inlet manifold trumpet velocity stack design

[Davezj]

Here is a great document that expains exact how and why inlet trumpets velocity stacks are designed the way they are and what sort of gains can be made from a well designed plenum chamber inlet manifold.

the main gist of the doc can be boild down to this,

basic comparison,
a straight pipe with no radius on the inlet compared to a straight pipe with a radius'd inlet. radius'd inlet pipe will flow about 16% more air.

ideal bell mouth inlet,
Optimum length of a bellmouth = exit diameter
Optimum entry daimater = 2.13x exit diameter
eliptical profile
corner radius = 0.08x entry diameter

for a 45mm throttle body:
exit diameter=45mm
length of curved section of rampipe=45mm (then it goes into straight pipe)
front of rampipe diameter=95.85mm
radius on front of bellmouth=7.66mm

best design for an eliptical bellmouth had 1.5% more air mass flow than a simple radius'd pipe
doesn't sound like much, but this is 1.5hp extra in 100hp....worth having

fully radiused end had next to no effect over a half radius
rectangular bellmouths are best avoided.

[Davezj]

this is a link to a thread about inlet manifold and plenum chamber design.

http://www.clubvr4.com/forum/showthread.php?70411-anybody-wanted-to-design-there-own-plenum-chamber-intake