Fit a larger capacity turbo
This mod is for those who want more than the approximate 135-145bhp that the standard turbo can flow enough air for. That isn’t to say that bolting a larger turbo on to standard engine will miraculously give you 150bhp because it won’t. You need both more air and more fuel to go with it. So in an ideal world you’d fit a bigger turbo alongside mods to increase the fuel (injectors/remap/rechip). If you want to play safe then fitting a larger turbo without adding extra fuel won’t harm the engine in any way so long as you keep the boost sensible (i.e under 28psi for a L series), however you won’t get all the performance benefits of it without adding extra fuel to match the extra air
There is no way I could cover all the details on what will and won’t work in an easily digestible webpage, so I’ll note down a few key points that you may wish to bear in mind
Boost level does not equate to airflow. For example a GT15 (standard turbo on the L series) running at 20psi will flow less air than a GT20 running at the same boost level People tend to get hung up on boost equating to power when it isn’t true. Airflow is what you are after, boost is just a byproduct along the way.
Many will say slap on the biggest turbo that you can find/fit. Well that simply isn’t true you need to size the turbo to what you want to use it for. Whilst a large turbo may give you bragging rights at the rolling road, in real world performance you may be outstripped by a smaller unit. For example you’d most likely pick a different unit for a race/track car than for a street car. The race car will spend its life at high revs, whereas the street car will spend much of its time at much lower revs where a smaller unit will be on boost sooner and so give better drivability.
So when sizing your turbo be realistic with what you want to achieve. If you want a drivable car then don’t go for the biggest thing you can find or you’ll only come back disappointed.
Types of turbo
There are essentially two types of turbo that you are likely to come across. A standard fixed vane turbo (which is what you already have fitted), and a variable vane one. These have developed in recent years and have accelerated the acceptance of diesels into everyday car use. They have advantages and disadvantages but are certainly worth considering if you are building a car for the street rather than the racetrack.
Variable vane turbos (sometimes called VNT) essentially expand the “size” range that a turbo is capable over. In real terms this means for a given power output, the variable vane turbo will spool up earlier and give a nice wedge of extra torque low down in the rev range than you’d expect from a standard turbo of the same size. They have the disadvantages that they are more complex, are more fragile and you have to control the vanes somehow to get good performance. I think it’ll be fairly soon that we’ll see controllers come onto the market to control variable vane turbos for the aftermarket fitment. Beware though that variable vane turbos are much more sensitive to exhaust gas temperature due to their more complex design.They don’t take nicely to just turning up the boost or adding a load of fuel to cause smoke. You have been warned!