I should have ended the sentence in post #23 ....."X-pipe is more likely causing a power LOSS"..... with, across the RPM range.

Many exhaust manufacturers that use X-pipes with unequal length tubes and report HP gains have dyno tested the X-pipe location so the scavange effect is occurring right at peak HP RPM.

These manufactures usually will not provide full RPM pull sheets comparing straight pipe vs H-pipe vs X-pipe results. In most tests the X-pipe will show a few more hp at peak RPM but an actual DROP in HP/torque at lower RPMs (peak HP #s sell exhaust systems/exhaust parts. Few buyers look into average HP/torque thru the RPM range) .

On tracked vehicles an X-pipe tuned for peak HP is a plus (the engine is kept near the optimum RPM to use the HP peak). Street vehicles operate thru the RPM range, a loss of HP/torque at lower RPM for a few more HP near redline is not a tradeoff I would accept.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!