Tuning the Subaru FB:
FB16E FB16F FB20B FB20D FB20X FRB20V FB25B & FB25D

"All you need to know about tuning the Subaru FB engine engine!"

The Subaru FB engine great bases for a tuning project and with carefully chosen motorsport parts like a remap, turbo upgrades and camshafts you will substantially improve your driving experience.

This pages aim is to examine the world of the options for your FB engine tuning and summarise the ultimate modifications.

We consider FB engine tuning and report on the best upgrades. Subaru FB engine are popular blocks and with the optimum performance upgrades like remapping, turbo upgrades and camshafts you will maximize your driving opportunities.

History, Power & Specs of the Engine

The FB was a NASP boxer engine with AVCS - Active Valve Control System overall giving a wider torque band than previous engine versions. By using lighter pistons and con rods, there is less noise and friction and the engine is more efficient.


  • 1597 cc DOHC, 2012+ EUDM Subaru Impreza XV 1.6i


  • 1599 cc DOHC, turbo, 2014+ Subaru Levorg


  • 1995 cc DOHC, 2011+ JDM Subaru Forester, 2012-2016 Subaru Impreza, and 2012-2017 Subaru XV


  • 1995 cc DOHC, 2017+ Subaru Impreza, and 2018+ Subaru XV


  • 1995 cc DOHC, 2014–16 Subaru XV Hybrid


  • 1995 cc DOHC, 2019+ Subaru XV Hybrid


  • 2498 cc DOHC, 2011–18 North American Subaru Forester, 2012-2019 North American Subaru Legacy, and 2012-2019 North American Subaru Outback


  • 2498 cc DOHC, 2019+ North American Subaru Forester, 2020+ North American Subaru Legacy, and 2020+ North American Subaru Outback

Tuning the Subaru FB engine and best FB engine performance parts.

Best FB engine parts

Just because particular tuning mods are are common on FB engine it doesn't mean it is good, we shall best tuning mods that will give your FB engine the biggest power gain return for your cash.

Altering your FB engine camshaft will make a dramatic difference to the engine power band. Choosing a higher performance camshaft profile raises the power band accordingly.

Fast road cams commonly increase the power throughout the rev band, you may lose a little low down bhp but your higher rpm power will be higher.

Motorsport cams, increase the higher rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

A Motorsport cam is not great on the daily commute, because the lumpy idle will make the car prone to stall and smooth driving at low rpm becomes impossible. If you are developing a track car this doesn't matter as you are in the high end of your RPM range anyway and that is where you want the power to be.

You should ideally optimize your torque band to your driving style so for a car driven daily stick with a fast road FB engine cam

Different FB engine engines respond better to mild cam durations than others.

The engine timing and injectors and fuel pump also have a large bearing on the bhp gains you'll achieve.

Extending exhaust or intake durations can alter the bhp band and on most engines the exhaust and intake durations do not need to match, although most cams and tuners use matched pairs there are some advantages to extending the intake or exhaust durations.

Please watch our video which covers the 5 principles of tuning your car. Be sure to keep up with our latest YouTube content and subscribe.

Best Engine Mods for your car

  1. Engine Tunes - engine tuning/remapping provides the most advantage in terms of cost savings,  aftermarket ECUs, and piggyback ECUs are all alternatives.
  2. Fast road cams are one of the most significant mechanical changes, but they must be installed by someone who knows what they're doing and they are not always easy to source but you might find a local firm to regrind a stock camshaft.
  3. Intake and Exhaust - Note that on their own these mods will NOT ADD POWER in most cases, but they can help enhance power after other mods by removing the restriction.
  4. Upgrades to turbochargers and superchargers - forced induction is the most efficient approach to increase air supply, allowing you to burn more fuel and make more power. It is one of the most costly upgrades but provides the best gains.
  5. Head work - The goals of porting and flowing the head are to get air flowing into the engine while removing flow restrictions and turbulence.

Typical stage 1 mods often include:
Sports exhaust header/manifold, Remaps/piggy back ECU, Panel air filters, Intake manifolds, drilled & smoothed airbox, Fast road camshaft.

Typical stage 2 mods often include:
Sports catalyst & performance exhaust, Ported and polished head, fuel pump upgrades, high flow fuel injectors, induction kit, Fast road cam.

Typical stage 3 mods often include:
Internal engine upgrades (head flowing porting/bigger valves), Crank and Piston upgrades to alter compression, Twin charging conversions, Adding or Upgrading forced induction (turbo/supercharger), Engine balancing & blueprinting, Competition cam.

Carefully think through your options and then source your parts and set yourself a power target to avoid wasting your time and money.

Mapping helps unlock the full potential of all the modifications you've fitted to your FB engine.

(In some cases, as the factory ECU is locked flashing is not an option, so an aftermarket ECU is the route to take, and many of these will outperform factory ECU's but make sure it has knock protection and that you get it setup properly.)

It will usually give you around 30% more power on turbocharged vehicles and you can expect to see around 15% on NA (naturally aspirated) engines, but your mileage may rely on the modifications you've done and the condition of your engine.

It is the whole point to any engine tuning job to pull more fuel and air into the FB engine engine

The intake plenum take the air from the filter and allow it to be sucked into the engine cylinders.

