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Tuning and modifying your car

Performance air filters

The simplest modification requiring next to no skill and with minimal cost is upgrade the air filter element to cotton gauze or foam, for about £30-40. K&N and Pipercross are the two biggest brands.

Both types of filter are sprayed with oil to attract and trap dust and the benefit of both is that air flows more easily through them than through a standard paper OE filter, thus releasing up to 2-3bhp. An additional benefit of performance filters is that they can be cleaned, re-oiled and used again, so you never have to buy a replacement element again! Cleaning instructions will be supplied with your filter, or refer to the company's website.

All air filter manufacturers make statements about their filters producing more air flow etc. but what really happens is that the performance filter produces a smaller pressure drop at a given engine speed than a standard filter. This means the engine sucks in air more efficiently and therefore the engine is more efficient. More efficient = more power.

The relative merits of cotton gauze versus foam are discussed by a cotton gauze filter element manufacturer here

Cotton gauze

K&N (US) are by far the biggest name in the performance air filter market both worldwide and in the UK. Green Filters of France, BMC Filters from Italy (not so well known but presently supplying the Ferrari F1 team), and Jetex Filters from Sweden also make cotton gauze filter elements. All claim different advantages in using their brand, the choice is up to you.


Pipercross from the UK are the second largest supplier to K&N in the UK. Ramair and ITG are also well-known.

Induction kits

For fuel injected cars the next step up (and more popular) is to install an induction kit. These require a little more skill but nothing more than a screw driver and small spanner. An induction kit is a cone filter placed as close to the air inlet of the engine as possible. In most cases you remove the original airbox which houses the standard air filter as well as the hoses and any resonator boxes which connected it to the air inlet. The cone filter is then usually attached directly onto the air flow meter which is usually mounted directly onto the inlet manifold. With the removal of all pipework and shortening of the air path, this means that pressure losses are minimised hence power can be increased.

However, the key to good power gains from an induction kit is how much cold air you can get to it. The colder the air then the denser it is and the more dense it is then the more mass of air you are getting into the engine. The ECU then matches the fuel input with this increased mass of air and more power is produced. So the message is: get as much cold air to the cone filter as possible and where necessary put in heat shielding of your own - we sell a range of Thermo Tec products to help with this.

As with replacement filters, there is the same choice of cotton gauze or foam filtering elements. Again the choices are K&N, Green Filters, BMC Filters or Jetex for cotton gauze filters and Pipercross, Ramair or ITG for foam filters. However, there are now also stainless steel mesh filters in the UK market from Quikshift and Powertec.

Quikshift have been in the market since 2001 when I met them at the Max Power show and we have supported them ever since (it is their induction kit that I use in my own Impreza). Powertec were launched at the Autosport Show in January 2004 where we were second in the UK to sign up as stock holding dealers. Prices of induction kits of any brand range from approx. £40-£100 and power increases are typically around 4-5 bhp. Some applications such as the Civic Type R are reputed to give 15 bhp!

Bolt-on filter kits

These are for cars with a carburettor rather than fuel injection. The original air box containing the OE filter is removed along with any associated pipework and the new filter simply bolts directly onto the carburettor. Again the advantage is a smaller pressure drop for the incoming air and hence more power. In some instances the carburettor will need to be rejetted so that the air-fuel ratio is still at the magical 14.7. We sell bolt-on kits from K&N, Green Filters and Pipercross.


If your car has fuel injection and an ECU (Electronic Control Unit) the next simplest step at reasonable cost is to fit a performance chip. Power gains for a non-turbo are around 10% and are a staggering 25-30bhp for a turbo. Especially in turbo applications you can't do better in terms of power per pound. Modern turbo-diesel engines such as the Audi 1.9 TDi respond particularly well to chipping.

We sell Powerchips at £150 for non-turbo cars and £250 for turbocharged cars. For most applications you need to post your ECU to Powerchips who will chip it and send it back to you the same day but in some applications such as most Fords, a module can be sent in the post which is simply plugged into the ECU. In naturally aspirated cars (non-turbos) a chip advances the ignition and puts in more fuel.

In turbo applications the chip increases the boost pressure or removes the boost limit to allow manual increase of the boost. Companies who program these chips make sure that these alterations are still within the design limits of the engine but they do point out that servicing the engine regularly is even more important than normal.


