Quote:
Originally Posted by The Radium King
jk - i'd say 74 mm on a 3.2 is fine. Porsche did it on the 987 3.2's and the 2004 special edition 3.2.
if you are feeling sporty, pull out the mechanical engineer in you: 3.2 litre engine displacement = 1.8 litres / rotation = 12,600 litres / minute at 7000 rpm = 210 litres / second = 0.21 cubic metres / second.
now you have three options for the size of path you want to push this air through:
inside diameter of throttle body:
986 = 67 mm = 0.0035 square metres
99x/987 = 75 mm = 0.0044 square metres (some say 74 mm, some say 76 mm; I used 75 mm)
GT3 = 83 mm = 0.0054 square metres
from here you can calculate the velocity of air in each case (unit analysis says simple division should work):
986 = 60 metres / second = 216 km / hour
99x/987 = 48 metres / second = 173 km / hour
GT3 = 39 metres / second = 140 km / hour
not sure what this tells me. I do know that it would take a heck of a lot less work to accelerate air to 140 kmph than 216 kmph. also more opportunity for ram air benefits with the slower airflow. but I presume there are other things at play; restrictions at the intake manifold itself, low rpm trade-offs, intake tuning effects, etc.
|
TRK! My gosh numbers..! How did you arrive at your swept intake volume per rev. Just out curiosity?
Well if you are able to stay clear of destructive turbulence at the higher velocities you would have more authority over the air flow...but this is not likely. The flow would start to die. I would guess that the GT3 size is suited to higher volume higher rpm operation more so than the other sizes, but at what minimum displacement??
CFD might put you in the ball park for the intake tract(s) but you have this crazy variable air pump below which makes calculative/modelling processes for the whole system ....crazy difficult.
You would need flow testing to get the real answers and this would have to be done with engine performance characteristics and goals. For the hobbyist all we can do is: take notes on driving impressions; time throttle response; time acceleration; chassis dyno before and after changes...
But alas this starts to feel like work! So we put it on and just try it!