This is as basic as it gets. As I understand it, a wastegate vents excess exhaust pressure before the turbo in order to prevent overboost. Also, a smaller inlet and drive turbine builds boost more quickly, but limits top-end horsepower because of the exhaust restriction that it causes, so people have to make a compromise between what rpm boost comes on, how much lag is acceptable, and how much top-end power they want. See where this is going? Why doesn't a wastegate (or if one big one wouldn't be precise enough, 2 smaller ones) eliminate the compromise? Obviously it doesn't. I just don't know why. Anybody willing to tell me without me needing to go get my flame suit? Thanks, Don.

 

Obviously some of the exhaust gas needs to keep going thru the turbine to keep the turbo spinning. Sure you can put 2 wastegates on there, but it still doesn't alieve the restriction of the smaller A/R turbine housing restricting flow. You can't just dump all the exhaust and not have anything supplying the turbo. The wastegate regulates the exhaust pressure to keep the turbo output constant. With a smaller A/R turbo the "nozzle" opening is small so the exhaust will have a high velocity when it hits the turbine blades thus resulting in quick spool, however at high rpms the exhaust is being restricted because of the smaller opening and the backpressure will have to increase to supply the turbine with enough energy (exhaust) to keep the turbine spinning fast enough to supply the correct amount of air to keep boost pressure constant. If you want more flow across a fixed oriface, the differential pressure must be increased! A large A/R turbo will have slower exhuast velocity hitting the turbine so it spools slower, however on the big end it can keep the turbine spinning with less backpressure, which can result in more power output from the engine due to decreased pumping losses. Know what I'm saying?

Bill

 

Too add to smoknz's explaination, the other way to "adjust" or "regulate" how the turbo responds is to instead change the A (area) of the nozzle opening, is to change the R (radius) or the length of the turbine blade or diameter of the wheel. a larger wheel will have more torque potential with which to turn the compressor side of the turbo.. this also requires less force to spin and will allow a higher rate of airflow with a lower restriction. everything should be matched to the output in heat energy the motor can supply (cubic inches) and effective air movement through the engine. this is where the science comes to play. most of this is information directly from "Maximum boost" but makes alot of sense. the other idea you have about the twin gate.. as stated above you can do this, but it will not change the fact that you still need to drive the turbine to maintain boost and it needs to continuously be accelerated during acceleration of the engine to keep up with the amount of airflow the engine requires through its power band.

the concept of VNT (variable nozzle turbos) and VVT (variable vane turbos is a better approach to accomplishing what your trying to do with wastegates. these turbos start off with a smaller A/R and have a solenoid that moves increasing the A (area) introduced to the turbine blades effectively reducing the restriction and allowing more exhaust flow accross the turbine at higher RPM's resulting in fast spooling and quick boost rise rates while also allowing the turbine side to breath without building excessive manifold pressures which creates high end horsepower. essentially a system thought out well enough could in theory eliminate a wastegate with enough movement in the Nozzle or vanes to bleed off the heat energy potential accross the turbine blades in terms of velocity. this would require some very intricate electronic controllers to actuate the nozzles of the turbine housing at the correct rate and in the proper correlation to the engine speed and boost pressures being created. I am sure in the near future as turbocharging is growing tremendously, more and more people are experimenting with the different technologies available, that these systems will be available. I for one am very excited with the direction performance is going! very exciting times!

Chris

 

That explains it perfectly. Thanks, guys. I had forgotten that the turbo will need to supply more and more air to keep the same amount of boost as rpm increases, and therefore needs more and more exhaust blown through it as rpm increases. If you bleed off the "excessive horsepower robbing" exhaust pressure, you will also rob the turbo of the ability to provide enough air to maintain the desired boost level, which just loses your horsepower in a different way.
I knew there had to be a reason, I just didn't know what it was. Thanks again, Don.