Intake runner sizing..

[Craig Pugsley]

Hi,

A semi-efi qn on a forthcoming carb-efi conversion..

I have a Mazda 13B street port that I am planning to EFI, and am
wondering about intake runner length/diameter characteristics for max
torque/max power.

For the un-initiated, mazda rotarys are a '2 cylinder' engine with a
primary and a secondary intake port. These ports commonly have different
opening & closing timing to spread the torque curve (to some extent).
For example mine has almost stock primarys (for good low down torque)
and large secondarys (for mid-high end power).

Is it true that the longer a runner the lower in RPM the torque peak
will be? If that's the case should the primarys be long and the
secondarys short?

Note that in the stock carburetted engines, the secondary runners are
bigger and longer than the primarys (as the 'primarys' are in the 'middle'
of the engine and the 'secondarys' are at the 'outside ends' of the
engine).

Any ideas or is experimentation the best teacher? (Hmm, go to the
plumbing supplies place and get lots of angled bends and pipe with
threaded ends for this one :-)

Cheers,
Craig.