QUESTION: So which size spacer do I get, 5/16" or 1/2"?
ANSWER: It depends. You WILL make power with either one, and both will make for a cooler sounding induction growl (the 1/2" slightly more so), but keep a few things in mind:
The 5/16" spacer makes good power 'under the curve' in the engines mid-range (4000-5500rpm) which is where the engine spends most of it's time. The 1/2" spacer moves the power further up the rev range (5500-redline) and is good for 1-2 more peak horsepower.
So which one? In our humble opinion we prefer the 5/16" because of it's excellent, usable, mid-range power gains, but go with the 1/2" if you are going with forced induction (or you just want the most "throaty" induction growl).
IMPORTANT NOTE: While the spacers make excellent power on the older VQ35DE engines, the REVUP engines receive little to no benefit from the spacer alone. However, combined with the MREV2 manifold no other naturally-aspirated bolt-on makes more power on the REVUP VQ35DE engines!