VTG - theory vs reality
In my experience with the stock TT, the VTG never delivered as promised. The boost never registered on the gauge or on my backside until 3k rpms. It did not rapidly peak at 1950 rpms as I expected from the Porsche torque plots, and never even reached the levels of boost advertised either. I never saw over 1 bar boost even in Sport mode. At one point, I theorized that Porsche simply programmed in a boost limiter until break-in was achieved on the odometer.
I have a theory as to why VTG did not deliver, and if my theory is correct, it explains why Porsche's claims are not observed in real world driving.
1)In the new VTG turbos, there is no more wastegate - no internal nor external - because boost levels are controlled solely by the VTG vanes. The vanes vary the effective size of the turbine in order to control boost. My theory is that the VTG vanes cannot react quickly enough to engine load at low rpms the way a mechanical sprung wastegate can. Using an analogy, I would compare the VTG control of boost pressure to controlling flow of water in the garden hose with the valve on the wall. I would compare the wastegate control of boost to controlling the flow in the hose by putting your thumb over the end of the hose. Adjusting the water valve control will produce less rapid changes than the instantaneous effect of moving your thumb across the end of the hose.
2)Some dynamometers can hold an engine at any constant rpm and measure its power output vs loading. If Porsche engineers put a maximum load on the TT engine and then slowly let the rpms increase from off idle, the VTG delay would be invisible, and we might see full boost and max torque at 1950 rpm. This would explain why Porsche's testing show the engine producing max torque at such a low speed.
3)The problem is that in the real world, the only way to duplicate that would be to test the TT in 5th or 6th gear and make it accerate from a near crawl either up a steep hill or towing a boat - the load on the unladen car on a flat surface is not enough to hold back the car so that it blows past 1950 rpms before the VTG can react. Recall that VTG is a technology borrowed from diesel trucks. Those engines do not rev rapidly, and are subject to massive loads. The diesel VTG vanes don't need to be quick at all. This would explain why the owners don't aren't able to reproduce the torque punch at such low rpms.
4)Since adding an aftermarket exhaust to my car, the torque and boost characteristics of the engine have become much closer to the Porsche charts. Boost onset is much faster, and is observable from as low as 2k rpms. The full boost will arrive as low as 2300-2400 rpms. I believe that by removing a substantial amount of backpressure, the VTG vanes can actuate more rapidly.
I have a theory as to why VTG did not deliver, and if my theory is correct, it explains why Porsche's claims are not observed in real world driving.
1)In the new VTG turbos, there is no more wastegate - no internal nor external - because boost levels are controlled solely by the VTG vanes. The vanes vary the effective size of the turbine in order to control boost. My theory is that the VTG vanes cannot react quickly enough to engine load at low rpms the way a mechanical sprung wastegate can. Using an analogy, I would compare the VTG control of boost pressure to controlling flow of water in the garden hose with the valve on the wall. I would compare the wastegate control of boost to controlling the flow in the hose by putting your thumb over the end of the hose. Adjusting the water valve control will produce less rapid changes than the instantaneous effect of moving your thumb across the end of the hose.
2)Some dynamometers can hold an engine at any constant rpm and measure its power output vs loading. If Porsche engineers put a maximum load on the TT engine and then slowly let the rpms increase from off idle, the VTG delay would be invisible, and we might see full boost and max torque at 1950 rpm. This would explain why Porsche's testing show the engine producing max torque at such a low speed.
3)The problem is that in the real world, the only way to duplicate that would be to test the TT in 5th or 6th gear and make it accerate from a near crawl either up a steep hill or towing a boat - the load on the unladen car on a flat surface is not enough to hold back the car so that it blows past 1950 rpms before the VTG can react. Recall that VTG is a technology borrowed from diesel trucks. Those engines do not rev rapidly, and are subject to massive loads. The diesel VTG vanes don't need to be quick at all. This would explain why the owners don't aren't able to reproduce the torque punch at such low rpms.
4)Since adding an aftermarket exhaust to my car, the torque and boost characteristics of the engine have become much closer to the Porsche charts. Boost onset is much faster, and is observable from as low as 2k rpms. The full boost will arrive as low as 2300-2400 rpms. I believe that by removing a substantial amount of backpressure, the VTG vanes can actuate more rapidly.
but nonetheless here goes. Is a change in the throttle set up necessarily a ecu remapping or can it be accomplished another way, without potentially voiding the warranty? I intend to eventually tune my car and probably will go with Ruf. I'm just unsure of what the tuning will look like...so forgive me if the answer to the question is obvious. 
Just trying to learn more. 
), stock vs. aftermarket exhausts, that made you determine that the reduction of backpressure is the most important thing to do on our cars to gain efficiency? 
