Since JMG Porsche LTD are local to me I asked their advice on the differences in use between an x-pipe and the under/over design of centre silencer delete pipes, Jon Mitchell very kindly wrote back with a highly detailed response which I will copy below hoping others may find his knowledge useful. Based on this I have decided to go the Topgear X-pipe route for my Black Edition C2 PDK.
Audio effects of an "under over" design.
Your engine has six cylinders, divided into two banks of 3 cylinders, with all the cylinders firing at different times.
With a "over-under" design of rear silencer delete, each side will be serving only three cylinders, with strong pulses travelling through the exhaust to the tailpipe, with a very low negative pressure wave between them.
With two tailpipes being about 4 feet apart, when the high pressure pulse leaves one tailpipe the sound pressure will expand outwards in all directions behind the car, including towards the other tailpipe.
With the two tailpipes both producing pulses, completely out of phase with one another, they can collide behind the car, which can cause some unusual effects, usually between 2500 and 4500 rpm.
With the speed of sound being constant (the speed the pulses move and expand) but the rate of them being delivered to the tailpipe being variable (depending on the engine RPM) these collisions will happen at different positions behind the car depending on rpm.
The effect of this is that at low rpm, the two tailpipes having individual pulses, completely separated, at a higher amplitude, with a low pressure wave between them, can result in a more aggressive idle exhaust note.
At higher rpms (typically 2500 to 4500 rpm), there can be a problem of resonance, where collisions between the two pulses behind the car, will begin to collide, which will be absorbed by the car as sound waves, and transmitted into the cabin as sound. Which can cause droning exhaust sound inside the car between 2500 and 4500 rpm, often described by some owners as the exhaust sounding "boomy" at cruising speeds.
This is less of a problem with cars like the 997 with the tailpipes separated (meaning collisions have lost much of their energy), compared to other models like the Cayman where the tailpipes are closer together (and the pulses still retain their energy and the collisions are louder)
Audio effects of a "x-pipe" design.
With an X-Pipe, both banks join momentarily in the exhaust, which allows the peaks of the pressure wave to equalise into the other banks low pressure wave, and the low pressure zones behind each pulse pull pressure from the other bank also.
This results in slightly lower pressure at peak at each tailpipe, and a less defined low pressure wave also, which removes much of the air pressure waves outside of the car, which in turn causes the collisions to be much reduced and little or no chance of droning/boominess inside the car at cruising speeds.
It also means that the sound at idle in particular will be slightly less aggressive and a warmer smoother sound.
Audio effects conclusion.
With either system, because the rear silencer has been removed, will result in a louder sportier note, with the "over-under" or "x-pipe" design, simply because the silencer has been deleted.
The "over under" design will be slightly louder at low rpm with a more aggressive note, with the "x-pipe" design sounding more aggressive and louder at higher rpm.
But much of this depends on the rest of the exhaust system, such as the manifolds and other silencing effects such as the catalytic converters.
Performance.
In short, a "over under" design will provide better low rpm torque, and a crossover design will provide improved top end rpm power, which is why race cars often use the x-pipe design, because they are designed to be closer to the rev limit constantly, whereas a road car will be pulling out of junctions and cruising, where low rpm performance is often preferred.
This is due to pulse length tuning, or the amount of time it takes the positive pulse to hit lower pressure air (either at the tailpipe in a over-under design, or the crossover and tailpipe in the x-pipe design.)
With an exhaust design, the time it takes for a positive pressure wave to reach the tailpipe (or crossover) will cause a negative pressure reflection to head back into the exhaust system, which depending on RPM, if it hits the an cylinder head valve just before, or as it opens, will help such exhaust gasses out of the cylinder, rather than the engine having to push them out, this is known as scavenging, and really depends on the exhaust system design as a whole.
As a complication to this, the standard manifolds are not a great design, and are different lengths, which effects the pulse width tuning, causing different cylinders to benefit at different rpm ranges, which does complicate things from a performance standpoint.
All of these aspects, effect the performance and why you will hear of exhaust systems as "Scavenging" or being "Pulse length tuned", and how they will effect power in different ways at different rpms.
Other effects.
With the advent of PDK transmissions, all of this becomes more complicated, as depending on your mode of driving the transmission will dictate high or low rpm bands to spend most of the engines's time is spent in, which then dictates which type of exhaust would benefit you most depending on your driving style.
All the best
Jon Mitchell
JMG Workshop Team
Telephone: +44 (0)1202 488800 – Website:
www.jmgporsche.co.uk – email:
[email protected]
JMG Porsche Ltd – 98 Cobham Road, Ferndown Industrial Estate, Wimborne, Dorset, BH21 7RE, England.