The shape and flow rate of the Intake manifold can make a large effect on to fuel mixing and power on the FB engine.

We often see intake manifold are crying out for performance upgrades, although a few makers provide reasonably well designed intake manifold.

Larger FB engine valves, doing some port matching and head flowing will also raise performance, and significantly will raise potential for increasing the performance increase on other mods.

Turbo upgrades

NA (naturally aspirated) engines need quite a lot of work when you add a turbo, so we have a separate guide to help you take into account the pros and cons of going this route on your FB engine

The more air you can get into an engine, the more fuel it can burn and uprating the induction with a turbocharger upgrade makes impressive power gains.

If a car is fitted with a turbo tuning mods are simpler to install and you'll see that turbo charged engines will have stronger components.

There are practical limits for every engine, with some being extremely strong and some just sufficiently able to handle stock power

We recommend you find these limitations and fit stronger pistons, crank and engine components to survive the power.

We've seen guys spending a loads on turbo upgrades on the FB engine only to suffer the humiliation of seeing the car catastrophically fail on it's first outing after it's been completed.

Large capacity turbochargers will usually experience low end lag, and small turbochargers spool up more quickly but won't have the peak rpm power band gains.

In recent times the selection of turbochargers is always developing and we commonly find variable vane turbochargers, permitting the vane angle is altered according to speed to lower lag and increase top end bhp and torque.

Twin scroll turbochargers divert the exhaust gases into 2 channels and feed these at differently profiled vanes in the turbo charger. They also help the scavenging effect of the engine.

It is not unusual that there's a limit in the air flow sensor MAF/MAP on the FB engine when considerably more air is being fed into the engine.

We note 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor limited power at a much lower level.

Adding a supercharger or additional turbo will make large power gains, although more complex to setup. We have this feature on twinchargers if you want to read more.


When you increase the performance you will need to ramp up to the fuelling.

More performance needs more fuel. We strongly recommend you to be generous with your injectors flow rate.

As a rule of thumb add another 20% when fitting an injector, which takes into account injector deterioration and gives a little spare capacity should the engine require more fuel.

We think this one is common sense, but you'll need to match your fuel injector to the type of fuel your car uses as well.

All the following flywheel power targets will assume an injector duty cycle of 80% and a base of 58psi of fuel pressure at idle.

4 Cylinder turbocharged engines

  • 58 PSI 340cc/min 200hp
  • 58 PSI 511cc/min 300hp

4 Cylinder NA (naturally aspirated) engines

  • 58 PSI 285cc/min 200hp
  • 58 PSI 426cc/min 300hp

4 Cylinder supercharged engines

  • 58 PSI 312cc/min 200hp
  • 58 PSI 468cc/min 300hp


Only look to replace your exhaust if your exhaust is actually creating a restriction.

On most factory exhausts you'll see your flow rate is fine even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust.

Don't go with the largest exhaust you can source you'll slow up the exhaust flow rate - the best for power gains are usually between 1.5 to 2.5 inches. It is the shape and material more than the bore size.

Common exhaust restrictions are in the catalysts installed, so adding a faster flowing race alternative such as a sports catalyst pretty much removes this restriction, thanks to it's larger size and surface area, and will effectively raise the performance to levels you would expect without having a catalyst installed, but keeps the car road legal.

Weak spots, Issues & problem areas on the FB16E FB16F FB20B FB20D FB20X FRB20V FB25B & FB25D

The FB engine engines are generally reliable and solid units, as long as you follow the manufacturers service schedules, and use a good quality oil to ensure longevity. Few problems should happen as long as they are regularly serviced and maintained.

Carbon build up in the head, particularly around the valves which will sap power or create flat spots, this is a larger issue on direct injection engines but should be looked out for on all engines. We have tips on removing carbon build up.

Some of our members have had issues with flat spots or glitches after applying mods and upgrades or tuning, this is not usually related to this engines design, so instead see our article on diagnosing flat spots and problems after tuning which should help you get the bottom of this issue.

Regular oil changes are vital on the FB engine, especially when tuned and will help extend the life and reliability of the engine.

If you would like to know more, or just get some friendly advice on Tuning your FB engine engine please join us in our friendly forum where you can discuss tuning options in more detail with our FB engine owners. It would also be worth reading our unbiased tuning articles to get a full grasp of the benefits and drawbacks of each modification.

Please help us improve these tips by sending us your feedback in the comments box below.

We love to hear what our visitors have got up to and which upgrades work best for you on your car. Which helps us keep our guides and tips up to date helping others with their modified car projects. Your feedback and comments are used to keep this page up to date, and help improve the accuracy of these FB engine tuning guides which get regular updates and revisions.

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2 Responses to “FB Tuning”

  1. John Davies-Cameron says:

    Excellent write up!! I was just looking to see if there were more interchangable parts and stumble on this and just kept reading! thnk you!

  2. Justin says:

    I own a 2021 Subaru Crosstrek Limited with 2.5 engine. Mods include Crawford Billet Power Block, Takeda CAI, Nameless Axle Back Exhaust and it’s been tuned.

    I am trying to find more performance parts but it seems like there’s very little available.

    Any tips on where I can find performance parts?

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