K04 to the bench

A theory I’ve be thinking about lately is that the turbine side of the K04 hybrid turbochargers is the limiting factor in the amount of power these turbo’s can produce.

This theory has been accepted for some time, and seems quite reasonable given the results obtained with K04 and RS6 style turbochargers.  I started thinking more about the subject when it occurred to me that continuing to upsize compressor wheels, or incorporate more advanced designs, would not be a particularly worthwhile pursuit if no matter the improvement to the compressor the turbine side put a ceiling on the power capable of being developed.

The dilemma of this situation became more clearer to me when I began to compare boost profiles of FrankenTurbo F21 turbochargers against the results I had recorded with the FrankenTurbo F4H turbo.  The trend with the F21 has been to operate with boost pressure around 24-25 psi.  The F4H turbo, with a slightly smaller compressor wheel could achieve similar results, but with noticeably quicker spool up.

It seemed that the larger compressor wheel was slowing boost onset but apparently not resulting in any more top end power.  Maybe the turbine ceiling had been reached?

Flow testing turbines is not something commonly done outside of rebuilding Variable Nozzle Turbochargers (VNT) but I did find a research study done as part of a thesis at Ohio State University that incorporated a flow bench into turbine testing.

With a few ideas in mind about what I might do to check my own turbochargers I built an adapter sized like the bell-mouth entry to the downpipe to attach my BorgWarner K04 turbochargers to the flow bench.

BorgWarner K04 Turbocharger on Flow Bench
BorgWarner K04 Turbocharger on Flow Bench

I then fixed the compressor wheel so that it and the turbine would not spin.  I left the wastegate closed and ran the flow bench from zero up to 30 inches of H20 depression recording airflow through the turbine at several points along the way.

k04_pressure_chartBy itself the information shown above doesn’t say a whole lot. Shortly I’ll have my K03’s back from being rebuilt and at that time I will put them on the bench to gain a comparison dataset.  At some point in the future I may get my hands on an RS6 hotside to be able to measure how much of a difference there is at these low airflows.

This post has some follow on testing with the wastegate door open.