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There is some great info and discussion in this long running thread.
From everything I have read here, elsewhere and my own findings I conclude the following.
On the intake side the biggest restriction is the standard airbox which has a tortuous and choked path for the airflow.
Replace with a 987 airbox or some other sealed system and that should help.
As for a larger throttle body and plenum, all I have read does not lead me to believe these are of any benefit and indeed seem to result in a power loss low down in the rev range.
Why is this? Well my thoughts are as follows.
From what I understand the electronic throttle body works by receiving a single from the DME that dictates how much it needs to open based on throttle position, load, engine speed etc.
If you have a larger throttle body then it just opens less (once it has calibrated it self).
As the throttle body is a larger diameter then although the air mass flow is the same as the standard smaller one the air velocity slows down which results in reduced cylinder fill at lower revs hence lower torque/power.
I know the intake neck after the plenum is unchanged so the air velocity in theory could return to normal but it won’t as it has decelerated too much to be clawed back in time.
When the revs climb the air velocity increases sufficiently to overcome this deficit and full cylinder fill is again achieved and possibly a small hp gain at very high revs.
Just my thoughts.
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