Modding tuning and best performance upgrades on the Subaru FB engine!

Tuners tips on "FB Modifications"

FB Modifications Guide

How to modify and upgrade the Subaru FB

"Comprehensive guide to tuning the Subaru FB engine!"

Popular tuning sites like TorqueCars are regularly getting posts about how to improve the FB from people inquiring about getting more power from the FB and wanting to know what are the greatest FB mods that work. So let us look into Subaru FB upgrades and outline the best mods that work on this great engine and point out some common pitfalls along the way.

The Subaru FB make a good tuning project and with the optimum motorsport enhancements like a remap, turbo kits and camshafts you will greatly maximize your driving pleasure.

In this article we look at FB tuning and point out the premier modifications for your car.

Just because particular mods are appear in lots of FB projects it doesn't mean you should fit it, instead we'll concentrate on the optimum mods that will give your FB the biggest power gain return for your cash.

Without us subscribing to the popular myth of 'if it's shiny and makes more noise it must be good' you seem to get in many tuning magazines and on car forums'.

FB induction kits

Improving air intake on the FB

You need air matched to fuel for an engine to work. If there is a lack of fuel then it will run lean, if there is a lack of air it runs rich losing power which risks damage to the FB. We'll pick up on fuelling later in this article but for now we shall look at air supply.

So Getting air and fuel into your FB is vital to any engine upgrade task.

It is not unusual that there is a limit in the air flow sensor (AFM/MAF/MAP) on the FB when a lot more air is being fed into the engine.

We see 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor was restricting performance at a much lower level.

Air Intake manifolds flow the during the engines suck phase from the air filter and allow it to be fed into the engine cylinders.

The size of bore and shape and flow rate of the Intake headers can make a large effect on to fuel delivery on the FB.

Commonly we find the intake are ripe for aftermarket tuning parts, although a few car makers provide fairly well optimized intake.

Increasing the FB valve size, getting FB port enlargement and head flowing will also raise bhp, & more importantly will raise potential for a greater bhp increase on other mods.

FB performance cam tips.

Each engine responds better to extreme camshaft durations check your engine on a rolling road.

The ECU mapping and fuel pump and injectors also will make differences on the power gains you'll achieve.

Altering valve durations can alter the power 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.

The cam profile plays a big part in the engines power output so cam upgrades make quite a large difference. The intake and exhaust durations will alter depending on the chosen cam profile, so large power band gains are on offer for cam upgrades.

NB: Fast road camshafts normally increase the performance over the rev band, you could sacrifice a little low down torque but the higher rpm power will be lifted.

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

A Race cam will just annoy you whilst driving around busy urban areas.

You should ideally optimize your power band to your usage of the car so for a car used daily stick with a shorter duration FB cam

Stage 1, 2 and 3 mods for the FB

Before we break the best tuning upgrades down into stages of tune, let's list the most effective tuning upgrades for you.

  1. Suspension Upgrades - always improve your cars handling
  2. Brake Upgrades - Even before adding power improve those brakes
  3. Intake - make sure the intake is not restricted, use filters and intake upgrades to improve this
  4. Exhaust - as with 3 ensure there is not restriction in the exhaust, cats are the usual bottleneck
  5. Tunes - remapping, piggyback ECU's and aftermarket ECU's can all make decent gains
  6. Fuelling - when you've increased power you'll need to match this with more fuel
  7. Turbo upgrades - Improving the intake with a large turbo and better flowing intercooler will be the biggest power gain you'll see (but one of the most complex).

This video guide to tuning cars is a great starting point to work from in your project.

Stage 1 FB parts:

Sports exhaust manifold, Fast road camshaft, Intake headers, Remaps/piggy back ECU, Panel air filters, Drilled & smoothed airbox.

Stage 2 FB parts:

induction kit, Ported and polished head, fuel pump upgrades, Fast road cam, Sports catalyst & performance exhaust, high flow fuel injectors.

