Here is a great article from Javad at 034. The basic summary is that with a large turbo a large wastegate is needed to run low boost on the street but not for the track.
http://www.034motorsport.com/article...rticles_id=128
Dispelling Wastegate Myths View all Tech Articles articles
Article by 034Motorsport
An explanation of wastegate function, bigger isn't always bigger.
By Javad Shadzi
The Massive Tial 60mm External Wastegate
Here at 034Motorsport, among many things, we spend our days helping people all over the world put together turbocharger systems to meet their engine power requirements. By far the most misunderstood component we find is the wastegate. Like many automotive tuning principles, wastegates are the reverse to intuition or common knowledge. Ironically they are simple devices, read further and we’ll explore some of the basics about wastegates.
Many we talk to are convinced that high power engines with huge turbos must need huge wastegates as well (bigger is better, right??!), we even see many tuners and race teams falsely implement wastegates much larger than are needed (Ever see an 800HP 4-cyl engine running 40psi boost with 2 huge wastegates on the header collector?) This myth extends to the misconception that internal wastegates also are not high performance and that external wastegates are the best way to control boost – which I’ll explain isn’t necessarily true.
THE BASICS
Fundamentally, wastegates are simply valves, the top of the valve is connected to a chamber isolated by a diaphragm or plunger, this creates an upper and lower control chamber. Turbocharger pressure acts on this chamber (usually the lower chamber to force the valve up and open), much like the effect of an air spring. The valve is kept shut by a spring inside the diaphragm chamber to keep the valve shut and also to offset the actuation pressure. In summary, the spring keeps the valve shut, and boost pressure forces it open, this is how the valve is controlled.
The basic task of the wastegate is to vent exhaust gas away from the turbo inlet, the less exhaust gas that flows through the turbo, the less boost pressure the turbo can produce. Thus the wastegate “wastes” exhaust gas and lowers boost.
A typical example of how a wastegate would function, specifically our Tial 44mm Wastegate and one of our favorites.
This particular wastegate is supplied “off the shelf” with a 2.0 BAR wastegate spring or about 14.7 psi atmospheric pressure. Thus, as the turbo begins to spool up from 0psi, the valve remains shut as the spring exerts 14.7psi on the valve. As the turbo continues to spool, boost pressure climbs past 5, 10 and finally by 15psi the pressure in the lower wastegate chamber overcomes the pressure of the spring and begins forcing the valve up and open. As the valve continues to open, more gas is routed around the turbo thus causing the turbo to slow down, and the turbo pressure falls into a synchronicity with the wastegate spring pressure and the 14.7psi of boost pressure is solidly maintained.
A wastegate cannot control boost any lower than the pressure of its spring, the above wastegate will never be able to provide a boost level below 14.7psi. In order to run lower boost, a softer spring would be required.
And now I know what you’re wondering…
HOW TO INCREASE OR MANIPULATE BOOST PRESSURE
Up to this point we’ve discussed how a wastegate works with only the spring pressure inside it and without any outside control systems. Using spring pressure is a good way to control boost as it results in flat and stable boost pressure. Base spring pressure won’t compensate for temperature variations (and thus boost pressure variations), and high levels of boost become ridiculous to control with only spring pressure as the spring rates go through the roof. Bolting the wastegate back together could require literally hundreds of pounds depending on the design of the wastegate. In almost any case, applications over 15psi boost, its recommended to use an outside control system to increase boost pressure. These systems always manifest it 2 basic ways:
1 – By bleeding off control pressure to the lower diaphragm and thus causing the wastegate valve to not open as far
-or-
2 – By pressurizing the top diaphragm and offsetting the boost pressure to the lower diaphragm, essentially lowering the pressure differential across the diaphragm and not allowing the valve to open as far.
We won’t go into too much more detail about these systems in this article, but this point simply emphasizes the way the wastegate works and how it controls boost.
DISPELLING THE MYTH
By this point you should have a good fundamental understanding of how wastegates work, and thus should be able to deduct why some of the myths are false.
Since a larger wastegate has the capacity to bypass more exhaust gas than smaller wastegates, in a given application, larger valves are actually better suited to less boost and lower HP applications. Saying a larger wastegate is necessary for more power is like saying cars with bigger brakes make more power, when in fact bigger brakes assist in making the car slower, not faster. The highest boost wastegate is no wastegate at all, with out a wastegate a turbocharger would produce its maximum boost capability at all times.
