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Turbo diesels need a good free flowing exhaust system, with minimum back pressure. However, since 1996 they have been fitted with a simple oxidation catalyst, primarily in my view to reduce the smell by burning off partially burnt fuel. This obstruction is responsible for a power loss. For example, the Peugeot 1.9 TD engine lost 2bhp when fitted with a catalyst. Not a lot, and quite possibly only this figure is lost on the BMW engined Rover 75, MG ZT and Landrover Freelander TD4. Certainly, decatting a standard engine will most likely lead to disappointment if a significant power gain is expected, though there are other advantages...

However, on a tuned engine I am confident that the cat is going to be a serious, power sapping restriction.

Apart from any power gains, cat removal also reduces turbo lag and gives a more instant, petrol-like throttle response.

By the way , not having a cat does not cause MOT failure as it has no impact on the smoke test.

The Sebring back box with twin 90mm tailpipes. They are double skinned so the actual bore is only about 50mm. They are a bit OTT on a 75, twin 75mm ones would have been better.

However, it is a TUV approved and homologated system and is no louder than the original, though it does give a lower, more sporty sound.

Even with the cat removed, the noise level is unchanged at steady speed and not too loud when revved.

The Sebring and original system compared. The Sebring system is designed to fit onto the original centre pipe once the old back box has been cut off about half way down the straight centre section.

The fit was perfect - it does not contact the body work or bumper even on bumps. It is a quality item and quite heavy too when you are trying to

get the rubber mounts over the hangers, lying on your back under it!

The old silencer was even heavier - probably due to 4 years of accumulated soot!

The section from the right of the cat to the far right white mark was retained because it has the hangers fitted.

So the decat pipe is just the length of the cat. However, the flexy pipe outlet is 65mm OD whilst the cat outlet is 60mm OD so the decat pipe needs a reducer.

The decat could be done retaining the original exhaust though this would defeat the object to a large extent.

Other back boxes could be used, but I suspect a straight through one would be too noisy and possibly boomy. This was the case when I decatted my 45: it was unbearable at 70mph with a Powerflow one fitted.

RIGHT: The hangers which are retained , with the pipe being cut off close to the cat (shown)

The cat was in good condition internally with no sign of melt down or broken honeycomb.

Big enough for 116ps, even 131ps, though I wonder how many bhp its costing, but not adequate for 150bhp though!

The original system is in one piece - there are no joints at all. If the flexy pipe fails, you have to replace the cat as well if you want to use a Rover spare part. (Part no WCJ101001 price £500) Otherwise you will have to buy a flexy pipe and have it welded in. These are available from Camskill.co.uk for about £60.

The rear section is part no WCG102831 price £142. I believe this fits in a similar manner to the Sebring.

1) Undo the 3 bolts holding the flange on at the turbo outlet elbow. Keep the stainless gasket, it can be reused.

3) Undo the bolt holding the silencer on and disconnect the front rubber hangers. The system can now be pulled out from under the car.

4) Carefully measure from the rear bend to the end of the Sebring and transfer this to the old system, deduct about 75mm to give the cutting point allowing for the old pipe to fit inside the new one.

5)Cut off the cat as close to the end of it as possible. this leaves a stub of about 30mm to the left of the hanger.

6) Cut off the flexy section as close to the cat as possible. This also leaves a stub of about 30mm.

7) Weld in the decat pipe, making sure the hanger is on the underside! Not like I did, the first time as you can see below!

My welding is not a pretty sight but there are no leaks! To correct my mistake I had to cut the decat pipe through and rotate the rear section 180^o

^{8) Remove the silencer clamp and fit the
Sebring. Next slide the front section into the Sebring pipe and then fit the
downpipe onto the flange loosley. Now refit the centre rubbers onto the hangers
and finally tighten the flange nuts, tighten the Sebring clamp. Refit the tunnel
cross members.}

The Sebring system is not cheap, but it fits perfectly, the tailpipes are level and parallel to the centre line and do not touch the bumper. Smaller diameter ones would have been more in keeping with the 75 I think. Its a heavy, solidly built system with chunky tailpipes.

Constant speed: Noise level inside is unchanged and from the outside is probably just a deeper note.

Accelerating: At low rpm, the low sporty note can be heard which gets louder as the revs rise. Even at 4500rpm, the noise is still less than from the engine.

The reduction in turbo lag is very noticeable. The only thing I missed when changing from petrol power to a turbo diesel was the instant response to accelerator pedal movement. All turbo engines suffer lag and a cat makes this worse. This was the case on my 45 and I now know that it is also the case on my 75. I would recommend a decat just for the lag reduction even if there was nothing else to be gained.

The other benefit is a nice free revving feel and willingness to reach the redline. This is reached in the first 3 gears with no feeling as though the peak has been passed. In fact it will hit the rev limiter in 1st and 2nd. It seems to need less throttle to maintain a steady speed.

There feels to be a bit more torque from as low as 1500rpm or its possibly because with the reduced lag, the torque comes in more rapidly. From 70mph, there is definitely more torque, with 90mph being reached very quickly.

Dyno runs planned when I've finished trying the various Synergy settings. I might experiment with new maps for it as well.

Results: Torque was 237lbs ft and BHP dead on 150 - a little less than expected, but this wasn't a direct before/after test so the actual hp gain could have been quite significant. However, the improved torque and reduced turbo lag are worthwhile alone to justify the fitting of the decat pipe.

Still not a bad result when barely a year ago, the car in standard form with 91k miles on it, recorded a feeble (but normal for a CDT of this mileage) 111bhp & 191lbs ft!

I am pleased with the results so far and look forward to the dyno tests. How close will I get to 160bhp!

I did 460miles on 10 gallons over the weekend. This was mixed roads, from motorway at 75ish, to the single carriageway A1 in Berwickshire & Northumberland at 65ish, with plenty of overtaking. It was very windy. The reduced turbo lag, improved torque and willingness to rev to the red line made overtaking numerous convoys of slower cars trailing behind HGVs, a doddle. Superb!