One of the earlier reasons for the "H" pipe, which Ford typically used on all of their '60s-era factory dual exhausts, was for sound issues, which could also relate to balancing the pressure pulses, side-to-side. Back when all of their factory dual systems used 2.0" pipe, no matter what the car or engine was (which I didn't realize until years later while looking at a then-current Walker Exhaust catalog. Same muffler on a 427 2x4bbl V-8 as a normal 390 300 horsepower T-bird.
GM didn't use an H-pipe, nor did Chrysler. IF the sound control issue was the real reason, then ANY crossover pipe/duct between the two sides would do, including the heat crossover passage in the intake manifold. But when a friend (as other "hot rodders" used to do) blocked the passage on his '79 Corvette, no appreciable change in exhaust note or sound level, that we dould pin on that one change.
The X-pipe is a more blatant effort to share flow, side to side, of the exhaust out-flow. According to a former Panther-body Ford website, the use of the X-pipe raised the frequency level of the exhaust. Using two X-pipes multiplied that effect. On a 4.6L V-8 LIncoln Mark, it went from normal rumble, to a higher pitched rumble, to a "scream" (in that progression) as the first and second X-pipes were added to the dual exhaust system.
Consider, too, that if you're going to use an H or X pipe, that automatically means that you've got to run the two exhaust pipes tightly parallel to the driveshaft. Which also works much better on IRS cars as the driveshaft does not go up and down, but remains "in its place", so to speak.
Also, the placement of the transmission tailshaft housing in relation to the rocker panel height will have a bearing of how the exhaust system hangs, which can also mean that ANY system with an H or X pipe could have to hang lower than the rocker panel, just to use them. I can tell you from experience, having to watch were one drives is a BIG PAIN with a lower-hanging exhaust system (as on my '77 Camaro after one muffler shop sold me some thicker-than-stock mufflers. It has one fist's-width of ground clearance, which even affected how I had to drive into my shallow-angle driveway without hearing scrapes! Not good--period!
Remember ONE THING about OEM's vs. aftermarket. IF the alleged improvement is enough the average customer will feel it and "feel value" in the change, it'll happen. Especially if it means more horsepower and/or better fuel economy (both VERY marketable areas). If not or any other compelling reason based in engineering facts, it either becomes optional or doesn't happen.
Example: For the '58 model year, Buick introduced their Triple-Turbine DynaFlow automatic transmission. Better torque multiplication off-the-line (now termed "launch"), for a mere $58.00 on the window sticker. BUT other than some possible exterior name plates, nobody knew your car had it. Spend that same money on two-tone paint or white wall tires? Instant recognition. People could SEE those things, but not a "hidden" transmission change. Buick spent $3M on that transmission, to see it last for three model years. People who had it liked it, but if the general public couldn't see it sitting on the sales lot, no interest.
The aftermarket exists to allow owners to upgrade/customize their vehicles, as desired. Nothing wrong with that, BUT sometimes the real benefits of such modifications lie in the customer's head. The added performance which many things offer will only show up on drag strip ET slips or in possible better throttle response, but usually NOT at the gas pumps, for example. If electronic ignition didn't offer some real customer benefits, it would not have been an option that later became standard equipment (as emissions regulations tightened, back then), for example. Helped the service industry as mechanics didn't have to spend as much time laying on fenders to change/adjust ignition points, too!
The ONE real benefit of an H-pipe is that should the front u-joint fail, the front of the driveshaft might contact those pipes first before it starts to rotate against the moving ground under the car. Same principle as a "drive shaft loop", just probably not recognized as such by drag racing safety inspectors.
Just some thoughts,
CBODY67
