Performance Modifications maintaining the dual nature role of the Forester just enhance the Forester's attributes without limiting any others by increasing the engine and suspension performance without giving up any ground clearance.
Suspension modifications such as a rear anti-sway bar (18mm) and Strut bars (front and rear) are also good, maintaining a factory ride and height but improving cornering. Engine Performance Upgrades can start with a Performance intake filter (such as K&N's filter charger, TRi Tuning's performance intake, or iSR Performance intake), an under drive performance pulley, exhaust systems (such as Strömung), and a header (currently only offered by Borla), are a good start for enhancing horsepower. If that's still not enough, brute force can be added with a Forced Induction setup.
Suspension Upgrades would include a rear anti-sway bar (18mm-22mm) and Strut bars (front and rear), as well as a good aftermarket spring and strut or coilover system upgrade.
Off the showroom floor, the Forester is designed to have moderate understeer. Understeer is the condition where you turn the steering wheel but the car continues to go straight, refusing to turn in sending you possibly off the road or into a curb. To help remedy this there are larger rear sway bars that upgrade the tiny stock bar to a thick 18mm, 20mm or 21mm bar.
The difference between the 20mm and 21mm is minimal with the 21mm giving you more oversteer compared to the 20mm, while the 18mm equals the front and keeps the vehicle more neutral. Sway bars, also known as anti-roll bars, reduce the car's tendency to lean over in turns yet do not degrade ride quality.
With the larger rear bar you can more easily induce off-throttle oversteer. This means that when in a turn and the car begins to understeer, you can simply ease off the throttle for a split second and the front of the car will begin to point in towards the apex of the corner. Once you're back on your racing line, ease back into the throttle to power through the turn. On steady state, high speed cornering, the larger rear bar keeps the car more neutral, giving you more control and speed through the corner. Installation only takes a few minutes with simple hand tools.
As your suspension works, the tops of the strut towers flex towards and away from each other reducing handling. To eliminate this, use a strut tower bar which simply bolts to the tops of the strut towers and braces them against any unwanted movement. This modification requires no drilling or permanent modification to the car and can be performed with ordinary hand tools in just a few minutes. Several manufacturers offer front strut bars, such as Cusco, STi, SYMS, and JC Sports.
As with the front strut towers, the rear strut towers also suffer from flexing under load. Due to the fact that the rear strut towers are not located near any major chassis structure, they tend to flex even more than the front therefore installing a rear strut tower brace can make a very noticeable improvement even when using the stock suspension. The bar runs across the rear cargo area directly behind the rear seats. Simply remove the two bolts that hold the bar in place if you never need to temporarily reclaim the extra trunk/cargo space. Rear strut bars are offered by Cusco, STi, and TRi Tuning.
ENGINE PERFORMANCE UPGRADES
Designed to exactly replace the restrictive factory air filter, the K&N performance filter is constructed with a pleated cotton gauze which improves air flow while maintaining a high level of filtration. Another unique feature of this filter is the ability to clean and re-use it by simply using the K&N cleaning chemicals and oil offered in their Re-charger kit. K&N even offers their famous 1,000,000 mile, 10 year warranty with this factory replacement filter, which means it quite possibly will be the last air filter you have to buy for your car. 50 State Legal (EPA and CARB).
Due to the delicate nature of the Subaru air flow meters, some tuners feel upgrading the in-box filter is your safest method of improving air flow into the engine without risking damaging your engine and/or electronic components. Other systems from various manufacturers, particularly those using foam elements, have resulted in many failures.
Another power gain in the intake track is to remove the secondary air filter in the large air box just before the intake manifold. This must be replaced with a gasket of equal thickness to the filter, but by doing so, you remove the filter's restrictions from the intake track.
Replacing the filter box altogether with a high flowing K&N filter is another alternative, and should yield a few more horsepower than just dropping in a replacement filter. Several manufacturers offer kits to adapt these filters to the MAF & MAP sensors. iSR offers the intake kit shown above, which only replaces the airbox and secondary air filter for better top end flow, nut leaves the stock plenum which provides for low end torque. A similar adapter is available from ?? which adapts a K&N type cotton element filter to the MAF or MAP sensor for similar performance.
