Richard Holdener new video, he tested the same compressor with a different A/R on the same engine. The larger A/R created more backpressure as seen by the boost vs. backpressure crossover point happens earlier with the larger A/R (the two 1.12 traces show the crossover point stays about the same at different boost levels). If you overlay the boost pressures of the 1.26 A/R on the 1.12 A/R then you'd see the back pressure would be higher with the 1.26 A/R. It's an interesting characteristic and not intuitive in my opinion. Richard says if you wanted to reduce backpressure then you'd need to increase the whole turbo size (including the compressor). I'd be curious to see the difference in turbo shaft RPM and also the EGTs for the same manifold boost between the A/Rs.

 

I'll have to watch this later. This is something that I've been a little curious about lately as I've started considering putting a turbo on my VR6 (2.8L).

Of course, they're playing with absolute monster turbos, while I'm looking at mid 50mm turbos with something like a 0.63 A/R.