Recently, I’ve had the privilege of tuning three similarly modified Focus ST’s very close together, with the only real difference between them being the wastegate actuator.
All three had the basic bolt-ons, though brands did vary. This alignment of the planets
allowed me to get some pretty close back to back data to compare the different
options for modifying the factory wastegate on the Focus ST.
Car number one came in with the actuator untouched and unmodified. Car number two came in with the factory wastegate preloaded a little more than stock. Car number three came in with a Turbosmart wastegate actuator upgrade, replacing the stock actuator.
When tuning the Focus ST on the stock turbo, you are pretty much just maximizing what the
wastegate can do. WGDC (wastegate duty cycle) is going to hit 100%, meaning it's working as hard as it can to keep the wastegate shut starting around 4000rpm and holding on to get as much boost as the turbo can give. This may sound harsh, but there's not much you can do through tuning to increase boost. It's physically limited by the hardware on the car.
Boost pressure on the small stock turbo falls off hard towards redline dropping as low as 10psi. This test was done at 5,000 feet above sea level which will yield different values on any given datalog. But in any environment, the same hardware limitation exists.
When you tighten the preload on the factory actuator, you can maintain a little more boost on the top end. By doing this with car number two, I saw an increase of about 2psi over car number one. The average boost pressure was higher
throughout the pull, as well. This is pretty cool for being a completely free mod! With a little handy work, you gain horsepower and torque as well as added power under the curve.
However, there is still some power left on the table. With the Turbosmart Actuator, you can see considerable gains in boost pressure throughout the curve. Low end boost needs to be
contained a bit more through tuning, but the top end can be set free for a whopping 6psi gain over the stock actuator.
If you decide to upgrade your actuator, there are two different base boost pressures available from Turbosmart. We recommend the lower, 7psi version.
Granted, we are forcing the stock turbo to work really hard. But despite this hard work, there
are gains to be had. You can see in the airflow data below that the Turbosmart Actuator
outflows both the stock and preloaded stock actuator. You can also see that the preloaded
stock actuator outflows the stock untouched actuator. This increase in airflow correlates to an increase in power as can be seen in this comparison between the three cars dyno plots.
In full disclosure, we feel you should not do this modification if you do not have a sufficient intercooler upgrade on your Focus ST. Pushing a small turbo this hard heats up the compressed air significantly. It's critical to bring down the intake temperatures as much as possible.
The other risk you're exposing the car to is premature turbo failure. It is fairly risky to push the turbo like this as it's simply not designed to spin this fast or produce this much boost at that turbo speed long term.
This post was originally published in September, 2019 but has been updated to reflect current parts and accurate information!