NA pre-cat removal: the four methods owners used

The NA has a small catalyst section (the "pre-cat") in the top of the downpipe, close to the manifold. It was a very early NA mod to remove it. This page records the methods owners reported. The photos in the 2020 thread are lost, and the other threads have no usable photos.

What owners say it does

• An owner in 2003 said figures of about 15 bhp had been quoted by other members. Another owner thought 160 bhp from the pre-cat alone was optimistic: with K&N induction, the pre-cat out, a sports exhaust and a remap he thought he had about 165 to 170 bhp.
• Several owners say the throttle response and revving improved: "the engine revs more freely", "sounds more aggressive", and one reports better fuel economy. One calls it the best performance mod he has done.
• Another owner bets the power gain on its own is not noticeable, but that it paves the way for other mods when airflow restrictions matter.
• An engineer owner points out that removing it should reduce pressure loss, but you may not see the full benefit: removing the brick leaves an expansion at the end of the runners, a space where gas recirculates, then a contraction back to the rest of the pipe, all of which add loss. See Sports cat and exhaust flow.

Method 1: take the pipe off and drill it out (2003)

One owner's outline. He says it takes about four hours to do properly:

1. Jack the rear and remove the rear undertray. See Jacking points.
2. Remove the heat shields from the top of the downpipe. A later owner says you don't need to: the three nuts at the top end go onto captive studs, so he wasted a long time removing his.
3. Remove both oxygen sensors.
4. With a long socket extension, undo the three bolts holding the pipe to the manifold (spray with penetrating oil first).
5. Undo the three bolts joining the downpipe to the rear silencer. They are often badly rusted: one owner says use lots of WD40, and if they snap, replace them.
6. Remove the whole pipe including the cat by sliding it off the big rubber grommet. The pre-cat is now visible in the top of the downpipe.
7. Drill a series of close quarter-inch holes round the edge, dipping the bit in oil often as it gets very hot. Then chisel the rest out with a cold chisel.
8. Wearing a dust mask, grind out any remains with a flap wheel.
9. Vacuum out all the dust and bits. Another owner adds you should remove the lambdas if you can, so no loose material is drawn back to the engine or blown into the main cat.
10. Refit in reverse, with copperslip on all the nuts.

Method 2: drilling in a vice

• One owner used a long 3 mm bit to drill all round the edge, then moved to larger bits until it came out. It took about 1.5 hours. He recommends a strong vice and water or oil on hand, as the bits heat up quickly.
• Another found this far harder: he got through about five drills (the catalyst melts onto the bit, so it ends up like a food blender), had to chisel the innards out with a long thin cold chisel, and says leave a full day. He still thought it worthwhile.

Method 3: hammer and stave (about 30 minutes, 2007)

1. Put the pre-cat pipe in a bench vice.
2. Take a steel or wooden stave 10 to 20 mm smaller than the pre-cat diameter (not much smaller, or the remains are harder to remove). Place it on the pre-cat and hit it hard with a heavy hammer. The structure breaks in a circle and slides down a little.
3. Screw a long screw into the middle of the pre-cat and pull the pre-cat out of its housing.
4. Break away the remaining material from the housing with a screwdriver. This is easy.
5. Optionally clean and polish the inner housing with a steel brush on a drill.

Method 4: wood screw (2020)

1. Drill an 8 mm hole in the middle of the pre-cat.
2. Screw a 10 mm wood screw into it with a cordless screwdriver.
3. Clamp the screw in the vice.
4. A few hammer blows on the flange and the pre-cat comes out of the pipe in one piece.

The owner who posted this calls it the easiest way, taking less than three minutes of work, without red-hot drills. His photos are not available. Another owner joked it brought "bolted horse springs" to mind.

After removal

• Sleeve the bore. One owner advises sleeving the old pre-cat area back down to the bore of the rest of the system, or you risk a loss of power. The engineer owner agrees: sleeve it down at a constant angle to meet the pipe diameter, so there is less space for gas to recirculate. Nobody posted the sleeve dimensions.
• Clean it out. Vacuum all debris, as above.

Two owners warn against fast-fit exhaust centres for this job. One says it is straightforward but takes care and experience, and worries that they won't clean the pipe out properly.