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.

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.

By 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.

While driving the other night I heard a bang sound followed by a drop in power.  Subsequent the car sounded a bit different.  Initial investigation showed that one of the two wideband O2 sensor bung plugs in the downpipe had come out.

That plug coming out seemed like an odd cause for dropping power so I continued to investigate.  An intake pressure check was good, I tested the wastegate lines and they were good.  When the car was idling there was no smoke or visible water vapor coming from the tail pipe.

I was still suspicious of a possible turbo problem so I hooked up separate pressure sensors to each compressor housing signal line and then went out to record the pressure in the turbo compressor housing.

The results are shown above.  The great disparity between the driver side and the passenger side (driver side is blue and passenger side gray), and neither matching the MAP sensor, has me concerned that the passenger side turbo may have failed.