Next up is a cam (or pair of cams in the case of most modern engines). The most popular option for road applications is a cam of around 270 degrees duration which Piper Cams call a 'Fast Road' cam and Kent Cams call a 'Sports' cam. These still idle OK and can give around 12bhp extra at the higher end of the rev range. The trade-off is usually a decrease in torque at the bottom end but this varies between applications.

Usually a chip combined with a performance cam will maintain torque at the low end as well as increased power over the whole range, so this is a great combination. Note that you do need to be a proficient home mechanic to fit a cam, so if you are not, you will need to get it fitted at a garage - allow around 2 hours for labour - most garages charge about £30 per hour.

Power boost valves

There's only one name in the market for power boost valves and that's FSE Glencoe. Contrary to popular belief these do not increase the power of your car but they do usually give you better throttle response.

A power boost valve is a replacement for the fuel pressure regulator in the fuel line which is found in the return fuel line to the fuel tank, so it is actually a back-pressure regulator. This often confuses people when fitting them because its more natural to think of controlling the pressure downstream of something than upstream.

What the boost valve does is allow the pressure in the fuel rail to recover more quickly to the required pressure when a sudden demand is made upon it when you put your foot down. FSE quote a figure of 1.7 times faster than standard. If you know what you are doing you can adjust the fuel line pressure with the Boost Valve - there is a screw in the top and you can purchase a fuel pressure gauge that attaches to the side of the valve.

It's kind of like playing around with jet sizes in a carburettor. We must point out though that you really do need to know what you are doing if you are going to adjust the fuel pressure. If you make it too low, the engine will run lean and this can cause things to melt such as pistons. boost valves come pre-set at the correct pressure for each engine.

Performance exhausts

Next up is a performance exhaust. Whereas induction kits and replacement performance filters are to do with reducing pressure drops on the way into the engine, performance exhausts are to do with minimising pressure restrictions on the way out. Some performance systems have an increased bore to reduce pressure losses but most retain the standard bore and have more freely flowing exhaust boxes.

However, just opening up the exhaust is not necessarily the answer. The extreme would be to remove the exhaust system altogether but, for those of you who have experienced it, if the exhaust breaks near the manifold, all power is lost.

For a normaly aspirated (non-turbo) engine in particular it is important to retain a certain amount of back pressure. It can also be important where the first exhaust box is situated. The reasons for this are all to do with exhaust scavenging which is to do with negative pressure pulses reflected back up the exhaust manifold to remove the last of the exhaust waste from the engine.

Having said all that, the situation with turbocharged cars is different. The turbocharger is positioned as close to the engine as possible in order to maximise its efficiency because it needs to be as close to the hot source as possible.

Because the turbocharger is spinning at such a high speed compared to the engine this means that any pressure pulses from the engine are evened out and exhaust scavenging is not possible to the same extent as it is in normally aspirated engines. Therefore turbocharged cars can increase their power much more easily by simply increasing the bore of the exhaust system and removing exhaust boxes.

So the outcome of all this is that typical turbocharged cars such as Imprezas and EVOs can be producing more power as they get mega loud due to reduced back-pressure (i.e larger bore systems and free flowing boxes) but a normally aspirated car will actually reach the point where it becomes less powerfull as it gets louder. This is where the performance exhaust manufacturers come in. They know what is best for each kind of car.

The biggest brand names in the UK are Magnex, Mongoose and Scorpion who all produce a large range of stainless steel performance exhausts for the most popular cars for tuning in the UK market. Other popular names are Piper, Sebring and Supersprint with Sebring and Supersprint manufacturing a huge range of part numbers and Piper (of the cams fame) manufacturing an impressive range of both mild and stainless systems and backboxes with a vast range of tailpipe options.

Other well known names include Janspeed and Ashley Competition Exhausts from the 70's who still produce a good range of performance exhausts and manifolds.


For normally aspirated cars in particular, the design of the exhaust manifold is all important. The standard manifold supplied on a car is a compromise between cost, power and space. However, a well designed aftermarket manifold will fit in the same space but be tuned to produce significantly more power.

This is achieved through carefully choosing the correct length for each of the manifold branches, thus maximising the cylinder purging through pressure waves. There are not actually that many manifolds available in the market. Magnex and Supersprint produce some but there are more available from Janspeed and Ashley Competition Exhausts although they do major on older applications like the Mk2 Escort.

