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 Ford never designed the Si to be a high performance model, but many things can be done to enhance the current engines, or if you so wish, change the engine completely. With the Si coming in two main types, the 1400 and the 1600, we can group basic engine modifications into both of these engines. The 1.4 8v in standard trim produces 75bhp and 109Nm of torque, with the 1.6 16v producing 90bhp and 134Nm of torque. Neither engine will ever set the world alight with its power, but doing fairly standard modifying techniques allowing the engine to breathe better and flow gases away faster will start to help. The first port of call will be to an after market manufacturer for either a high-flow panel filter, or a full induction kit. A panel air filter will fit into the standard filter box, and sells itself as a lifetime filter, so you never have to replace it again. It can be simply washed and put back into the air box, therefore negating the need to ever buy another filter. K&N is a typical manufacturer and they guarantee their filters for 150,000 miles. A stage on the panel filter is a full induction kit, this comprises of a replacement pipe going to the throttle body, and a conical filter. The shape of the cone gives a greater surface area and less resistance to air, allowing it to freely enter the engine, without compromising the filtration it offers.  Once the car is receiving more air, it's necessary to expel the fumes without resistance. There are many companies who can replace your exhaust system with a free-flowing stainless steel one. They are simply designed to follow the path of your existing system, but without the compromises in manufacturing that Ford would've had to make for budgeting purposes and are made of higher quality stainless steel to reduce the effects of rust. Many claim to be rust-proof, but nothing's perfect! With most cars running electronically controlled engine management, it is possible to change the ignition timing and fuelling to release more power from the engine. As a result, fuel economy will suffer, but you can't expect more power, and keep the same fuel economy. Someone like Superchips provide 'off the shelf' chips that contain new maps for the engine management to run off. By altering the parameters, the engine will receive more fuel, so together with the more air you're giving it will create a bigger bang, therefore more power! Superchips quote a 7bhp increase over standard power, but vary depending of the cars current state of tune and health.  The next step is to fit performance camshafts to the engine, baring in mind the Zetec is twin cam, so will cost a fair amount for a marginal increase in power, in the best of circumstances up to 12bhp, but in the real world behind the marketing ploys, an increase of 4/5bhp is more likely. Camshafts can be bought separately, or in kits where you receive two camshafts and new followers to fit, which is considered good practice. They adjust the timing on the engine and how long the valves are open or closed for varying the timing can make the engine work more efficiently within a certain rev band. The cams tend to leave valves open for much longer, and tend to enhance the engines performance higher up the rev range. These are considered fairly normal modifications to the engine to create a little more power, and can generally increase the power from 10%-20%. Most of the parts can be bought 'off the shelf' and fitted by any confident DIY mechanic, but be wary with the fitting of camshafts as timing errors can be made, so it is suggested Vernier pulleys are used to allow accurate setting of the timing. Changing the exhaust manifold is certainly one way to release any power that might be vacant as design flaws by Ford means that its may not flow quite as well as it could. Currently no one makes a manifold specifically for the 1.6 Zetec, but Ashley may have one under construction and be available soon. However, there are companies who make manifolds for the 2.0 Zetec, and it would be easy to find someone who would make a quality, well designed manifold for you at your request, just don't expect it to be cheap.  All the Fiesta Si's came equipped with a catalytic converter, and can be somewhat restrictive to the escaping exhaust gases. Magnex offer a de-cat pipe, that is simply a hollowed out cat, without the honeycomb centre. To the unsuspecting MOT tester it looks although it has a cat on it, but it doesn't. It must be warned that without a cat on your car, it may not successfully pass emissions tests, which will lead to MOT failures. Following on from this stage of tune, further modifications can be made which are considered more serious by augmenting the size of the valves; the head can be gas flowed - a process in which the path of air into the combustion chamber is made to be as smooth as possible. The block could be bored out to give a larger capacity and the compression