W210 320 CDI Decat

[Robster_1981]

Since owning my CDI I've been itching to remove those two big cat's under the car to unleash a bit more power/sound out of that beast like 3.2 engine.

At first, I was planning cutting them out with a grinder and welding in two straight bits of pipe in their place, but, I found a company on Ebay that sells repair pieces for exhausts with the swaged ends, which looked ideal. So, I emailed the company with my intentions, and the guy replied saying he would like me to take a few photo's so he could design a full decat system in return for a discount on the job.

Off I went under the car with my phone camera and a tape measure and drew up a sketch with possible designs for the decat. we are now at the stage where they have requested me to remove my exhaust (centre section) and send it to them so they can copy it using stainless steel, then send it back to me. This is going to cost £150 which seems quite reasonable, but it does seem to be a lot of messing around on my part.

I am half considering it, and half thinking I should just revert to my original idea and cut them out and weld in some mild steel tube.

What do you guys think?

 

[trapperjohn]

You could cut out the cats, empty them and weld them back on? If you are based anywhere near Warrington Pipewerx do a cracking job of decatting and they have all the sleeve bits and wotnot to hand.

They did my 124 300D 24V. (They took out the back cat and centre box) Centre box is going back in as its too "raspy" without. Passes MoT first time every time on emisions BTW

 

[Robster_1981]

I'll have to look them up, I'm only in Manchester. I was thinking of cutting out the cats to gut them. Quite a bit of work, but would cost £0

Have you noticed much benefit since having it done?

 

[trapperjohn]

In honest truth no. But at least I didn't have to buy a new cat and middle box which were about done.

Pipe Werx - Car and Bike Performance Exhaust and VAG Motor Engineering Specialists

 

[Robster_1981]

In honest truth no. But at least I didn't have to buy a new cat and middle box which were about done.

Pipe Werx - Car and Bike Performance Exhaust and VAG Motor Engineering Specialists

Thanks John. I might just have a hack at the pre-cat next to the turbo first and see what difference that makes since it is pretty easy to take off and clear out.

If it's not going to make a huge difference, then is it really worth it I ask myself

Thanks for the info

 

[taint]

I wouldnt clear out the pre-cat because you might end up with less power and more turbo lag, because of the size of the pre-cat the exhaust gas will not take a direct route out and end swirling around in all sorts of directions and cause turbulance which will result in unwanted back pressure if you empty it out. The best bet with the pre-cat is if you empty it, you gonna have to put a pipe in there to make sure the exhaust air is not lost in that big cavity of a cat.

 

[Dieselman]

I disagree. With the ceramic honeycomb removed ht egas will flow more freely so the turbo should spool a little quicker.

 

[mejermy]

I've done this on my W211 220cdi - first cat removed, attacked with a cold chisel and refitted empty, and the mid section soot trap removed by taking off the section, cutting open the top of the box, removing what is really only chicken wire, and brazing up the box again. Because the box is cut on top the alteration cannot be seen when inspected on a ramp. I've also done the first cat only on my d-in-law's car, same as yours. Mine now gives up to 45mpg on a run at legal speeds, and the d-in-law's around 35 when driven by my son in similar conditions (but he's inhis 30s and still has a bit of the Fangio mentality, as many of us did).
On a higher mileage car I would urge you to do this, together with the EGR shunt and the swirl flap removal. D-in-laws were disgusting at 190,000 miles, mine very bad at 150,000 miles.

 

[amg_Charged]

Leave it alone its a diesel, why butcher a perfectly running car? It you want and need more power and a better sound, another car would be my choice

 

[taint]

sorry late night posts and all, i meant low pressure not back pressure,

ok by emptying out the precat you now have a big empty chamber yes its true that rate at which exhaust gases flow would be increased as it enters but it will not travel in a straight route and will bounce around in the empty pre-cat as the gas fills it causing turbulence and reduced exhaust pressure which will slow down exhaust gas velocity, and reduce the scavenging effect you normally get so the engine has to use a little more energy to push out the gas.

Now dont get wrong a emptied out pre-cat will perform better then a semi blocked pre-cat. but you have to remember that the pre-cat is huge for a reason to maintain a good constant flow rate (when complete) without the turbulence and if you where to add up the space the gas travels through in in-between the honeycomb it should be the same size as the flange entrance. basically a hollowed pre-cat will not perform better then an new unblocked pre-cat because of the adverse effects of having a massive empty chamber, this is the reason well designed performance exhaust back boxes are not just hollow cans.