'69 VS. '70-'71 Spec Mag MFI stacks

[Scott Clarke]

The '69 stack was 45mm ID at the top surface, and, if memory serves, 38mm at the bottom. The '70-'71 stack was 42mm at the top and the same bottom diameter as the '69. The '72-'73 stacks had dimensions like those of '70, but were plastic. Why did porsche change in '70, making the stacks smaller at the top when at the same time displacement increased? Anyone have any theories?
-Scott

[69911e]

For those obsessed with weight; You will also find the 69 stacks are much lighter.

[jpnovak]

I often wonder if the decrease in CR and emissions were the reason. A smaller stack will have a higher intake velocity for a given volumetric flow rate. The air would then expand more as it entered the heads thus cooling and helping suspend the fuel for better burning.

maybe the increased intake velocity helped with the lower compression engines that still used the same cam specs. The overlap of an E or S cam should have a higher compression to make much torque at the lower rpms. smaller intakes would increase the velocity and momentum allowing more air to pack into a cylinder.

I could be way off, just dreaming.

[Scott Clarke]

Jamie-
It sounds like you are saying that the '69 stacks would flow more CFM at high RPM, even though the ID at the base is the same. I guess this makes sense, and I like your theory about the CR issue. I thought that CRs were about the same between '69 and '70, but Porsche could have been (correctly) anticipating future reductions.

[mpdevelopment]

The 69s 2.0liter was just closer to a race engine. The factory got more and more conservitive as time went on. Those stacks fit the very peaky nature of the 69s engine.