Prices for a stainless steel performance exhausts start at around £350. Top of the range exhausts can release around 8-10bhp.


The first step for most people is to fit lowering springs. This is often to make the car look better with the wheels closer to the arches but from a handling point of view they lower the centre of gravity of the car and are uprated, usually by about 25%.

This means there is less roll in the corners and the tyres retain better contact with the road. Many springs nowadays are also progressively wound which means that their poundage rating increases as they are compressed. As you go over smaller bumps the springs can feel almost standard and a comfortable ride is retained but when you corner hard the springs stiffen up to reduce roll. The best of both worlds!

There are basically two price groups for springs. There is a large group of manufacturers who produce springs at around the same price of around £70 - £100. We sell most of these brands, namely Apex, AVO (Chassis Dynamics), Jamex,PI, Spax and Ventura (used by Bilstein).

There is then a second group of more expensive springs at around £130 - £150. The biggest name in the UK is Eibach and the other is H&R - both from Germany. They both say that their springs are more expensive because more development goes into them with each set designed and tested on every individual car. We certainly get a lot of good feedback from customers who fit these springs.

Shock absorbers / dampers

Many people fit uprated shocks some time after they have fitted their springs while others will go the whole hog and fit a full suspension kit complete with dampers and springs. While fitting springs on their own is OK you certainly don't get the best out of the suspension because uprated springs do make the suspension work harder.

Forces are higher and the speed of oscillation goes up just like tightening up a guitar string. In some circumstances standard shocks are fine - for instance its quite common for BMW owners to fit Eibach springs with standard dampers.

Shock absorbers are basically a plunger in an oil-filled tube with holes in the plunger to let the oil through. The 'rate' of the shock absorber is related to the size of these holes - obviously the smaller these holes, the higher the rating and the harder it is to move the plunger. There's obviously more to it than that but its the basics.

You will also hear of gas-filled shocks. This does not mean it is only gas inside - they are still mostly oil but the unit is pressurised with gas. Companies who produce gas shocks say the gas takes away some of the harshness sometimes associated with uprated shock absorbers. The two biggest names in the shock absorber market are Bilstein and Koni, both with a long heritage in racing and rallying, especially Bilstein. Both do two ranges of shock absorber for the road.

Bilstein do a very large range of OE shocks which are popular with people wanting to firm up their suspension a little bit by replacing worn out shocks with the high quality Bilsteins. Bilstein also do a range of uprated sports shocks which are around 25% uprated. They are not adjustable - Bilstein say they know what rating the car needs (they are from Germany!).

Konis on the other hand are adjustable and their two ranges are 'Special' and 'Sports'. The Special range adjusts from standard to approx. 30% uprated and the Sports range adjust from approx. 20% uprated to very hard. Bilstein and Koni shocks tend to be around the same price - a rough average is around £100 per shock absorber.

There is then a second stream of shock absorber suppliers who are all around the same price but are cheaper than Bilstein and Koni. The main brands that we sell are AVO and Spax. All AVO dampers are adjustable while Spax has two ranges of shocks - one adjustable and one non-adjustable. All AVO and Spax shocks are adjustable on the car (Konis are also adjustable on the car where the adjustment can be made at the top of the shock but their other shocks need to be adjusted off the car). Some of the spring manufacturers also sell full kits of springs and shocks but don't sell shocks on their own. Among these we have Apex, Eibach, Jamex and PI.


Fitting uprated bushes is often the last thing people do when uprating their suspension. Perhaps its because old bushes can be a pain to get out and the new polyurethane ones difficult to get back in but if you know what you are doing and know some 'tricks of the trade' its not too difficult. Especially if you've got an older car, fitting new bushes can transform your car's handling because the original rubber bushes do get tired and worn which allows too much movement between suspension parts.

We supply bushes from Powerflex and Superflex which are made of polyurethane. This material does not deform anything like as much as standard rubber bushes but still manages to absorb much of the vibration and noise from the road. It is also self lubricating so that parts can rotate within the bush (most rubber bushes need to twist because they are firmly attached to both suspension components).

The best bushes to upgrade first are usually those associated with the steering such as the track control arm bushes. The benefits of this are very noticeable with a sharper steering response. The rest of the front suspension bushes are usually next on the list to further control the steering and also to allow the suspension geometry to do its job.