CASE STUDY
One example of wastegate sizing constraints is in our very own 80tq project. In various configurations we were forced to change wastegate sizes to achieve different boost control strategies.
Configuration: GT40R turbo with .95AR, stock 30mm Audi wastegate, free breathing 20v 5-cyl turbo. With this configuration, the lower boost limit was 18psi at 5500rpm, boost would quickly taper to over 30psi by 6500rpm on simply spring pressure with no outside control. The reason for this boost taper increase was that the small 30mm wastegate could not bypass enough exhaust gas to maintain springe pressure boost, causing boost to increase. Counter-intuitively, this very small wastegate was perfectly suited to running 30psi or higher at 600 crank HP and above. The only reason to put a larger wastegate in this application would be to lower boost. By fitting a Tial 44mm wastegate we could maintain a 20psi boost curve all the way to redline, a 60mm wastegate could have been fit to run lower boost curves in the 10psi range, or about 300 crank HP. With such a large turbo and a free flowing motor, large amounts of air go through the motor, thus large amounts must be bypassed to control boost. With a smaller turbo, like a GT3071R for example, the stock Audi wastegate is effective at holding 20psi to redline since overall airflow levels are so much less.
GT25R with built in Internal Wastegate
THE DIFFERENCE BETWEEN INTERNAL AND EXTERNAL
This is another commonly misunderstood area; common knowledge has firmly established that external wastegates are superior and most just run with that “knowledge”. Both are very effective and reliable ways to control boost, most OEMs use internal in their turbo systems.
Internal Pro’s:
-Compact, the wastegate valve is typically integrated into the turbine housing, the actuator bolted to the compressor housing, gases are vented around the turbo inside the turbine housing.
-Supplied with many turbos direct from the manufacturer, properly sized and engineered.
-Eliminate the wastegate inlet tube in many fabricated manifolds that is usually the failure point (the tube that goes from the header collector to the wastegate inlet, since the wastegate is so heavy and expansive forces great, it will stress the welds on the collector and cause them to break and fail).
-Eliminates a need to dump exhaust back into the downpipe or exhaust system.
-Effective boost control with external devices.
-Leak proof design, no problems with loosening hardware.
Internal Con’s:
-Due to the way the wastegate vents directly behind the turbine, the venting can cause turbulence which can adversely affect boost control and turbo flow. Proof of this effect is rare and its effect seen more rarely.
-Spring rates inside the wastegate are difficult to change and require a new actuator.
-Turbo clocking not as flexible as the wastegate actuator mounting forces the compressor housing into specific locations
-Limited in size, typically internal wastegate valves measure in the 25mm range, which doesn’t make them as useful for very low boost applications.
External Pro’s
-Large range of sizes means flexible boost tuning options, smallest are 30mm and go as large as 60mm. Multiples can be used for high-waste applications
-Easy to change out spring rates to set base boost pressure.
-Flexible mounting options as the wastgate can be mounted remotely from the turbo
-Limitless brand and size options on the market to suit any turbo and wastegate, choices can be made independently of each other.
-Wastegate dump can be facilitated far from the turbo turbine outlet reducing turbulence problems.
-V-banded wastegates like the Tial 44mm are leak proof and stay tight.
External Con’s
-Can be large and bulky, difficult to package depending on the application.
-Require a vent tube in the collector of the manifold, which can often be a point of header failure.
-Models with 2-bolt flanges have chronic problems keeping hardware tight; we’ve seen them literally fall off.
Bottom line, both are effective ways to control boost and have their pro’s and con’s, don’t discount internal wastegates as a reliable, solid way to control boost. Ironically, internal wastegate’s biggest flaw is that due to their smaller size, they are not as useful for low boost applications. The other big criticism of internal wastegates, that they cause turbulence behind the turbine wheel, is minimized as boost increases due to less flow out the wastegate and more through the turbo.
If you have specific questions don’t hesitate to contact us, for a full line of wastegates we offer, see our wastegate section.
This article was published on Wednesday 06 February, 2008.
Originally Posted by TighTT
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