The size of the intake runners varied over the years but not the port size on Z heads.
The typical Z intake port is 34mm in diameter.
The
cross sectional area is 908mm˛
The typical runner diameter on an early Z carbed
intake manifold is 34.5mm. It matches the port in the head.
The cross sectional area is 935mm˛. (This combines to a total cross sectional area of 5905mm˛ for all 6 intake ports.)
The typical runner diameter on a later Z EFI intake
manifold is 30.5mm. Obviously, it is narrower than the head port!
The cross sectional area is 731mm˛. (This combines to a total cross sectional area of
4384mm˛ for all 6 intake ports.)
The EFI manifold runner is a restriction when you compare it to carbed intake manifold and when you think about its interface with the head.
The typical carbed Z intake manifold mouth diameter
is 47.2mm. Since there are two, the equivalent cross sectional area is 3499mm˛.
This equates to 582mm˛ per port cross sectional flow.
Since the combined cross sectional area of the same manifold's runners is
5905mm˛, the smaller mouth is a 40.7% restriction.
The correct mouth and
throttle body diameter needed to match output cross section is
61.3mm.
The carb manifold runner is a restriction when you compare its input cross sectional area to its total output cross sectional area.
The typical EFI Z intake manifold mouth diameter is
52mm. The cross sectional area is 2124mm˛.
This equates to 354mm˛ per port cross
sectional flow.
Since the combined cross sectional area of the same manifold's runners is
4384mm˛, the smaller mouth is a 51.6% restriction.
The correct mouth and
throttle body diameter to match output cross section is
74.7mm.
The EFI manifold runner is a restriction when you compare its input cross sectional area to its total output cross sectional area.
Bottom Line
-
EFI manifold runners are a restriction and do not match head ports
-
Carb manifold's runners match head ports and flow better than EFI manifolds
-
Both EFI and carb manifold mouths flow less than the sum of their runners. ( However, the intake duration for each port is only 1 stroke of a 4 stroke motor AND it is proportionate to how far open the valve is so it is can only draw up to 25% of its maximum draw. Thus comparing input and output cross sectional areas is not significant).
Suggestions
-
acid port EFI runners
-
use bigger throttle bodies and port the manifold mouths
-
two 52mm TB's on a carb manifold would make a nice "poor-man's" setup to maximize stock parts performance (of course EFI bungs would need to be fabricated).
RacnJsn95 of
Zcar.Com's EFI injector flange welded to carb runners and dual TB.
1 fast Z of
ZCar.com and BMC discovered that manifolds from maxima LD28 motors have
larger runner's and mouth.
The LD28 runner dia is 35mm (same as the Z head and Z carb manifold runner) The
LD28 runners are longer too. They just need a TB plate and injector bungs
attached.
BMC make custom intakes for Z's
Lone
Wolf Performance make a highly recommended high-flowing custom intake.
