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VTT Turbo Basics: WGDC -what it is, what it means, and why it sometimes doesn’t matter
In the beginning... Let’s start by describing the function of a wastegate. A wastegate is a device that diverts some exhaust flow around the turbine on a turbocharged engine as a means of regulating the speed of the compressor wheel. Wastegates can be internal to the turbo housing (like on OEM N54 turbos), or external (like on N54 single turbo kits).
Figure 1: VTT OEM/Stage 1 manifold/housing with internal wastegate
Figure 2: VTT upgraded wastegate valve pieces
WGDC and spool? For spool, we’d want to run a very high WGDC until the turbo spools, then we’d drop WGDC down into something manageable to maintain the boost in a steady-state configuration.
WGDC evaluation. We at VTT like to use WGDC as an indicator from tune-to-tune on the same car, or hardware change to hardware change as a rough guide to gauge system flow deltas (changes). If you changed hardware on the inlet side of things and WGDC goes down in order for you to reach a target boost, then you did the right thing! If your WGDC seems abnormally high to hit a given target boost the first thing you’re going to want to look for is boost leaks.
Boost leaks will cause you to need more WGDC to reach your target, increasing shaft speeds (never good) and if the leak is bad enough you’ll miss your target, all the while possibly overspeeding your turbo (very bad!). Having general ballpark knowledge of your expected WGDC ranges is useful when understanding the state of your turbo motor and diagnosing issues. One of the things we’re working on at VTT is publishing expected WGDC ranges for a given boost/rpm target to help with troubleshooting issues.
You cannot, however, use WGDC as an indicator of system capacity unless it’s at 100% (nothing more left). We’ve all heard the proclamations “I made 30 psi on just 47% WGDC! Plenty left in her!”. Well, maybe. Remember that WGDC is merely the percentage of time that full vacuum is applied to closing the wastegate actuator. We can’t tell anything at all about airflow from WGDC, we don’t know anything about shaft speed based on WGDC (except more WGDC should mean higher shaft speeds), and we don’t know anything about system efficiency based on WGDC. Not only that, but WGDC numbers can be “fudged” easily.
Figure 3: Wastegate actuators; different lengths for different applications
Final Thoughts. System capacity and ultimate horsepower capability requires an understanding of compressor maps, shaft speeds, and of the complete engine; reducing flow restrictions on the intake/inlet side to give the compressor easiest access to air, reducing flow restrictions on the compressor outlet (hot side charge piping and a free-flowing FMIC), and on the exhaust side of things removing as many bottlenecks and restrictions as possible to reduce backpressure will always help. Efficiency and shaft speed aside for a moment, when you increase WGDC and your boost doesn’t increase, you ran out of turbo, if the WGDC is 90% or if it’s 40% it doesn’t matter -you have maxed the turbo. If you want more it’s time for some hardware changes (do a boost leak check first though!).
The turbos and components should always be matched to the power, reliability, and response goals of the customer. From 100k+ stock motors to fully built dedicated track cars, let us know if we can help you sort out a package that will deliver what you’re looking for.
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