Northstar Engines and System Technical Discussion Discussion, Is It Possible To??? in Cadillac Engine Discussion; Bore an intake manifold from a 99 concours? I know stock is 80mm, and 70mm for TB, is it possible ...
I believe something similar was done in here a year or so back. Net result was a lot of work for no gain: The stock N* intake system already flows more air than the engine needs at WOT, and larger TB's tend to introduce a low-mid RPM flat spot due to reduced air velocity screwing up the MAF or flow signal.
If you want a little more intake noise to make it sound faster, (forget the so-called CAI's) rig a 3 1/2" - 4" tube from the bottom or side of the existing air box and run it down to just behind the lower grille area on the driver's side. Several people in here have done that and are happy campers. (The stock (panel ?) filter is plenty big, but if you want a bit more surface area, check out the WIX numbers for your car.)
Don't forget: An engine is actually an assisted air pump. Barometric pressure pushes air into the engine, depending on butterfly position, to try and fill a constant vacuum. The engine does not "suck" air in. Rig a good vacuum gauge to the manifold and observe the readings at WOT. Anything less than 3-4" of vacuum is as wide open as it gets. Even if the TB were as big as a sewer manhole opening, the engine can't get any more air through normal aspiration (NA). When you're tooling around town or putting down the freeway at 60, manifold vacuum is high, say 16-20". When you slam the throttle open, vacuum drops radically, the engine goes rich to compensate, and atmospheric pressure tries to force air into the vacuum created by increasing engine speed. Very elegant. Even with a blower, the intake will always run in vacuum until a significant power demand occurs. You actually lose HP at low-load road speeds with a blower due to the mechanical load.