YES!! THIS^!! You have to basically DOUBLE the CFM when switching from a properly built/tuned engine with a dialed open plenum single plane to a 2x4 tunnel ram. 340=2x750, 440=2x850, etc. We ran 2x660s on our FED rail 383 (Offy TALL tunnel ram) and I felt it needed 2x750s, we went to a 471" stroker and put a M1 (short runner, bigger plenum) tunnel ram on it with two 850s and I wanted to try 2x1000's on it!!