Let’s first go over how the wastegate and actuator work. The purpose of the wastegate is to maintain a particular boost pressure at the intake manifold by regulating the speed at which the turbo spins. It does this by regulating a pathway/door which diverts exhaust gases AROUND the turbine. The faster the turbine spins, the more boost the turbo generates.

The more exhaust you divert through the wastegate (the more you open the wasetgate door), the more you slow the turbo down and this is how you lower boost.

This wastegate is pushed open or held closed by the wastegate actuator and a number of other variables such as exhaust pressure. The actuator is a boost actuated diaphragm inside a canister with a rod sticking out as shown below. Boost pressure is applied to the canister and once the boost is higher than the spring pressure inside the canister, the wastegate actuator rod starts to move and open the wastegate.

The actuator doesn’t open the wastegate fully at a given single pressure. It starts to open it at a cracking pressure (also known as spring pressure) and then the more pressure you apply to the canister the more it opens the wastegate door. The amount of boost pressure this canister sees is regulated using the boost control solenoid which is controlled by the ECU and the tune running on the motor.

In spite of the ECU controlling boost pressure going to the actuator, the amount of boost a turbo can hold at any engine speed (RPM) is determined by a large number of factors. Some of these are the turbo itself, the engine displacement, the flow parts installed, the ambient temperature and more importantly for this discussion the spring inside the wastegate actuator and the preload on this spring. On OEM wastegate actuators we can’t change the spring inside the canister because they are sealed. On aftermarket actuators and external wastegates we can change the internal spring. Therefore, the only aspect that we can change on the OEM actuator is preload. Shortening the wastegate rod using the adjustment screws adds preload to the internal spring. The reverse removes preload. When you increase the preload you do two things. One is that you increase the boost pressure needed to start opening the wastegate. The second is that you  lower the opening range of the wastegate door.

Increasing preload or changing the actuator with a higher spring pressure model (such as a Turbosmart pictured below) results in more boost! And that’s a good thing – right?

Well – the answer is a little more complicated.

First of all, keep in mind that making these kind of changes almost always requires that you get your tune adjusted. Reliable boost control is extremely important for the reliability of the engine. Secondly, you can introduce boost response issues such as boost spiking and boost creep. Most OEM turbos won’t suffer from these issues but it’s very important to make sure the boost of your vehicle reacts smoothly and is under control after you make any changes to the system.

Finally you have to make sure that more boost actually results in more power. When building and tuning a car, you have to look at the engine as a package. Changing boost will affect other variables such as fuel requirements, cooling requirements, and whether or not everything is still working efficiently.

A Turbosmart actuator on a Focus ST with the OEM K03 turbo will hold over 20psi to redline for example. The OEM actuator will only allow 13-15psi. In experimenting with this we found that holding more boost to redline makes the car quite knock prone even when using E85 mixes and mild timing. The turbo looked to be well out of its efficiency range and we kept dropping the boost until we were at 18psi at redline. Even then the motor was knock prone especially up top. Increasing the boost on the OE turbo in the upper RPM (past 6000RPM) is not worth the penalty in heat and timing reduction especially on pump gas.

Another interesting bit of information was that without modifying the fuelling tables the short term fuel trims (STFT) became quite negative in the upper RPM (in orange in the graph below). The car calculates fueling based on boost pressure/manifold pressure and this drop in STFT indicated that for the boost measured the airflow was actually less than the ECU was calculating and expecting.

This is another indication of the lack of efficiency of the K03 at those boost levels. The higher we pushed it in terms of boost, the more negative the trims.

The Virtual Dyno plot below shows a similar story. This was on E85, stock actuator (red) vs Turbosmart (blue). There is a big difference in boost which results in a nice midrange increase in torque. However look at 6000 RPM – there the increase in boost makes no difference in power and we are probably driving the OEM turbo much too hard and reducing reliability. We had to reduce timing by around 6 degrees up top to keep the car from knocking even on 3.5 gallons of E85 mixed in the tank. Things got toasty and inefficient.

Conclusion on this? A little bit of extra boost is going to give you a nice bump in midrange torque. Adjusting the preload on the OEM actuator on the Focus ST carefully can give you a little bit more boost, but don’t overdo it. If you modify the boost control system and are not sure of what to look for make sure you have your tuner look over the data coming from the engine after the change. We offer the datalog report service for situations such as these.

