What is the best combination of Manifold insulator and Plenum spacer for Ecotec V6 3.8L Engine?

For N/A (naturally aspirated) applications we recommend the 25mm manifold spacer and 12mm plenum spacer.

If you are running boost we recommend a 25mm plenum spacer and 12mm manifold spacer, as the thicker plenum spacer does offer even better air distribution between cylinders, which is more critical in boosted cars making lots of HP.