X/H Pipe

What is the purpose of an X pipe and H pipe in an exhaust.
Based on our experiences and judgement an X pipe or H pipe are generally fitted on V Block based vehicles. The X pipe is fitted at the start of a sports/ performance system to help balance the exhaust pulses from either side of V-style engine. We have tested systems with and without and X pipes and have discovered that by fitting a crossover section on a tuned V Block the cars produced much more torque and some BHP as well as giving better MPG. Test were carried out on a M62 Engine Tuned with an Emerald ECU.

In order to understand X pipe and H pipe benefits you have to understand what Exhaust Scavenging is all about.
Positive exhaust scavenging occurs when exhaust pulses of an engine are tuned to the performance exhaust system in order to efficiently dispose of spent exhaust gasses from each cylinder. As exhaust gasses are produced, they exit the engine in pulses and create a small vacuum. This vacuum acts and helps pull the next pulse of exhaust gasses and so on, this helps a complete a full exhaust gas cycle. An X pipe forces these gases from each independent cylinder to converge together in the same direction, when the gases work together they speed up the flow which in return creates are greater scavenging in the exhaust. It is essential to use the correct bore (bigger is not always better with naturally aspirated V Block engine), a true X piece and a good ECU tune (carburettor tune on older cars) to achieve gains.
Whether to choose an X pipe or H Pipe depends on your goals. Whether you want low end torque or top end power.
Where to place your Cross Over Section? This is largely dependent on your application, as you may have clearance issues, transmission positioning and chassis design. Where possible it is best to place the X pipe as close to the front of the system as possible. It is best for the gasses to converge as soon as possible. This is not a universal law, as it is possible that neither would be efficient on your exhaust, your stock system may consist of Y piece or a 2 into 1 section which also promotes scavenging.
Much of the preference to fit a cross over pipe is the sound that they produce. X pipes in an exhaust system generally produces that V8 rasp or gurgle that you find on more modern V8s such as AMG models (C63 E63 S63 etc.) or BMW M cars (V8 M3 V8 V10 M5 M6 etc.).
H Pipes are common on Old- School Muscle cars to produce the classic rumble particularly on Idle. Each of the above application have there on purposes on the road. Some are just cruisers others for performance.
To fabricate our X pipes we use mandrel bends. Using 4 x stainless steel true 90-degree mandrel bends we cut the bends at specific angles on our band saw. The bends are then prepped and welded together as pairs, followed by another precisely calculated cut. Once the pair have been sanded and straightened out they are tig welded to achieve you X pipe.  Our X pipes are also tested to ensure that the follow of the pipes is correct, the results are that the air travelling through the inlet will come out the opposite side. The cross over in the middle is what helps cause the back pressure or scavenging.