Both TRi Tuning and Weapon-R Intake offer an all inclusive kit for the Subaru Forester. Quality, sound, fit, and finish describe the both intakes. The filter provides well for such a high flow unit. The sound is pure joy, and with an average horsepower gain of 13-15hp on a '98 2.5 liter motor, this intake isn't just sound. Pull from 4,800rpm to redline is improved substantially. As for mileage, some have reported up to 2mpg improvement in the sub 3,000 rpm range at highway speeds. Put your foot into it though, and with any performance unit, mileage will drop. The major difference between the two is that the Weapon-R actually loses some performance below 4,800rpm. The TRi Intake has a velocity stack attached to compensate and provide for low end torque.
Built specifically for the North American-spec Forester, the Strömung exhaust systems were dyno proven to provide the largest gains of all the systems tested recently by Sports Compact Car magazine. Featuring smooth mandrel bent tubing and precision laser cut flanges, the Strömung exhaust systems fit perfectly with no additional welding or cutting required.
The Strömung performance exhaust systems are currently available in mild steel construction with a single tip 89mm tip for a more aggressive look and sporty exhaust tone. This system will provide an additional 4-6% increase in HP over stock throughout the entire powerband.
At this time, Strömung is the only company offering a complete cat-back exhaust system for the Forester. The only other choice is a custom-made exhaust utilizing a universal performance muffler such as one by 3A Racing.
The stock Subaru headers are extremely restrictive and clearly built to save costs, not power. To remedy this inexcusable design flaw, there is the Borla Stainless steel headers. These headers feature nice, smooth mandrel bends with a much more effective layout which improves mid and high RPM torque and power. Currently, Borla offers a 2nd generation header with an improved design to offer better power gains and to address a rubbing problem found in the 1st generation headers. These are considered superior, for obvious reasons.
Not only do you gain performance but you also get the "boxer" sound the Subaru vehicles are famous for. This will transform the sound of the car under acceleration from a docile purr to a menacing rumble that sounds unlike any 4 cylinder engine you've ever heard. Sound quality at idle and cruise is increased only slightly.
The Borla headers are designed to bolt directly in place of the factory header system and therefore should be compatible with everything from 100% stock cars to car equipped with aftermarket turbo kits such as the Minnam or LUDESpeed system.
BORLA headers are built of aircraft-quality T-304 stainless steel - stainless
inside and out, not mild steel, not aluminum, not coated aluminum, not just
wrapped with stainless on the outside for show. Coating techniques, such as
aluminizing, can somewhat prolong the life of mild steel, but coatings suffer at
the weld points. And comparing aluminized steel to T-304 stainless steel is like
comparing gold-plated metal to solid gold! T-304 stainless steel has the highest
chromium content, making it the most resistant to corrosion and therefore the
most desirable material for long-lasting exhaust system.
Constructed from high quality billet aluminum, these power pulleys are CNC machined for a precise fit. Horsepower gains are realized throughout the entire powerband with an increase in peak power of approximately 6-8 engine HP. Most gains are a result of how much lighter this pulley is compared to the factory unit (4.5lb savings!!). Additional gains are a result of the slightly smaller diameter which underdrives the accessories for additional power. While this underdriving has no adverse effects on the reliability of the engine or components, it should be noted that when idling during heavy electrical load (ie: A/C on, upgraded stereo, etc) some owners might experience slight dimming of their headlights. Once RPMs rise back above idle, headlight intensity will return to normal. This is a very small tradeoff for the increase in performance and throttle response gained from this pulley.
TURBOCHARGED FORCED INDUCTION
For even more power, at this point the Forester owner will want to start looking at Forced Induction. Turbo kits are available and even with a mild street tuning will take engine output well over 220 horsepower. Brake upgrades would be a good idea at this point as well.
Turbocharger Kits are offered by JC SPORTS and LUDESpeed. Both offer a bolt on kit that will give you a mild increase in power and torque. It starts with 5 psi of boost and is easily upgradable to accommodate 8-10 psi. At 5 psi you can expect an increase in horse power and torque of around 40-50%. This is quite an increase if you look at it as horsepower per dollar. This is a complete turbo kit that will include all necessary parts and hardware. There is NO damage to the vehicle and you can return it to stock at any time (great for a leased vehicle). One of the most impressive upgrades you can do to your SUBARU. This is what SUBARU of America should have done years ago, instead of hiding behind the unprovable excuse that the WRX would not pass USA emissions laws. Please note that cars equipped with turbo kits REQUIRE PREMIUM UNLEADED FUEL (due to the higher compression caused by forced induction) and it is recommended all turbo kits be installed by a qualified and experienced technician.