It depends what budget you are planning to spend. It also hugely depends on whether you are going to use the car for everyday purposes or just use it for the weekend drive.
If your budget is limited than you cant do much except for installing an performance air replacement filter and a sporty exhaust. I must tell you though, that if you install the latter wisely (not just for the sake of putting them on) you will gain a noticeable improvement in power.
Eg: the best way to optimise the air filter is by installing it (via extensions if need be) somewhere where it can suck cold air. Some people have even gone through the extent of cutting four 2 or 3 inch slits in the front mudguard so as to allow cold air to enter directly onto the air filter. But all this is useless if you dont cover the plastic connection (between engine and filter) with reflective foil. This, so as to reflect the heat away or else the engine will end up sucking hot air anyway. I couldnt be bothered with that, so I just stuck mine directly to the engine (removed all piping & extensions). The above, if done properly should give you a 4 bhp boost.
With regards to the exhaust dont just buy one and weld it on. NO! Remember that the standard exhaust pipe incorporates two silencer boxes and another one containing the catalytic converter. Rip everything off and replace with a straight pipe which will connect directly to the back box except for a small silencer box in the middle. This will increase another 4 bhp.
Some other cheap mods will be to replace your sparking plugs with a set of Splitfire Platinum plugs (which claim to give an extra 1bhp per plug) as well as replace the High Tension Leads. If you buy a good set of HT leads, it will see you with an extra 2-3bhp.
You can do all the above if you are on a limited budget and/or if you plan to use your car everyday. Mind you....you can also install a nitrous kit and still use the car everyday. The nitrous kit will relieve you from quite alot of money, but it will provide you with an extra and instant 50bhp at the touch of a switch. The advantage of this is that you can still use the car as normal without consuming more fuel or reducing reliability. You can even strip off the kit if you want to sell the car. You can of course set the nitrous kit to blow much more than an extra 50 bhp, but for that you need to do heavy engine mods. For a 1.6 or 1.8 engine, an extra 50bhp is still safe. You will have to keep an eye on how to use the nitrous....its a burden on the clutch (since it kicks in instantly) as well as the exhaust. I have already replaced 3 exhaust because of the nitro.
You can also change reprogramme or change the CPU, which can give you an extra 10-15bhp, but that will effect fuel consumption badly.
Last but not least, you can get a turbo fitted which will see an added 60,70 or 80bhp (depending on the engine capacity)....but....there are many buts.....
1. It is very costly
2. You need to change almost have the engine (pistons, driveshafts, injectors, radiator, modify bonnet for air ventilation, reprogramme CPU, etc)
3. Fuel consumption will be effected badly
4. Reliability may also suffer.
5. Insurance price hike
6. As far as I am concerned, you cant remove the blower. You'll have to change engine if you plan to resell otherwise it'll be difficult.
...in the end it will be more worth while investing to stuff in a coupe engine.
Since I live in Malta, I wont not able to give you any cost suggestions as I am not aware of the costs in the UK.
There....now be a good boy and two a couple of the above mentioned mods.
