The first modification most popularly advocated to improve an engines performance has to be a change of air filter or induction system on fuel injection engines.One of the most well known makers of replacement filters etc.,apart from Pipercross has to be K&N.As I have used K&N's filters in various applications I will review their products here.

The Technical Bit - An internal combustion engine is basically nothing more than a large air pump running backwards.If you were to apply a force to the crankshaft,driving the pistons they'd draw air in through the intake and pump it out the exhaust.A standard mass-produced engine while having to conform to certain casting,construction and build tolerances for obvious reasons will not be built to the exacting specifications of a race engine and this lower standard of build quality is where a lot of engine tuning produces power.Despite this last statement modern mass-produced car engines aren't slung together it's just that to effectively turn out engine after engine on the production line compromises between ease and speed of manufacture and high build quality have to be reached.Also a race engine is purpose built for one purpose while your everyday car engine has to perform under vastly different conditions all day long,in extremes of hot and cold,in bumper to bumper traffic jams and on the motorway cruising it has to do all these equally well and equally quietly,smoothly and with the minimum of fuss and then start all over again in the morning while giving good power and fuel returns.

The main way to tune a car is to make it 'breathe' more easily.The four strokes of a four stroke engine are induction - compression - power - exhaust where on the induction stroke with the inlet valve open and the piston moving down the cylinder bore draws air through the filter and carburettor,where the vapourised fuel is also drawn out then the air/fuel mix travels through the inlet manifold,
inlet port in the cylinder head and into the cylinder to fill the vacant space left by the downward travelling piston(a fuel injection differs in that having no carb the fuel is injected usually in the inlet manifold and mixed that way).On the compression stroke with all valves closed the upward travelling piston compresses the air/fuel mixture into a tight space.The power stroke is when almost fully compressed the highly combustible mixture is ignited by the spark plug and the resulting explosion and violent expansion of the gases forces the piston back down the cylinder bore producing the power stroke.Finally travelling back up the cylinder bore and with the exhaust valve now open the piston pushes the spent gases out of the exhaust port into the exhaust system thus completing the exhaust stroke before the whole cycle is repeated over and over again.So this drawing of air/fuel in and forcing of exhaust gas out can be seen very much as engine breathing therefore anything that helps get more air'fuel in will give more mixture for power also more in demands the ability to get more out.This is where the performance filter comes in being the first part of the induction system.

Modern engines having to conform to emmisions constraints,noise levels,fuel economy figures and reliability etc. tend to have an air filter that sacrafices power for good all round performance.Thus replacing the stock filter for an aftermarket free-flowing version is a tried and tested first step to upping your power.

Unfortunetly,it's not that simple though most the magazines and adverts would tell you otherwise because good engine performance requires good combustion and the more oxygen in the air/fuel mix the better the combustion.The colder the air mixture the denser it'll be and more will be drawn in compared to a hotter air mix so factors like under bonnet cooling come into play thats before we get into such things as intake pulse tuning but that's anot
her story.

The Carb Filter - K&N make a massive range of filters for all types of engines,though I'm only reviewing non-turbo charged(or naturally aspirated)petrol engines.For carburretor engines they make a pancake type air filter which usually has a chromed top plate with K&N stamped on it with a matching bottom plate bolted to the carb and the filter sandwiched in between.The filter is oiled and usually guaranteed for life.This type of filter will give an increased induction roar especially at higher revs and usually gives a slight increase in top-end power,I've used them often before in several carbbed Fords and for those who have read my review on the Ford Sierra may not be suprised to hear that I have this type of filter on my Sapphire.The main advantage of this filter is the ease of fixing - incredibly straight-forward just unbolt your old housing and bolt this one in it's place though the oil breather pipe from the rocker cover to the oil seperater in the old housing has to be either fitted with a special K&N oil breather filter or fixed to the underneath of your main K&N or if you're like me you could lead your breather pipe to a home-made catch tank consisting of an old small coke bottle tie-strapped to the inner wing with the pipe which led from the oil seperator to the inlet manifold blocked off - obvious enhanced look from the chromed case,increased sound and slightly increased power.The disadvanteges being the risk of drawing in hot air in summer and carb-icing in winter where when the temp. drops so low that the cold air freezes the atomized petrol it the fuel mixture making it drop out of the airflow causing the engine to run either roughlly or even to stall completely,if this becomes a problem you can run a hose from the filter to around your exhaust manifold to give it heated air.These filters can sometimes upset your fueling and the carb may need adjusting or rejetting but this isn't usually a problem.

