Back pressure is EXTREMELY dependent on exhaust manifold and turbine
configuration.   There is NO typical value!  Mistral has measured full-power
backpressures up to 1.8 bar (23-24 psi) which is unacceptable, resulting in
a significant loss of engine power, and as low as 1.2 bar (17 psi) which
does not optimize turbine efficiency.  By changing manifold configurations
(pulse or constant-pressure system), turbine scroll (single or twin-scroll)
and turbine inlet configuration (A/R ratio), back pressure and turbine
efficiency can be adjusted to the desired compromise.  Believe me, it ain't
trivial.
I recommend highly the following reading:  Diesel Engine Reference Book,
Chapter 2, The Theory of Turbo charging, p. 28-71); it doesn't have to do
with diesel engines only and it is quite comprehensive.

Francois Badoux

Hi Francois!
Can Francois or anyone tell me what the A and the R in A/R represents?
Also with the low bar/psi which doesn't give optimum turbo efficiency - does
the turbo need optimum efficiency for turbo normalizing at altitude. I
thought we would be looking at running the turbo less efficiently (with
reduced back pressure) to save the turbo from overspinning and overheating?
George ( down under)



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