There are many performance filters on the market that will replace your car's stock air filter. There may be variants in design and construction materials but they will all produce the same benefits, increased air-throughput resulting in increased engine power (up to 5%). They will also last longer than a conventional filter as they can be cleaned and re-used.
Performance Air Filters
Standard exhausts compromise top-end power to reduce noise levels and ensure smooth power curves. Performance exhausts gain extra power by eliminating the compromise. Replacing standard baffled boxes with straight through silencers reduces restriction and increases power. Large bore pipe-work and modified manifold tract lengths are also used to maximise flow. There are many types of exhaust. Which you want and what your car needs may be different, so choose wisely.
The most cost-effective way to upgrade your exhaust is to replace the back box only, as this is where the original exhaust suffers most restriction. This modification will increase power, improve looks and emit a deeper sound. Peak gains rarely breach 5bhp, but you should be happy with the increase in throttle response.
Performance chips remove the allowances set by the manufacture to improve areas such as fuel economy. Aspirated cars gain around 5 to 10bhp and extra torque. On turbocharged cars including diesels, the chips also increase turbo boost pressure, which ups the power dramatically. The only disadvantage with a mass produced chip is that it has a set program and cannot be adapted to ensure that any other modifications that have been made to aid performance are running at their optimum.
Intercoolers work like a car's radiator, transferring heat. In this form, they cool the temperature of the intake air. This cooler air is more dense and will allow for extra fuel to be added so when detonated will produce a greater force.
Induction kits are designed to replace the car's restrictive air-box system. These kits can offer power gains of up to 10% by drawing in plenty of cold air. But be aware, if they are unable to draw on cold air you will not benefit from the design. It is essential the kit is set up with adequate cold-air feed pipes and is protected from heat. Also check that the oil used with the kit will not have any effect on the operation of the air flow meter, as on some models it can like later Volkswagen Group vehicles.
Performance systems that stretch from the catalyst back are more expensive, but offer improved peak power gains of up to 8bhp.
Replacing your car's iron manifold with a tuned tubular steel version can offer good gains. Quality manifolds can deliver good improvements in both mid-range torque and peak power. They are rarely cheap, but will transform the output of highly tuned engines.
CATALYSTS & PIPES
The catalyst causes major restrictions to airflow, reducing your car's total power output. By fitting a 'Cat pipe' the catalyst is removed and replaced by a straight piece of tube. But it is illegal to remove a catalyst from a car registered after 1986. Another option is to fit a performance catalyst. This keeps the car road legal but with the benefits of the extra power.
An ECU (electronic control unit) can be mapped to run all of the modifications on a modified car. To maximise the possible gains of running a larger output turbo and up-rated injectors a mapped ECU is the only answer. If a team who know their electronics do mapping, the results can be awesome.