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FWIW, H-pipes were developed because they were less expensive and lasted longer than resonators. Back in the '50s it became obvious V8s with separated dual exhausts developed a resonance at cruising RPMs. GM started adding resonator-eliminator/mufflers behind the axle. Because these ran so cold, they rusted out quickly. Ford developed the H-pipe to eliminate resonance. It cost less and didn't rust out. The X-pipe was developed first for NASCAR about ten years ago. It seemed to make more power with the extremely long duration cams they run on the speedway.
There are those here and on other fora who swear the X-pipe to have made large HP gains measured on the chassis dyno. Guess I just haven't had the right setup on enough cars, but I haven't seen the big numbers.
thnx, jack vines
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