Originally Posted by triple88a
why dont you just buy a right waste gate and enjoy boost with a gate thats suppose to open at whatever psi you want it to open?
What the above would do a lot of time is boost creep since it doesnt open all the way or if youre using one for too much psi it will always be loose and have a lot of lag since the waste gate will not be shut completely.
your wrong, read this by Joe Perez
Here, we see the entire pressurized intake system. I have marked three points of interest. "A" is a point after the compressor but before the intercooler, typically the little hose nipple that comes stock on most compressor housings and is the classical boost reference. "B" is a point after the intercooler but before the throttle body, which is where my boost controller is referenced to. "C" is a point after the throttle body, within the intake manifold itself.
Now, putting aside arguments of boost threshold and such, can we all agree that, all else being equal, a boost controller is going to try to achieve the pressure to which it is set, at the point in the system being measured? And we'll also agree that the boost gauge on the dash is always fed from a point after the throttle body, such as the brake booster hose, the cruise control hose, etc.
Good. Let us assume that we have a hypothetical perfect boost controller (and a perfect wastegate) and that the boost controller is set to 12 PSI.
We'll start with point A, since that's where most people are referencing their wastegate to. In this scenario, the boost controller is always going to try to achieve 12 PSI at the compressor outlet. At relatively low speeds, the flow through the intercooler is relatively little, and thus the pressure drop across the intercooler is relatively small. Assuming WOT then, the pressure in the system at points B and C will be fairly close to 12 PSI. As RPM increases, the magnitude of the pressure loss across the restrictive intercooler also increases. So while the boost controller is ensuring that 12 PSI exists at point A (before the restriction), the pressure at B and C will decrease by the amount of pressure drop across the intercooler. By redline, we may be down to 9 or 10 PSI at points B and C, despite the fact that there is still 12 PSI at point A.
Next, we move the boost controller's feed to point B, and re-run the test, still at WOT. As airflow increases and pressure drops across the intercooler, the boost controller works to maintain 12 PSI at point B, and so we see 12 PSI being reported on the boost gauge, assuming zero pressure drop across the throttle body. (Give me this one for the moment on faith, and I'll come back to it.) Now, since we're still seeing 12 PSI at points B and C, and yet pressure drop exists across the intercooler, this means that the pressure at point A is steadily increasing. Under the same assumptions as in the first test (that 2-3 PSI of drop occurs across the intercooler at WOT at redline,) the pressure at the compressor outlet will have risen to 14 or 15 PSI by this point. This, of course, is because the boost controller is ignoring point A, and looking only at point B. Some extra heat is going to be generated owing to the fact that the compressor is now working harder than before, but as a percentage of the total heat being generated, this is relatively inconsequential.
Ok, now we'll move the boost controller to point C, and we'll also change one other assumption- we're no longer going to be at WOT. We'll be going up a fictitious hill (or accelerating past a fictitious truck on the highway) and so you are modulating the throttle with your foot to achieve, say, 8 PSI. What's important here is that we're added a second restriction in the system- the throttle plate. And there's going to be pressure drop across this. If boost starts to creep up, you're going to close the throttle a little, increasing the pressure drop across the throttle, and holding MAP at 8 PSI. Because you are actively holding MAP below 12 PSI, the boost controller is never going to reach its activation point, and the wastegate is going to remain completely closed.
Now the problem is that at this load condition, you're generating more than enough exhaust gas to spin the turbo well beyond 8 PSI. If you were to measure the pressure in the system at points A or B, you might find that you've got 20 PSI. Or 30 PSI. Or 40? Who knows, really. A lot is going to depend on the size of the turbo, the exhaust system, etc. But it's going to be a hell of a lot more than 8 PSI, or 12 PSI, or even the 15 PSI we saw at the compressor outlet in scenario B. And this is going to generate a shitload of heat. And heat is the enemy. Granted, reducing the throttle opening is going to reduce the volume of gas available to spin the turbine, and there will be an equilibrium point somewhere, but remember that the whole reason turbuchargers work in the first place is that there is presumed to be a significant excess of exhaust gas available at all but the lowest load conditions- otherwise, we wouldn't need wastegates, and for that matter, the damn thing would never spool up.
"But" you ask, "won't this also be the case at part-throttle conditions in scenarios A and B?" And the answer, of course, is no it will not.
We'll step all the way back to scenario A. You're climbing the same hill, at the same part-throttle condition. You're making enough exhaust gas to spin the turbo up into orbit, however the boost controller is going to make sure that there is never more than 12 PSI in the pipes at point A. Doesn't matter if you are working the throttle to regulate MAP to 8 PSI, or 10, or 4. The pressure in the intake pipes will never exceed the boost controller's setpoint, because the boost controller is watching the intake pipes, not the manifold. Once the pressure in the pipes reaches 12 PSI, the wastegate will open to ensure that it does not exceed this. Thus, the compressor will never generate more heat than it would in a non-throttled run.
Same goes for scenario B. The pressure after the intercooler will never exceed 12 PSI, and the pressure before it will never exceed 12 PSI plus drop across the intercooler. And since the mass of air flowing through the intercooler at this part-throttle condition will be less than it was in WOT scenario B above, the pressure drop across it will be less and the pressure at the compressor outlet will be less.