This is what I'm thinking, and I've heard some technical eplanations from diesel experts with the right words that back it up 100%.
The turbine has a huge effect on diesel performance, possibly more than any other single component [assuming decent injectors]. The turbine is spun by exhaust gases/heat thermal expansion. It must fight against drawing air through filter and piping to intake manifold. Up to a point, more fuel fed to cylinder increases the expansion of gasses at combustion. More fuel, more pressure. That expanding wave then goes out the exhaust manifold and passes over the exducer side, spins the wheel, and out the downpipe, exhaust/muffler and into atmosphere.
Where an engine is allowed free flowing exhaust by larger pipe diameters the exhaust gases can more easily transmit their power to impeller. An exhaust brake works against this principle and by restricting exhaust it greatly slows the engine's revolutions. A smaller exhaust housing on the turbo transmits more of this power directly to impeller. A larger volume compressor side, with correctly sized impeller, forces more air during the process for a given amount of exducer's energy exchange. After that the key is to allow these gases to get out of the way as quickly as possible to allow the incoming exhaust gas to do its work with the least resistance i.e. back pressure.
Turbo diesels do not want exhaust back pressure because they're not petrol engines, and because they use an exhaust driven turbo. Exhaust back pressure benefits in a gasser engine have to do with exhaust wave exiting exhaust pipe and the relationship of positive/negative pressure wave inversion in pipe, which can cause a beneficial extraction effect at correct diameter and length. The same inversion wave theory applies to diesel, but the restriction by back pressure gives no benefit, but instead slows down the exhaust/turbo process.
If this is not so, can somebody please explain how restricting a turbo diesel exhaust can improve performance? How can slowing down the gases working in and exiting the turbo help the engine to make more power? Why would it help on bottom end when the turbo hasn't even spooled up yet? Thanks,
JimmieD