Removing Restrictions

Remove any restrictions from the air intake system

What do I mean restrictions in the air intake? Well there are a number. The biggest restriction and probably the most obvious is the air filter. However that is there for a reason so don’t just remove it! I’ve covered air filters elsewhere so I’ll not say a lot here, other than the easiest way to measure the performance of the air filter is to measure the pressure drop across it under full load/full throttle. This will give you a direct measure of how much the air filter is resisting airflow. The lower the resistance the better!

The other restrictions in the air intake are a bit less obvious, well some of them might be. Next on the hit list is the intercooler. This isn’t straightforward to improve though. Most people will say bolt a bigger intercooler on, and in some respects they are right, if the number of cores in the intercooler is increased and the cross section of them remains constant, then the resistance to airflow will drop. However bigger intercoolers are usually physically bigger which means the cores are longer which increases the resistance to airflow of them, so bigger isn’t necessarily better. Like air filters the resistance to airflow is the key factor. The pressure drop across an intercooler is often measured to give an idea of how well it flows. Note that this is not a measure of how well it cools the air! Better flowing intercoolers often cool less efficiently, like most things in life it is a compromise but we’ll come onto that in the next section. For standard power the original intercooler is fine, even for mildly tuned engines their performance is adequate. The compromise that you need to make is between the resistance to airflow, compared with the air intake temperatures. If you are looking for 150+bhp out of the L series then you going to have to look at a bigger intercooler or some other method of cooling the air intake, (people have managed 145bhp on the standard intercooler).

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