I will post a couple of comments from the Garrett People and the Borg Warner people about pressure across the inlet and outlet of the Turbine.
Garrett:
"http://www.turbobygarrett.com/turbobygarrett/turbine_housing_AR_and_housing_sizing
Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to breathe"" effectively at high RPM, adversely affecting peak engine power."
Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.
When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.
Here's a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system.
Imagine two 3.5L engines both using GT30R turbochargers. The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:
1. Engine #1 has turbine housing with an A/R of 0.63
2. Engine #2 has a turbine housing with an A/R of 1.06.
What can we infer about the intended use and the turbocharger matching for each engine?
Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more "fun" to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high back pressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.
Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 since Engine #2 will be operating at high engine speeds most of the time."
So basically Garrett is singing the same old song: "higher engine speeds, smaller A/R housing will result in high back pressure, which can result in a loss of top end power"
Borg Warner:
http://www.turbodriven.com/en/turbof...gnTurbine.aspx
Borg Warner says that Higher Pressure across the Turbine is not necessarily a bad thing:
The turbocharger turbine, which consists of a turbine wheel and a turbine housing, converts the engine exhaust gas into mechanical energy to drive the compressor. The gas, which is restricted by the turbine's flow cross-sectional area, results in a pressure and temperature drop between the inlet and outlet. This pressure drop is converted by the turbine into kinetic energy to drive the turbine wheel. Energy transfer then happens (from kinetic energy into shaft power in the turbine wheel, which is designed so that nearly all the kinetic energy is converted by the time the gas reaches the wheel outlet.
The turbine performance increases as the pressure drop between the inlet and outlet increases, i.e. when more exhaust gas is dammed upstream of the turbine as a result of a higher engine speed, or in the case of an exhaust gas temperature rise due to higher exhaust gas energy. The turbine's characteristic behaviour is determined by the specific flow cross-section, the throat cross-section, in the transition area of the inlet channel to the volute. By reducing this throat cross-section, more exhaust gas is dammed upstream of the turbine and the turbine performance increases as a result of the higher pressure ratio.
A smaller flow cross-section therefore results in higher boost pressures.
The turbine's flow cross-sectional area can be easily varied by changing the turbine housing. Besides the turbine housing flow cross-sectional area, the exit area at the wheel inlet also influences the turbine's mass flow capacity. The machining of a turbine wheel cast contour allows the cross-sectional area and, therefore, the boost pressure, to be adjusted. A contour enlargement results in a larger flow cross-sectional area of the turbine.
For a high overall turbocharger efficiency, the co-ordination of compressor and turbine wheel diameters is of vital importance. The position of the operating point on the compressor map determines the turbocharger speed. The turbine wheel diameter has to be such that the turbine efficiency is maximised in this operating range.
The turbine is rarely subjected to constant exhaust pressure. In pulse turbocharged commercial diesel engines, twin-entry turbines allow exhaust gas pulsations to be optimised, because a higher turbine pressure ratio is reached in a shorter time. Thus, through the increasing pressure ratio, the efficiency rises, improving the all-important time interval when a high, more efficient mass flow is passing through the turbine. As a result of this improved exhaust gas energy utilisation, the engine's boost pressure characteristics and, hence, torque behaviour is improved, particularly at low engine speeds.
I personally tend to agree with Travis Q and the Borg Warner people in that High Turbine Pressure Numbers do not necessarily always result in Top End HP losses. Basically you have to look at what you are trying to do in each application and then go from there. "Rules of Thumb" are ok but actual data that says "if i do this then I actually got this" is always better. "One piece of data is worth 1000 Opinions" (James Clarke)
Tom Vaught