Turbo size, pressure and tq/hp figures ? 2

It is a matter of matching the turbocharger to the engine, considering the entire operating range, and the parts of the operating range that are most critical. What you end up with is an overall turbocharger efficiency characteristic as a function of engine speed and load. The backpressure imposed by the turbocharging system is an inverse function of the overall turbocharging efficiency. A wastegate, when open, as the name implies, reduces turbocharging efficiency.
A relatively small turbocharger will tend to offer high turbocharging efficiency at lower speeds and loads, and the efficiency will drop off at higher speeds and loads, especially if there is a wastegate that opens up at higher speeds and loads. Conversely, a relatively large turbocharger, because of its inability to provide significant boost at low speeds and loads, will offer low turbocharging efficiency at these lower speeds and loads, but at higher speeds and loads, its efficiency will excel, especially if matched without a wastegate.
But remember, matching is critical. A large compressor does you no good if the engine breathing is such that it is running into surge. On the turbine side it is also important to have the right combination of A/R and trim, for the application, to realize the highest potential efficiency. And of course the turbine needs to be well matched to the compressor as well, especially with respect to diameter, lest the turbine efficiency be penalized by a too low tip speed ratio.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

the system will make max power when it is set up so that it reaches max boost at the rpm where the engine would make max power na (or a little above). compressor+turbine efficiency. boost should be higher than backpressure. (pressure ratio can be considerably higher at that point than if it were kept roughly constant through the max torque range.)

Hello Ibrahim, I realize that your question has already been well answered by the two previous responders. But I thought it an interesting question, and would like to answer it in a sightly different way.

The turbocharger will allow the engine to make maximum power when the design point of the engine (air mass flow, manifold pressure, manifold temperature) allow the pressure ratio and corrected mass flow of the turbocharger compressor to fall directly on the maximum efficiency spine of the compressor map.

Concurrently, to take full advantage of the engine pumping loop power, the wastegate must have just arrived at the fully closed position.

And finally, the compressor wheel diameter / expander wheel diameter ratio should be such that the turbine expander is loaded to it's peak efficiency point. For a 50% reaction turbine expander (that is - most turbochargers with radial wheels), that means the expander wheel tip speed should be about 70% of the theoretical tangential velocity of the gas entering the turbine wheel. This is sometimes referred to as tip speed (U) over isentropic spounting velocity (C). (Or more theoretically, the square root of 50%). See wiki explanation.


My advice is this: pay primary attention to the turbo compressor match (aligned corrected flow and pressure ration to the middle of the compressor map), but don't forgot the turbo expander match (wastegate should not be "wasting" any exhaust gas energy).

Best regards.