Removing heads with engine in car in Engine, Transmission, Drive Line, Etc Posted Friday at 04:48 PM 1 hour ago, tony-muscle said: I was thinking about this, so I am wondering if Pence's opinion still holds true for a 408 that in theory is breathing about 16% more air than a 351, or about 1,000 rpms? I would think so. Remember, 4V ports are huge and should be more than capable of flowing whatever your 408 can throw at it. My main point, there is something else going on here. You aren't falling off a cliff at 5K RPMs because of valve shrouding.