Sports cats (100 and 200 cell): are they worth it?

In 2007 an NA owner asked if anyone had done back-to-back testing of a standard system against a sports cat (a 200 or 100-cell unit), as he had read they free up little for a high cost. The replies are summarised here. Treat the numbers as owners' estimates.

What owners reported

• One owner says a weld-in sports cat improved performance over the standard cat. His was a ready-made unit under £100. The section he photographed is downpipes he made for a Weapon R 4-2-1 manifold, with the cat welded in, but he says you could simply cut the standard cat out and weld a sports cat into the standard pipe.
• The same owner says it gives a couple of bhp, but the engine response is improved and the exhaust note is deeper.
• Another owner, who had seen the rolling-road data, says 1 to 2 bhp isn't worth £100 in his opinion.
• Another owner who recommends a sports cat says his evidence is seat-of-the-pants only, though it changed the air/fuel ratio on his wideband. He thinks Thorney may have some Milltek data.
• A 2013 reply put the sports cat at 2 to 5 bhp when paired with 2.5 inch pipework, against 5 to 10 bhp for a 4-1 manifold. Another owner says the parts most people change do very little in the grand scheme, and that the manifold and U-bend are the key ones. See NA exhaust tuning: which parts restrict.
• An owner who wants an inexpensive route cut out the main cat and welded in a 200-cell sports cat early in his tuning and says it made a noticeable difference.

Fitting advice

• One owner says to fit the sports cat to the biggest entry and exit pipe diameters you can find, at least 55 mm.
• Another owner whose pre-cat was already removed asked whether swapping the standard cat for such a unit would give gains similar to a roughly £500 sports cat section. The thread doesn't answer that.
• Fewer cells flows better, but there is less catalytic coating. One owner says some 100-cell cats set the engine light if the second sensor is badly located. See Post-cat lambda sensor replacement.

Why a pre-cat removal gives less than you might think (one owner's analysis)

An owner who says he has worked on exhaust flow design explained in 2007 that the flow through a catalyst is channel flow down each cell, and the pressure loss depends mainly on the cell diameter and the brick length, so fewer cells per inch means lower pressure loss.

• The pre-cat sits close to the ports, so the gas velocity there is very high, and the flow can't be evenly spread across it for a single valve event. Removing it should reduce pressure loss.
• But you probably won't see the full benefit: you leave an expansion at the end of the runners, a space where the gas recirculates, and a contraction back into the rest of the exhaust, all adding loss.
• The main cat sees slower, smoothed flow, but on the NA the gas turns through two 90 degree bends before the cat, which he expects hurts the flow uniformity.
• Sleeving the old pre-cat area down to the bore of the rest of the pipe, at a constant angle, should reduce the space for gas to recirculate. See NA pre-cat removal.

Header design points from the same thread

• For a DIY 4-branch header, pay close attention to how it pairs with the exhaust ports. You don't want an exact match: you want a step from the ports up to the larger header tubes, because matching them exactly would lose power. That is one owner's view, and another owner who read it already knew of the anti-reversion idea.
• The same engineer ran a computer flow model of a 4-2-1 layout that joins cylinders 1 and 3 and 2 and 4 and follows the current exhaust path, and found uneven flow between cylinders 1 and 2 and 3 and 4 because of short secondaries. He planned to use open space under the boot and a 1-2, 3-4 layout, which would only work with a Piper or Larini style backbox. No result was posted.