Can any power or delivery gains be expected on my N/A (Naturally Aspirated) VS V6 with the use of a 25mm plenumspacer over the 12mmplenum spaceror even not having a spacer?

As stated it is recommended to use the 12mm plenum spacer for N/A applications. The 12mm plenum spacer improves air distribution between cylinders as well as increasing plenum volume resulting in low end torque gains. Engine performance is improved when utilizing a plenum spacer compared to the standard plenum:

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