Keep a very close eye on knock/timing corrections. This is especially true on pump gas as more boost (especially past 6000RPM) results in more knock. If you have poor pump fuel this may not be worth the penalty you will take in terms of timing and heat/inefficiency. Finally, driving the OEM turbo (or any turbo) to inefficient boost levels is likely to affects its long term reliability.

Drag racing is something a lot of the ST community enjoys so here are some of our findings in drag racing the ST.

We didn’t take the car drag racing when it was stock or close to stock. At our local track a stock ST runs in the 14s on street tires. You can take a stock ST and drag race it all day long without worrying about breaking things or being overly hard on the car. Seat time will improve times more than anything. At the same time keep in mind that the low profile tires on the ST are made for cornering and not drag racing. You will get lots of wheel hop and 60 foot times will be high. This doesn’t result in the best overall quarter mile times.

As you increase the power, you will see incremental improvements especially when coupled with seat time and traction! Keeping the stock K03, a car with E85 or WMI and all bolt ons and a good tune is capable of low 13s at around 105-107mph with the right tires. For tires you can either get a drag radial like the Hoosier Drag Radial DOT tire or something even more aggressive like a Quick Time Pro.

The Drag Radials are sized the same as normal street tires. A common size is the P245/45R17 (https://www.hoosiertire.com/specdrag.htm#DOT Drag Radial). These need to be fitted to a set of 17″ rims. I recommend getting the smallest diameter wheel possible (on the later ’14s and ’15s that’s a 17 rim to fit the brakes) Remember that in drag racing the most important part of your run is the launch and 60 foot. This is why people talk about it so much. Whatever you save on your 60 foot will double by the end of the quarter mile. For example if your 60 foot is 2.2 seconds and your quarter mile is 14.0; if you cut that 60 foot to 2.0 seconds your quarter mile time will drop to 13.6.

This is why drag tires are so important. If you have lots of power and can’t put it down, you won’t have an ET that is representative of that power level. Most people on street tires will have a 60 foot of 2.4-2.2 seconds. On drag radials you can consistently be in the 2.0-1.9 range. Your trap speed (if you don’t miss shifts) is representative of your power level.

Jeremy, one of the guys that works for us from Memphis uses drag radials and here is one of his runs. He has a lot of track experience and consistent times. He is running a GTX2867 and 2 gallons of E85 in this run.

For my car, I went with a more aggressive tire – a Hoosier QTP slick (https://www.hoosiertire.com/specdrag.htm#QUICK TIME PRO). The size is 26 x 9.5 x 16. It is a little tall even for 400whp. It hooks up very well and I run 12-14psi of pressure in the tire. Since it is on a 16″ rim there is a lot of sidewall flex meaning it can absorb the impact of a launch quite well and not slip. This tire should be able to get well into the 1.8-1.6 60 foot range depending on how aggressive you launch.

When we took the car to the track the first time, it was already on the big turbo (ATP GTX2867), Stratified aux fuel, etc. It was making around 380 ft lbs and 400whp but flat foot shifting was not working as it should have been. With a launch around 4000 RPM, managed a 12.6 second run.

Knowing there is more in it, sorted out some of the FFS issues, increased torque and went back. While the car was making a solid 400 ft lbs+ of torque the clutch was still stock with 23000 miles on it.

On the first pass at this power level, the clutch finally couldn’t hold the torque.

The shock delivered by the 5000 RPM launch overwhelmed the OEM clutch which is not surprising. Later that day once the car cooled off, the clutch would again hold fine on street tires. So on slicks, at 400+ ft lbs of torque launching can overwhelm the clutch. Otherwise and so far everything has been holding together great. We have an upgraded COBB rear motor mount as well as Bilstein B14 coilovers installed which help keep the car and engine planted while still being very civil in daily driving.

What’s next? A Spec Stg3+ clutch (sticking with the OEM dual mass flywheel) and a Quaife LSD (this is more so for auto-x). The engine has been very reliable so far even with the abuse of R&D as well as daily driving the car which is great to see. While the ST is better suited for going around turns, you can certainly have a lot of fun with it at the drag strip. So get out there and race your ST, you won’t regret it!