You need to look at the wastegate duty required to meet the boost target.
A more restrictive intake (pre compressor) will result in a
lower pressure at the compressor intake. The post compressor pressure will be the same regardless (i.e. that set by the ecu mapping). To make this post compressor pressure the wastegate soleniod will be driven to a duty cycle (i.e. controlling the amount of exhaust gas that bypasses the turbine). The pressure ratio across the compressor will increase as the pre-compressor pressure falls, hence the compressor will be required to do more work, which does 2 things
1) heats up the charge air more - so for the same IC efficiency the plenum air temp will be higher
2) requires more work from the turbine
to meet this greater demand for turbo shaft power the wastegate will be closed more, allowing less exhaust gas to bypass the turbine via the wastegate, and increasing pre turbine pressure. This increase in pre-turbine pressure also has 2 effects:
1) means the piston must work harder to expel the burn gasses into the exhaust manifold on the exhaust stroke, which reduces crankshaft torque
2) reduces the pressure ratio across the engine (plenum pressure is the same as before, exhaust manifold pressure is higher) so actual airflow through the engine will reduce, limiting torque
This is further complicated by the ME engine management actually requesting a target air mass flow, not a plenum pressure, so it will actually have to increase plenum pressure to restore the target airflow, which also causes all the above effects.
This is why the more boost you run the harder and harder it is to make more power, i.e. the 1st 5psi boost increase gets you 20bhp, the next 5psi only 10bhp etc.
Pre compressor pressure losses are a real
problem on high boost turbo engines, and most turbo's will loose approx 1.5 - 2 bhp per kPa increase in pre-compressor pressure loss.
(still awake at the back?? lol)
I do have a BMC kit but i dont know why and if it is beneficial
