Quote:
Originally Posted by TiAgX5
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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The general consensus is that an H-pipe equalizes exhaust pulses. A X-pipe both equalizes the exhaust pulse and improves scavenging. Depending on the application and the rest of the exhaust system components, the addition of an X pipe may, can, might improve torque and or horsepower. The following video provides a demonstration:
The XPipe Explained by Ed Hanson at Ed Hanson's Muffler Service. - YouTube
There are some applications like a low torque high winding engine, where the addition on an X pipe may decrease the performance of an application. Since getting from point A to point be is a combination of torque and horsepower the X-pipe could decrease torque even when it increases horsepower resulting in it taking longer to get to point B.
Since a H-pipe does not improve scavenging, an X pipe, assuming on that particular application it improves performance, will generate better numbers than a H-pipe, except in low torque and high winding application where the X-pipe is more likely to be a negative. There are also cases where a H-pipe will generate lesser numbers in those same applications.
The benefits of scavenging depend on the pressure of the exhaust flow combined with the amount of vacuum that follows an exhaust pulse. As RPMs increase the pulses are closer together resulting in decreasing vacuum. Less vacuum means less scavenging which means any benefits of scavenging decrease as RPM increase. At the top of the RPM range a X-pipe is the least, if at all, beneficial.
The best numbers because of an X-pipe will come from the farthest upstream the exhaust system it is possible to install the X-pipe.
It is important to know how exhaust changes impact torque and horsepower as the best changes will be those that are most effective where you need improvement, i.e--torque get you going, horsepower keeps you going.
I have been unable to find dyno numbers that are meaningful. The ones I found either changed more than just the X pipe between runs or the difference was within normal dyno error.