compressor size of the F21 and capacity to hold at least 25 psi out to red line I would expect a proper F21 setup to perform even better than the example below toward the top end – ie past 5000 rpm.
What is of most interest to me in this particular comparison case is the lower rpm region, below 4000 rpm. I’ve seen boost logs of F21’s and noted that boost comes on more slowly than with the FT F4H-BT which was very similar to the K04, not surprising given the closer sizing of the compressor wheels in the F4H and K04. I am a bit surprised at the torque differences shown by this F21 car and the K04 car. I’ve seen other K04 dyno logs from EPL tuned cars as well as 034 tuned k04 cars and the torque curve is typical. With only one F21 log to compare it’s good to be cautious in taking these results as representative, but assuming this is a fair comparison there is a decent shortfall in torque from the F21 as compared to the K04 in the sub-4000 rpm range.
This particular rpm region is significant when the car is driven in what I consider a conventional manner, that is one where high rpms aren’t being held often and downshifting in order to accelerate is not desired. The torque advantage of the K04 will translate into a perception of greater response, though the F21 is no slouch, still there is around a 50 ft-lb difference at 2500 rpm which grows to a 75 ft-lb difference through approximately 3500 rpm.
These results while being quite interesting haven’t swayed me to eliminate one turbo or the other – they do highlight how much turbocharger selection can affect the end driving experience and the importance of establishing a priority of what criteria are to be met.
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