For many, the rear bushes are the last to upgrade simply because they do not show such an immediate effect as the front ones do for the steering. Some cars steer at the rear as the car rolls on cornering and uprated rear bushes can help limit this.


Whereas suspension kits are popular because a lower car looks cool and performance exhausts make the car louder as well as more powerful, brakes are often the last thing on people's modding lists because its perceived that they don't do exciting things to a car. However, good brakes can enhance your driving experience considerably as well as provide safer braking. Safer braking is perhaps a side benefit with properly modded cars being able to stop way quicker and more securely from legal speeds than standard cars but better brakes also allow later braking into corners and will resist the dreaded brake fade when worked hard on twisty back roads or the track.

How brakes work

Car brakes work by converting kinetic (movement) energy into heat energy. When the brakes are applied and the brake pads are pushed against the rotating brake discs the resulting friction between the pad and disc surfaces produces lots of heat with temperatures up to hundreds of degrees. This heat is dissipated to the atmosphere, mostly by radiation, but also by convective heat transfer directly to the surrounding air.

Too much heat

If the brakes get too hot you will experience problems. The friction surface of the pads can bake, resulting in a smooth glaze which drastically reduces friction. Resins within the pads can also vapourise, resulting in a gas layer between pad and disc. This also reduces friction.

Less heat

Most modern cars now have vented discs on the front which vastly improves convective cooling with air passing through the two halves of the disc drawing heat with it as it goes. On more powerful cars it is now also standard practice to have vented discs on the rear too.

If you're being really serious the discs can have shaped cooling vanes between the two halves of the disc drawing air out from the centre to the outside like a fan. These vaned discs can be extremely expensive as they have a machined internal finish rather than the usual 'cast' finish i.e. as it comes out of the sand cast mould.

Performance pads

The simplest brake improvement is to fit performance pads. These have a higher friction coefficient than standard pads and can run up to much higher temperatures. If your discs are up to coping with these higher temperatures then you gain from the greater temperature difference between the discs and the surroundings because heat will then be transmitted at a greater rate and the car will then slow down quicker.

We sell a large range of performance pads with lots of different compounds from good fast road pads up to pure race pads.

What's the catch?

As always in engineering there's a catch. Very high specification pads don't always work well from cold and they can be too noisy for road use. Have you ever heard a rally car at the end of a stage? The brake squeal can be ear splitting.

The majority of pads we sell do not fall into this category, working well from cold with no squeal, but some of the race compounds do. There are however some pads on the market, particularly Pagid, which are essentially race pads but will work well from cold and are not too noisy. These pads are made from expensive materials and you tend to pay up to 3 times more than for more 'normal' performance pads.

Grooves, holes, and dimples

It's always a good idea to fit performance discs along with performance pads. Performance discs come with a mixture of grooves, holes and dimples which deglaze and de-gas the brake pads when they get hot, thus keeping the friction level high.

Holes also enhance cooling by increasing air flow through the discs as well as increasing the overall disc surface area for heat transfer. Black Diamond offer a full range of choice with drilled-only, grooved-only and drilled and grooved discs. EBC and some others have opted for blind drilled holes (or dimples) along with grooves.

Other manufacturers such as Tarox and Red Dot specialise in grooved discs with their most expensive discs having 40 grooves. As well as deglazing the pads, the increase in surface area with so many grooves will also help cooling.

The hole debate

With the advent of ever more powerful cars quite a debate has emerged about drilled discs. Those against drilled discs will tell you that the drilled holes are the source of cracking under extreme heat and that the holes are totally unnecessary.

Whilst it is true that heat stresses are maximised at sharp edges, if discs are well enough made from high quality castings it is not an issue - look at high end performance Porsches and Mercedes - a number of models have discs with multiple drilled holes.

Of the brands that we sell you will see that Black Diamond, Tarox and Red Dot drilled holes all have chamferred edges which reduces heat and mechanical stress. We have fitted Black Diamond Combis (drilled and grooved) discs to a number of our own cars and friends' cars and have always found a great improvement in braking with no problems whatsoever.

The only disc warranty claims that we have seen from any brand of brakes have been as a result of sticking calipers producing massive heat because the pads are jammed against the discs or discs being used outwith their limits, e.g. cars running twice their normal power with drilled or grooved discs of standard size where they really should be running larger diameter discs.



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