Decisions decisions

[Michael6457]

You can get new replacements front cats from @Elementian most members decore the rears but the fronts can still fail with the decore rears in place, if you decore the fronts not sure how this works out with the front and rear O2 sensors

 

[Lpgc]

if you decore the fronts not sure how this works out with the front and rear O2 sensors

It works out very good, no different.

The front O2 sensors are between the engine and front cats so they're not affected anyway.

The rear O2 sensors are fitted between front cats and rear cats so normally see the oxygen level in the exhaust gas after exhaust gas has been through the front cats. With no front cats fitted they see the same exhaust gas as the front O2 sensors but more of an averaged (over time) signal than the front O2 sensors. On some vehicles this would cause an error code because the main purpose of the rear O2 sensors is to check that the cat is working properly but on E51 Elgrands it doesn't cause any error codes or effect mixture or effect how the engine runs... It seems as if the rear sensors on E51's are only fitted to 'tick a box' - The ECU doesn't use the data from them, I use the data from them when I run electronic tests to see if front cats are fitted/working/blocked/etc.

The above is why particularly for people who's E51 is LPG converted I don't see why they spend extra money on aftermarket front cats when they could simply remove, decore and refit the original front cat sections. If you run on LPG you're allowed much higher emissions than if you run on petrol, while at the same time if you run on LPG the emissions are lower anyway. Once you've got the front cat section removed from the car it's an hour or two's job to decore it, versus buying a new front cat section and pay for the expensive platinum / cat material inside it. You've got to remove and refit front sections whether you refit the standard (decored) section or a new aftermarket section, so if you decore the original section there's the extra hour or two to decore it but it will go back on and mate up to the rest of the exhaust as standard. If you replace with an aftermarket front section you won't have to decore it (obviously) but you've got to pay for that new section and won't know how well it fits and mates up until you fit it.

No cats at all on my black E51, the front pipes fit as well as standard because they are standard except for being decored, I've seen aftermarket front sections that contact/touch parts of the car underneath that they shouldn't contact. The aftermarket rear part of the exhaust that was on it when I bought it is louder than standard while at the same time flows worse than standard so restricts power a bit (not surprising though given the middle box of the aftermarket system is a lot smaller than standard), the opposite of what I want and I would like to fit a standard middle box and back box if I can get one. My silver Elgrand has no cats fitted, standard but decored front end, standard middle end, Rider back box, quieter than my black one and also flows as standard (because it is standard, just decored) so doesn't restrict power. Black one is my daily driver so I wish I'd swapped rear ends of exhausts when I had the opportunity (when I fitted another engine in the silver one) but now that's done I might as well just buy a standard rear exhaust for the black one rather than swap exhausts on 2 Elgrands.