3.0 MFI Intake Port Size

[1972_911T]

Im planning on building a 3.0 MFI engine and want to know what intake port size to use. The original RSR had a 43mm port but still only used the same 49mm valve used on the SC and 3.2 that had 39mm (big port SC) and 40mm ports. As I will be using bored out 911 E stacks with a throttle size of 42mm it seems pointless matching the original RSR port size and my planned engine wont be reving as high so im sure this wont be required. Will it be best to open the port up to the same size as the throttle (42mm) or is it best to go a bit smaller and stick to the 3.2 port size of 40mm? Id read some where that the throttle plate size should be at least 2mm bigger than the port.

Thanks Steve

[boba]

What will be the use of the motor, race or street? High HP high revs or more torque mid range drivability?
Bob

[1972_911T]

Mainly street motor so more torque mid range drivability.

Steve

[boba]

Then I would recommend the smaller size to increase velocity.
Bob

[1972_911T]

Ive just done a few calculations and the RS has a 36mm port the 3.0 is 11% larger in displacement but a 40mm port gives 19% larger area than the 36mm of the RS. Would I therefore be better going with a 39mm port which is about 14% larger by area to better match the RS characteristics or will it not make that much difference?

Steve

[boba]

This is all about flow, to make the correct determination I would advise finding someone with a flow bench and flow the heads and then match the cam, intake, and headers. Once you have the data on the engine you are going to build (compression, head flo, piston, stroke) any of the cam suppliers can recommend the optimum profile to meet your objectives.

[Eagledriver]

With the 42mm throttles I'd go with standard big port SC ports (39mm). You'll find a nice selection of cams to go with this combo and unless you want to rev above 7000 RPM you won't need bigger ports.

-Andy