Tuning the Subaru FB16 / FB20 / FB25

"Comprehensive guide to tuning and performance parts on the Subaru FB16 / FB20 / FB25 engine!"

Herein we review FB16 / FB20 / FB25 tuning and highlight the greatest upgrades. Subaru FB16 / FB20 / FB25 are good project engines and with a few sensible modified upgrades like ECU maps, turbo improvements and camshafts you will positively improve your driving experience.

Just because particular modifications are are common on FB16 / FB20 / FB25 it doesn't mean its worth having, so we'll best modifications that will give your FB16 / FB20 / FB25 the best power gain for you money.

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

NB: Fast road camshafts normally increase the bhp and torque across the rpm range, you might lose a little low end bhp but top end will be better.

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

In a road car must carefully try to match your torque band to your driving style.

You'll never find a FB16 / FB20 / FB25 Race cam is a pleasure to live with when driving in heavy traffic.

Different FB16 / FB20 / FB25 engines respond better to different camshaft durations so set your engine up on a rolling road.

The engine timing and injectors and fuel pump also have an effect on the bhp gains you'll get.

Longer valve 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.

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Stages of Tune

Stage 1 mods: Remaps/piggy back ECU, Panel air filters, Intake headers, Sports exhaust manifold, Fast road camshaft, Drilled & smoothed airbox.

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

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

A remap helps release the full potential of all the parts you've fitted to your FB16 / FB20 / FB25.

It will usually give around 30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but your mileage usually rely on the parts you've applied and the condition of your engine.

Forcing more air and fuel into the FB16 / FB20 / FB25 engine is the aim to any car tuning task.

Headers carry the air from the filter and allow it to be drawn into the engine cylinders with fuel for the squish phase.

The size of bore and shape and flow rate of the Intake headers can make a substantial effect on to fuel mixing and power on the FB16 / FB20 / FB25.

I usually find intake headers are in dire need of aftermarket tuning parts, although some makers provide well optimised intake headers.

Big valve conversions on the FB16 / FB20 / FB25, doing some port work and head flowing will also boost bhp, and as an added benefit will permit a greater bhp increase on other parts.

Turbo upgrades

NASP 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 FB16 / FB20 / FB25

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.

When the engine is fitted with a turbocharger parts are going to make more power and you'll see that turbo engines are built with many forged and stronger components.

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

Research these restrictions and fit more solid crank and pistons to survive the power.

We've seen drivers spending a loads on turbo charger upgrades on the FB16 / FB20 / FB25 only to suffer the humiliation of seeing the whole thing explode on it's first outing after it's been completed.

Big capacity turbo units tend to suffer low end lag, and smaller turbo units spool up quickly but do not have the peak end bhp gains.

In the last 10 years the range of turbo chargers is always increasing and we are seeing variable vane turbo chargers, where the vane profile is altered according to speed to lower lag and increase top end torque.

Twin scroll turbo chargers divert the exhaust gases into 2 channels and direct these at differently angled vanes in the turbocharger. They also improve the scavenging effect of the engine.

You'll commonly see there's a restriction in the air flow sensor AFM/MAP on the FB16 / FB20 / FB25 when considerably more air is being sucked into the engine.

Going up you'll find 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor sapped power at a much lower level.

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

Fuelling upgrades and mods

You will need to ensure that the engine is not starved of fuel so should uprate the fuelling when you start extending past 20% of a performance increase.It makes sense to be generous with your flow rate on the injectors.

As a rule of thumb add 20% capacity when fitting an injector, this takes into account injector deterioration and allows some spare capacity should the engine need 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.

Exhaust upgrades and mods

You only need to to replace your exhaust if the current exhaust is actually creating a flow problem.

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

Sports exhausts will certainly help air flow from the engine but avoid an exhaust that is too big or you could will reduce the flow rate. Stick to 1.5 to 2.5 inches for best results.

Usual exhaust restrictions typically happen through the catalyst and or DPF filters which manufacturers fit, especially as they start to get worn and carbon builds up, so adding a higher flowing high performance alternative will help avoid this restriction.

Weak spots, Issues & common problem areas on the FB16 / FB20 / FB25

The FB engine engines, if regularly serviced and maintained, are generally very reliable and have few issues.

Regular oil changes are vital on the FB engine, particularly when the engine has been modified and is putting down more power than the manufacturer intended.

For more information 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.

We need your help improving this article, so please send us your feedback in the comments box below and pass on any tips, points or facts we have wrong or have not covered.

We really like hearing from our readers, and hearing about which upgrades were the most effective for them, it helps us improve our recommendations and articles to reflect current trends in modifications and ensures that our FB engine guides and tips are kept up to date.

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This article is flagged for an update, so please pass on specific tips below that I can include. All articles, and content was written by me, Waynne Smith TorqueCars founder. Some outlines are just too general so I appreciate your feedback and suggestions. This entry was filed under Subaru. You can leave a response below or join our forum to discuss this article and car modification in detail with our members.

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