ratio could be raised both giving the possibility of more power. However, these alterations to the engine are considered somewhat wasted on the 1.6 16v engine due to restrictions in the design of the engine, and the sheer expense of some of these processes. The suggestion that the engine is changed and replaced with one of the larger Zetec units of which there is abundance. The first step up would to be to use the 105bhp XR2i 16v engine, this can be very easily uprated to the RS1800 specification 1.8 engine too which kicks out 130bhp. The final engine that would slot straight in is the 2.0 litre engine found in only the Mondeo. This produces 136bhp in standard trim and a bucket load of torque to go with it makes this the obvious choice for those seeking the largest power available to them. All of the modifications mentioned above can be applied to this engine and produces some satisfying results, creating a pretty good 'bang for buck' ratio.  If big power is to be achieved, thought must be applied to the aspiration of engine. A normally aspirated engine sucks air into the combustion chamber through one throttle body. A popular way of getting more air into the engine is by blowing air into the inlet, by using a turbocharger. This works on the exhaust, with the escaping gases spinning a turbine which is connected via a shaft to a compressor wheel which spins the air and blows it into the inlet at pressure. In order to cope with this, the engine must be altered to handle the extra air. The compression ratio must be lowered because the air is being forced into the combustion chamber, the air entering is denser and when compressed by the piston it can cause excessive pressure in the chamber causing detonation, creating problems such as head gaskets blowing. The bearings of the turbo run on a bed of oil so oil must be routed to and from the turbo from the engine block, therefore these must be drilled and tapped into the block. Management must be changed to allow for it to up the fuel mixture when the car begins to boost. The Fiesta RS Turbo management can be used to for this, or an aftermarket system such as OMEX or Alpha, but expect to pay for these systems, as they don't come cheep! More information can be had by discussing this with an engine builder of tuning company, and if you're serious about turboing your Zetec expect excellent results, but at a cost. A 1.8 turbocharged Zetec can produce 200bhp easily, with this climbing to 280bhp without too much stress. The 2.0 Zetec can produce upwards of 250bhp with ease, as if you push enough money towards it over 500bhp can be produced, a prime example being Sunny Khalsa's 2.1 Fiesta Zetec Turbo.  Other ways of aspirating the engine is by using Carburettors and throttle bodies. A 2.0 Zetec on twin 40 Weber carbs can produce around 160bhp if you fancy being old school. Alternatively, using one throttle body per cylinder power can be increased to close to 200bhp, with the main advantage of throttle bodies over carbs being they are much more tuneable and produce less emissions. And emissions are the main problem when using carbs. The strict ruling over emissions in post 1992 cars, means the car must adhere to these rules, otherwise the car will never pass MOT, so bare this in mind when thinking of using them. As with all performance tuning, the want for bigger power outputs always create other problems. The first thing that has to endure the brunt of all this turning power is the clutch and gearbox, so these must both be uprated according to the power your delivering. The standard clutch should be able to handle up to a 20% power increase without problems, but beyond this a stronger one would be required with stiffer springs within to handle the extra power without slipping, baring in mind a stiffer clutch will require greater pressure to depress it. After this a paddle clutch may be needed, but this is only really for when changes in aspiration have been made, and massive power is to be handled. Over about 170bhp a paddle clutch would be advised, but due to the nature of its design would make for very difficult and unpleasant urban driving, and I suggest you get down the gym and start working out! Next on the list of destruction is the gearbox. The Fiesta shares its gearbox with the mk4 Escort amongst others, which is useful as the Escort RS Turbo gearbox has a limited slip differential to help combat torque steer. This item would be an idea when turbocharging your Zetec as when coming on boost the car will want to travel in many different ways, apart from straight on! The Fiesta diesel box, and the RS1800 box are the only ones with uprated internals to cope with greater power, so are often used in normally aspirated conversions such as the 2.0. Delivering big power means higher speeds which means a greater expectancy upon the braking system, see the braking section for possibilities in this area.
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