Hi peeps,
I was doing a little research on twin scroll turbos just because i was curious about how they worked (not because i'm upgrading again) and came across this tidbit of information that was explained very well. I'm sure allot of you already know this, but i wanted to post it for those looking for some clarification and also for those trying to push 24psi out of their K03 with MBCs and diode mods. Okay... it's mostly for the K03 people pushing their little turbos to the limit.
Enjoy.....
Phil
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Common turbo myths dispelled
To make more power, you do not want only more boost. Boost is only a measure of intake pressure. Everything else being equal and within reasonable limits for the setup, more boost makes more power only if the turbo is operating in an efficient range of performance. If you were to increase the boost to the point where the turbo is trying to move too much air, it actually reduces performance. This is because past a certain point of diminishing returns, a turbo is basically blowing hot air. This hot air creates intake air pressure and more boost because boost=measure of pressure. Again, back to the idea of volumetric efficiency, you want the maximum mass of air for the engine. Unless the air can be cooled sufficiently by the intercooler, the density of the air can be less than it would have been at a lower boost level. Modern cars will compensate by using their computer and sensors to adjust the timing. In other words, in this case more boost has reduced the amount of power! Lastly, the VW TDI car computer has air temperature and pressure sensors and a program that will prevent increased power by just turning up the boost. You need a chip or other performance enhancement to increase fueling.
It is a common mistake to equate boost, or intake pressure, with denser air. Referring to the ideal gas law as a simplified example, PV=NrT, if you raise pressure, temperature would also have to increase, assuming the other variables remain the same. Also keep in mind the above paragraph about operating a turbo outside of it's islands of efficiency. People often get so caught up with learning about the quick power gains from more boost pressure that they forget that the ultimate goal of turbocharging is increasing air density, not pressure. In designing the engine as a whole system, you want to create the same amount amount of power with the least amount of boost to reduce stresses on the engine and turbo and to keep air moving at an optimum speed throughout the intake tract. Some ways of doing is is to change camshafts to allow more air into the combustion cylinders, changing the combustion cylinders by boring, making the diameter of the cylinders wider, or stroking the engine and making the length of the cylinder travel longer. If the intake piping is too long and bent, this adds air resistance and pressure not associated with creating power. Shortening the intake piping, making the transitions between piping as smooth as possible, and and routing the piping as straight as possible will reduce the amount of required pressure to produce a certain amount of power, increasing reliability and efficiency. A rough rule of thumb is that each 90o bend in pipe adds as much resistance to airflow as 25 ft of straight piping. Of course, this depends highly on diameter, smoothness of bend, etc., but this should reinforce the idea that short straight piping is best for flow.
However, adding a larger turbo or turbo components does not mean the engine will make more power. The turbo is regulated by sensors, computer feedback, and solenoids set to control the boost at a certain pressure. The source of where the computer measures the pressure from is normally the intake manifold or some spot right before the intake manifold. Everything else being equal (load, rpm, etc), one large turbo and one small turbo will flow identical pressures of air at a given psi but remember that psi is just a measure of pressure, density is what matters and is what makes power! In other words, to flow a certain amount of air, where a smaller turbo may have already passed it's maximum efficiency and is blowing mostly hot expanded air, a larger turbo will still be operating in its area of maximum efficiency and is moving cooler air at the same psi. Again, assuming that one turbo is stressed too much and the other is in it's peak efficiency, they are both giving the same psi but not the same density of air. I will state it a third time for fun - 20 psi at 2500 rpm is always 20 psi at 2500 rpm, the difference between an efficient turbo and a turbo blowing hot air is the temperature of the air coming out of the turbo which affects density. There are also other factors that effect this such as the size of the turbo housings, backpressure, etc. You want to select a turbo which balances responsiveness with moving your desired mass of air. Again, this is a consumer level article so do a lot more research before crunching the numbers. Garrett turbo's website has some more info on calculating airflow there. Turbo manufacturers often have published graphs which show where the turbo is operating most efficiently.
Also remember that the control systems and sensors for the turbo are normally located after the intercooler in the intake manifold or piping. If there is an air leak in this section of piping, the turbo has to work even harder to make up for the lost and provide the same reading to the sensors. Because it has to work harder, it may even be operating outside of it's optimum efficiency range and creating excess heat compared to if there was no boost leak.
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Originally Posted by AudiA4_20T
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