::: Golf GTi - Improving Air Flow :::
The first step in aftermarket tuning is to buy a free flow performance air filter and exhaust. The filter costs about £20 and the exhaust upwards of £250. It is possible to improve the airflow into the engine further by modifying your air box. This will help extract the most from your filter and is a completely free mod. We won't begin to claim that it drastically improves your power output but it gave a noticeable improvement on a k-jet Gti (although this would be magnified by the other performance mods already in place) and so is worth a try. If nothing else the mod gives the car a greatly improved induction roar.
NB : Having improved the air flow with the filter and air box mods you may need to increase the fuelling to compensate otherwise the engine might run slightly lean.
POST 88 Mk2 8v:
The air enters the air box trough a highly restrictive baffle on the front face of the air box (designed to reduce engine noise). Disconnect the rubber air intake pipe from the air flow meter and remove the rubber hoops that hold the air box in place (located on the front passenger side and rear of the box). Remove any other vacuum hoses and then remove the air box from the car. Un do the clips that hold the bottom of the box to the top and separate the two halves. The restrictive baffle should be clearly visible on the front of the box. It should be removed by gripping the lower half of the box in a vice and prising the baffle off with a screwdriver. This will dramatically open up the airway. If you already have a performance filter now would be a good time to wash and re oil it; if not now would be a good time to buy one. For further improvement try drilling as many holes as possible down the wing side of the box. Do not drill down the engine side of the box, as this will cause hot air to be drawn into the engine rather than cold air. If there is room then it would help to drill more holes in the front of the box. Also take care not to drill through the strengthening ribs as this will weaken the box. Refitting is then the reversal of removal, and if you haven't already then remove the plastic trim that sits behind the grill and mates with the now obsolete baffle.
PRE 88MK2 / 16v [K-Jet injection]
The air enters the box through the side of the air box via a very pointless looking plastic 'thing' with a very small air hole in the bottom of it. Jack up the car and remove the front driver's side wheel. Remove the wheel arch liner by undoing the Philips head screws. You will then see a large black plastic 'thing' sitting behind the wing. Unclip the top half of the air box and remove the rubber rings that hold the lower part in place. Separate the lower part of the box from the tube that runs into the inner wing and remove both the tube and lower half of the air box. Discard the tube. Remove the big plastic thing from the under the wing and bin it. If you are concerned about neatness then plug the large square hole in the inner wing with a VAG blanking plate as available in most scrappies on 1.3s and driver's etc. To further improve the airflow you should cut two largish rectangles out of the front of the air box and then drill further holes down the wing side. Do not put any holes in the engine side of the box, as this will cause hot air to be drawn into the engine and do not drill through the supporting ribs. Refitting is the reversal of removal remembering to put the air filter back in the air box when refitting.
Contrary to popular belief, fitting an induction kit (cone filter or equivalent) to a forced aspiration engine where the air is being forced into the single most expensive part of the whole car is not always to be recommended.
Free flow filters achieve their 'free flow' by (obviously) letting in more air (or offering it less resistance) the problem here of course then is that larger granules of cack can make it through the filter and start roaring around within the delicate vanes of the supercharger.
To improve airflow on a G40 the single most important modification is to remove the restrictor flange within the lid of the standard airbox. This flange was (allegedly) placed there by VW during development to starve airflow in the final part of the rev-range so the car hit the limiter less!! - as we all know this is the place where the G40 is at its neck-wrenching best, so get out your file / Stanley knife and trim away the flange from the shoulder-spur at the top of the airbox until the whole pipe is the same diameter throughout.
Whilst you have the airbox open, pull out the silencer trumpet that connects the airbox to the fairing around the headlight for a more convincing roar, discard all of the fairing gubbins to ensure more air flows from around the headlight, and finally garnish with a good quality aftermarket panel filter (if you are prepared to slightly shorten the life of your re-built supercharger!).
Disclaimer : Matey-Matey accept no responsibility for any of the information contained within this document or the accuracy thereof. It is intended as a helpful guide and is solely based on personal experience. The authors also wish to stress that the methods highlighted are centred around personal opinion and there may be other equally credible ways of performing this conversion.