Tuning tips and articles Engine tuning Transmission tuning Care care Intake & exhaust mods Improve handling Forums

Stage 3 FB parts:

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

The FB engine blocks respond well to mods and thanks to their popularity there are increasing numbers of mods and performance parts out there.

A remap helps fully realize the full potential of all the parts you've done to your FB.

You will typically expect to see gains of approximately 20-30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but figures achieved often vary depending on the parts you've carried out and the condition of your engine.

Turbo modifications on the FB

To get a turbo working on a NASP engine there is a lot involved, from lowering compression ratio to improved fuel supply and mapping, so we have found a great guide to helpavoid the common pitfalls or end up with an unfinished project because you were not told what you were getting into by doing this on the FB.

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

When a car has a turbo already fitted mods are going to make more power and turbo charged engines already contain better components.

There are practical limits for every engine, with some being incredibly solid and some only able to handle stock power

See where you'll find these limits and install more solid crank and pistons to survive the power.

It's not unheard of people spending a lot of money on turbo upgrades on the FB only to watch the engine throw a rod on it's first outing after it's finished.

Larger capacity turbo chargers will usually suffer no power at low rpm, and smaller turbo chargers spool up quickly but won't have the peak rpm engines power gains.

Over the last 20 years the choice 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 power.

Twin scroll turbochargers divert the exhaust flow into two channels and push these at differently profiled vanes in the turbocharger. They also boost the scavenging effect of the engine.

Beefing up the air intake by adding a supercharger or additional turbo will help you achieve very large torque gains, although more challenging to get working. We have this feature on twinchargers if you want to read more.

Tips about fuel supply on the FB

You will need to ensure that the engine is not starved of fuel so will have to increase the fuelling when you start exceeding 20% of a performance hike. At this point we flag up the need to over specify your injector capacity.

The accepted safe increase is to add 15% when fitting an injector, helps cope with injector deterioration and provides a bit of spare capacity should the engine need more fuel.

Don't forget that different fuel grades will usually require different settings, a high octane fuel will burn more efficiently than lower octane fuel for example. A car will trim fuel supply to keep within the ideal air fuel mix but at the extreme ends you may need to adjust your injector capacity, or the mapping or you'll get flat spots and potential lean or rich fuel issues..

Recommended FB performance exhaust upgrades

Only look to increase your exhaust system with an upgraded one is if the current exhaust is creating a restriction in flow.

On most factory exhausts you should find that your flow rate is still good even on modest power gains, but on a serious tuning project with a big boost in power you will certainly need to get a better flowing exhaust.

Do not go with the biggest exhaust you can get you'll reduce the exhaust flow rate - the best for power gains are usually between one and a half to two and half inches. Your exhaust design, the bend angles and construction are what have more effect on flow rates than your pipe width to a certain extent. So the exhaust design is a big factor.

Usual exhaust restrictions are traced to the filters installed, so adding a higher flowing sports alternative will help avoid this restriction.

Catalyst removal (NB:catalysts reduce harmful engine emissions) is illegal in most countries and regions.

Faults issues and things to look out for on the FB

Like most engines an FB, should have few issues if it gets properly serviced and looked after.

We cannot stress enough the need for oil changes with the correct grade of oil on the FB, failure to do this will exacerbate engine wear, on a tuned FB this is more important than ever and close attention should be paid to your maintenance schedule.

If you are interested in learning more or getting impartial tuning advice for your FB why not pop over to the forums at TorqueCars where you can chat about FB tuning options and read about others projects who may be planning to do similar modifications on their FB.

We need your help to complete and improve this page, so do give us your feedback in the comments box below.

We want to know your experiences with parts on your FB, it helps us improve our tips and keeps us current so please use the comments box to drop a tip.

If you liked this page please share it with your friends, drop a link to it in your favourite forum or use the bookmarking options to save it to your social media profile.

Check out TorqueCars new YouTube channel, and see their awesome new content...


Please use our forums if you wish to ask a tuning question, and please note we do not sell parts or services, we are just an online magazine.

Help us improve, leave a suggestion or tip

Your Constructive comments on this article