STVA stands for secondary throttle valve actuator. Before fuel injection, these bikes used a CV (constant velocity) carburetor. On the fuel injected engines, Suzuki calls this technology "Suzuki Dual Throttle Valve" (SDTV). It's been in use since their first fuel injected motorcycles, but the purpose behind it has been there longer.
I won't go into too much detail, but the basics of it are pretty simple. The speed at which the air moves into the engine has a pretty big impact on its performance. The velocity helps pack a little more air into the cylinder as the piston reaches the bottom of the stroke. You also have to consider how all the EFI sensors work to measure the air going into the cylinder to calculate the fuel injector timing. When you quickly open the throttle, you rapidly change the pressure. The intake air pressure sensor sees this, and increases fuel. At the same time, the velocity of that air has slowed. What you get is a hiccup in power of the engine. Ideally, you want maximum power delivered smoothly.
So that's why the SDTV system exists. The sensors that feed the ECU let it know you slammed the throttle open. The bike makes instant power! The ECU has control of both the air and fuel going into the engine. Which gives you smooth power delivery.
"Ok, that's nice but I ride very smooth, and have very delicate throttle control." I've heard that phrase 1000 times. Even better ones are "Ben Spies didn't use it on his AMA superbike", or "it's there for emissions". The Suzuki engineers aren't the dumbest people around. I'm pretty sure they knew what they were doing when Suzuki invested the money to develop this system. Ben had a team of engineers around him and full factory support. Something most of us will never even come close to having. When us mere mortals have that little error light on the dash come with FI flashing at us, our bikes do something Ben's bike doesn't. It reverts to a failsafe mode. I know guys who've taken the secondary plates out, wired them fully open, etc. The bike doesn't fall on it's face at 6000rpm like some of us have experienced when the STVA fails in the closed position. But, it does stay in the failsafe mode.
Failsafe mode is Suzuki's way of doing their best to get you home if something goes wrong. It's really just software in the ECU. When an error is detected and the light goes on, the program goes into effect. Fuel injection is pretty simple when you look at the big picture. It measures the air going in, and sprays the right amount of fuel to match that air. Where it gets complicated is in the measurement. On the GSX-R, there's an air temperature sensor (IAT), an atmospheric pressure sensor (AP), an air pressure sensor inside the intake (IAP), and some good old math that allows the ECU to estimate how much oxygen is going into the cylinder based on the density of the air. In failsafe mode, these values are no longer read from the sensors. They are substituted with a set of values that Suzuki knows will make the engine run. Without these real world measurements, your bike is running maybe a little rich, maybe a little lean. Regardless, it's not giving you it's full potential. Even if you want to remove the plates and rely on your own throttle control, you need the sensors to let the ECU make the air density calculations.
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