| I am referring to the volume of gases that flow out of the engine at low RPM - the sub-2.0" diameter of the turbine exducer is a small orifice and the turbine itself is nested in that same bore which adds to the resistance of flow through that port, however, there is significantly less restriction with such little flow of exhaust gases as compared to when the engine is singing at higher RPM. The gases literally just slide by the turbine. As a crude example, hold a turbo in your hand and blow into the turbine housing inlet, and then blow out into free air - you will notice that it is no more difficult to blow into thin air as it is to blow into the housing and through the turbine. This is due to the fact that the turbine will not begin to offer significant resistance to the flow of gases until there is appreciably more volume. The resistance of flow through the turbine isn't linear either, so at lower airflow, it offers significantly less restriction. Consider this as well: A stock turbo as compared to the GT28RS. Lets even put a .64 A/R housing on the GT28RS just to make that as similar as possible. It should be very clear which one of these setups is going to produce the most backpressure (resistance to flow) across the entire RPM band. This is because the larger diameter wheel with less blades simply flows better. Another interesting thing to note as well:
The Z1 Stage3 (AKA GT675RS) and the Z1 Stage2 turbocharger (no longer offered) ONLY differ by 1 single blade on the turbine housing - the original Stage2 has 10 blades vs. the GT28RS's 9-blade design. The 10-blade variation creates more backpressure (resistance to flow), however, it also produces equal torque to horsepower, whereas the GT28RS usually has less torque per equivalent peak horsepower. These turbochargers have identical compressor wheels and housings as well - the only thing different is that the Stage2 had a 10-blade wheel in a OEM 5-speed .63 A/R housing (which is a little more restrictive than the 5-bolt .64 A/R 5-bolt variant of the GT28RS). All I am trying to say is that I have seen some striking variations from what is considered common knowledge in reference to exhaust systems and what actually occurs in the car in the real-world scenario. Perhaps I am confused, I dont know yet, but the results I have seen from various setups where things have changed in ways not explainable by the simple exhaust theories are leaving big questions in my mind and opening up new ideas to test. Interestingly enough, since I came up with the idea of using some sort of variable backpressure in the exhaust, it has come to my attention that GM is now doing this in the new Z06 corvette for 2006. It is my opinion at this moment that there are many things that can be done to an exhaust system that will allow a dynamic response in the exhaust to aid the powerplant's pumping abilities throughout the entire RPM band, rather than just creating a tuned sweet-spot at one RPM interval. Consider it analogous to Nissan's VTC in our engines - it was created to give the best of low end grunt as well as top-end power. The actual dynamics that change in the engine when the VTC switches from one mode to another are there to aid in moving air through the engine. Some of the logic behind those dynamics actually appear counterintuitive at first, but then you come to realize from the results that something not as apparent is going on and making a bigger impact than you first imagined could have existed. I think there is a good bit more potential in the exhaust system that will require a little more mechanical gadgetry to tap....
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01:40:06 12/28/05