You
have two options on an injection engine.Fuel injection engines usually have a black plastic trunking running from the throttle body to the air-flow metre/charge temp. sensor housing and after this is normally a large plastic airbox which contains a flat filter element.The bottom part holds the panel filter and has a small air intake under the filter while the top part has a larger pipe to the air-flow metre/charge temp. sensor housing they seperate just above the actual panel filter and are usually clipped together.One option is simply to replace the stock panel filter with a K&N replacement filter element.The other option is to replace the whole airbox assembly with a K&N57i induction kit.

The Replacement Panel Filter - This option is cheaper than the induction kit and obviously easier to fit but by itself I've found it to have little effect except to make the car a little heavier on fuel.As ever their are pros and cons to be weighed up on all sides and the way to make this option work is to cut plenty of holes approx 1/2-1cm in diameter in the base of the airbox below the panel filter.The important point to note when doing this is not to cut above the filter but also cut on the sides away from the engine as the idea is to get as cold air as possible in through the filter.If you run a 2.5-3inch flexible hose from a cold air source preferably away from heat like the radiator/exhaust system straight to the lower inlet of - or around the base of the airbox to where you cut your holes you should begin to notice an improvement.I've found this solution to work particularly well on my fuel injected Vauxhall's the Cavalier SRi's seem to work nicely and the holes give it a lovely sound but I stress that you try this at your own risk as it is easy to make a mess if you don't know what your doing,also if your MOT's coming up it's probably best not to do it till after.The injection systems on the Vauxhall 8v engines work well because t
he engine calculates load from Mass Airflow as opposed to Manifold Absolute Pressure like some other cars and therefore the fueling can,to a degree,cope with the increased airflow seeing as the fueling is metered by airflow.The advantages over the induction kit is that despite the unwieldy look of the large plastic airbox they flow suprisingly well,and you can smooth the airflow within the airbox above the filter,but they are also very good at shielding airflow from engine heat.They also look standard when going for an MOT,if uncut and unpiped,and won't have the tester looking deeper into your engine bay as well he might if you've got an induction kit or flexi-hoses piping air to and fro.If this option is fixed up properly I've found it to make the car more responsive and willing at mid-high revs and though the actual power increase as with any of these mods won't be outstanding the added driveability makes it worthwhile.The key is not to let it breathe engine heat which will make the car 'bog out' under throttle and make you wish you'd left it well alone.

The 57i Induction Kit - This is the more expensive option and more noticable when the bonnet is lifted.It usually consists of a large cone filter with a chrome end stamped with K&N which bolts onto a seperate tube with a metal fixing to clamp it to your inner wing.The cone fits into one end with a jubilee clip and the airflow meter housing into the other end once again fixed with a jubilee clip.There is usually a 2-3inch flexi-hose to pipe cold air to the cone and this set up needs cold air especially with 16v engines.The filter tends to sit where the airbox did and this in my experience causes most of the problems as it's just in the right place to draw engine heat.There is also some talk about these systems stiffling intake pulses which the airbox helps to produce but I've never seen it proven and after a lot of fiddling with one of these kits got it to work nicely
on my 16v Cavalier SRi after fitting an extra length of pipe about 8-10inches long between the cone and the fixing tube to take the cone away from the engine I also fitted extra cold air pipes to get as much cool air to the cone.Maybe the extra length did encourage intake pulses either way the effect was more dramatic after modifying the 57i with the car picking up more cleanly and being a lot more responsive on the throttle though she was more sensitive to the weather i.e in the cold she felt flat and when it was damp she just flew - the extra oxygen and denser air that you get when it's raining would probably explain this but the effect was noticable

Price - K&N 57i for Vaux Cav/Cal/Vectra @ £60 Nova/Corsa @ £47 - Ford Mondeo @ £60 Ford Fiesta @ £55 Ford Escort @ £55
K&N Elements from £30
K&N Pancake Carb £40-50

In conclusion a performance air filter is a good place to start when improving your engines performance.There are no power figures here but when properly set up the panel filters and induction kits do make small increases in power though it's more the driveability that is improved.The same is true but to a lesser degree with the carb versions but the main thing is if you have not done these modifications before while they are not hard to do a little forward planning when it comes to installing them can reap the benefits while just throwing them on can mess up a nice engine.I would recomend them but would also say that there are other makes out there bar K&N but they are the ones I've used in the past and have no experience of say the PiperX Viper for instance so can't tell you whether it's good or not.Happy Modding BB.