Modifying a Focus ST (Part 1)

Author: Jon Walker

I often get asked a question such as "What is the first mod I should buy for my Focus ST?" The answer is usually, "It depends (on plans, budget, likes/dislikes, etc.)" but there are a few staples that I can say are well worth doing regardless of what your plans are for the car. I’m planning on doing this over a few articles as I want to get into the technical aspects/science behind the upgrades (some data if I have it readily available). On top of that I will give my own recommendations on which products to use based on my own personal experiences, and the experiences of Focus ST builds I've helped with, or have innate knowledge about.
I'm also going to be discussing performance upgrades and not visual upgrades. If you would like to modify the visual aspects of the car, I believe your own personal taste trumps all. Don't let other people tell you what looks the best. All that matters is that you enjoy your car. Do I agree with modifications that hurt performance? No. But just because it’s not my taste, doesn’t mean you shouldn’t do it. Just don’t expect me to comment on how awesome it is.

I've always said the best approach to modifying a Focus ST is to think long term for your car. What is your end goal (amount of power and/or use for the car)? Try to develop a parts list and work from there keeping an eye for high quality second hand parts and good deals. For example, if you think that at some point you will get a big turbo, why purchase something that you will have to replace when you finally do add that large snail? It doesn't matter if its years in the future Those parts will be hugely beneficial to your car in the meantime with very few (if any) drawbacks. Will they be optimal for the stock K03 turbo, perhaps not. However it will be once you've finally upgraded to that high powered turbo and that's all that matters. When building a higher horsepower car, doing all the little things right goes a long way in keeping your car on the road. I learned the lesson of planning ahead the hard way. I’ve purchased and installed multiple exhaust setups, intakes and FMICs. I lost money each time I had to resell something. Trust me, you will too.

#1: Keep the temps down with an upgraded Front-Mounted Intercooler (FMIC)

If you follow my build thread at all (can be found over on It’s the Practical Enthusiast Build) you may notice I didn't purchase an upgraded front mount first. In fact, I purchased quite a few things before the FMIC (APV3, Springs, BOV, Exhaust (x2), Downpipe, Intake (x2), Snorkel, Wheels, Shifter Bracket, RMM). The reason I didn’t get an FMIC first is two fold. One: my own ignorance. I didn’t fully realize how terrible the stock FMIC is. Two: I started my build on what I liked, and what I could find good deals for. I didn’t think long term. I always thought that I might add a larger turbo in a few years but didn’t care what I needed in order to optimally run one and that logic is evidenced by my parts purchases.
When I finally realized just how terrible the OEM FMIC is, I couldn’t wait any longer to buy one. One muggy summer afternoon on my commute home, I noticed on my Cobb Accessport V3 (APV3) that my charge temps were significantly higher than I realized. The ambient temperature that day was in the mid 90s (*F) and it was humid. I was stuck in stop and go traffic and my charge temps were creeping higher and higher and eventually broke 200*F. That’s not a typo… TWO HUNDRED DEGREES! Thats 100+ degrees over ambient, and that’s bad. Yes this is stuck in traffic, not on a track, but I have seen data from tracked Focus STs with the OEM FMIC and it’s not a pretty picture.
It’s pretty well understood that higher air temperatures mean lower peak horsepower/torque. However, if you're a geek like me, the science behind that is a bit more interesting. When an intercooler can’t keep charge temps in a safe operating range, the air/fuel mixture in the cylinders is exposed to enough heat and pressure that it causes knock. Knock is essentially a uncontrolled detonation of a pocket of air/fuel mix in a cylinder. If this detonation is allowed to continue under extreme conditions (wide open throttle pulls for example which we all do a lot of when tuning the car) or over an extended period of time it can lead to damaged or completely destroyed engine parts.
Now that I know all of this information, I firmly believe upgrading the FMIC on the Focus ST is one of, if not the, first modification you should make. As for which FMIC to go with, it depends. If you are going to keep the car mostly stock and not push it much, almost any aftermarket FMIC will help. (There are a few that I’ve seen lots of people have issues with that I personally wouldn’t choose, mainly the DIY ones as, while they do help, I’ve seen charge temps still 50* over ambient). If you are going to push the limits of the OEM K03 turbo, but not go beyond that and upgrade the turbo, then I’d recommend the the 3.5” core FMICs. Most of them are reasonably priced and do very very well on the stock turbo. Once a bigger turbo is installed they will still do well, but the larger core FMICs (5.5” cores) are the most efficient you can install. I started with one of the 3.5” cores and on the stock turbo saw no more than 10*-20* increase in ambient temperatures even on a 3rd through 5th gear pull. However, once I decided to get a big turbo, I kept an eye out for a good deal on the 5.5” core FMIC and snagged one. Now, even with the GTX2867R installed on my car charge temps stay no more than 3* above ambient. Below is a chart from one of my pulls from tuning my GTX2867R turbo with the 5.5” core FMIC and a full silicone intake (which I will discuss in a later article)

Notice how over the pull charge temps actually drop from 66*F to ambient (63* F). That is what I call efficient and that is why I believe the 5.5” FMIC is the best route to go.

In the next article I’ll discuss what I believe should be the 2nd and 3rd upgrades for the Focus ST, an Accessport Tuner, and a Rear Motor Mount.