de-cat my derv

[mrk]

im still kinda undecided what to do, he says it will be £80 for a backbox removal and new tail pipe but only £70 for a de-cat. i can obviously change my mind up till the point it gets done but ive heard there wasnt enought back pressure on the mk3 tdis to run them also i heard the management light wood come on.

 

[devonmikeyboy]

Removing the CAT is totally pointless as you will not get any power gain,throttle response,noise etc. Removing the CAT on a diesel car will not bring up a cell light as there are no censors to do this, people telling you this are petrol tuners as that happens on petrol cars not diesels. You also don`t need to worry about backpressure on diesel cars as they run differently to petrol cars, thats petrol tuners applying petrol tuning to diesel cars again. To get the most from modifying your exhaust all you need to do is remove the stock backbox and replace it with a free flow item.

 

[mrk]

so if i was to remove my cat on a tdi ibiza there would be no increase in noise?

 

[devonmikeyboy]

so if i was to remove my cat on a tdi ibiza there would be no increase in noise?

Very little. The CAT is designed to remove particles not noise. The backbox is designed to remove noise not particles. Work it out for yourself.

 

[mrk]

ive got an upgraded box on the car just now,but it was only a wee crappy universal s/s box. so if im looking for an increase in noise im best off just removing the box and putting in a straight bit of pipe plus a new tail pipe and just leave the cat on

 

[devonmikeyboy]

If you want lots of noise then remove the backbox. Personally i think it sounds shite and will boom like hell when on the motorway. The type of backbox you have and the type of tail pipes ie twin round are quiet but a single large oval is loud will make a lot of difference.

 

[mrk]

im going for a large oval with no backbox and no centre silencer and im gonna leave the cat on

ps devonmikeyboy

was reading the review on your a3, thats some machine man lol

 

[devonmikeyboy]

im going for a large oval with no backbox and no centre silencer and im gonna leave the cat on

ps devonmikeyboy

was reading the review on your a3, thats some machine man lol

For the cost of removing both boxes it might pay to ask how much it would be to just have a new exhaust pipe made from CAT back removing CAT and both boxes in stainless steel. Friends have had that done and depending on area and who does it should cost about £100-£200 all in. It will be very load though so bee warned but if thats the sort of thing you want then thats what i would be looking at doing.
Thanks for the compliment about my car i do try. The write up is 1.5 years old now so had lots of new toys fitted since then. Car is getting worked on at the moment but told them i want it back in two weeks as they have had it for several months to play with.

 

[Muttley]

devonmikeyboy wrote

The CAT is designed to remove particles not noise.

No, the 2-way catalyst on a diesel engine catlyses the oxidation of carbon monoxide to carbon dioxide and of hydrocarbons to water and carbon dioxide. The hydrocarbons are from unburned/partly burned fuel and while it is true that soot particles are also from partly burned fuel, as solid particles they are not in a form that the catalyser can help to oxidise. Particulate traps are fitted to some newer diesels.

The catalyst will provide some back pressure, but nowhere near as much as the turbine. Turbocharged engines are already extracting a lot of energy from the exhaust, providing significant silencing as a byproduct. Removing the catalyst and/or silencer boxes will affect the noise made by the car a lot more than the power it produces.

Any mods to the engine's gas flow, from the air filter inlet trunking to the end of the exhaust pipe, should be accompanied by a remap to make sure that positive benefits are gained.

 

[Clone]

Devonmikeyboy - so what you are saying is - there is no real need for a turbo back exhaust? Instead just get an after market backbox?

 

[devonmikeyboy]

Devonmikeyboy - so what you are saying is - there is no real need for a turbo back exhaust? Instead just get an after market backbox?

Basically yes. All aftermarket exhaust systems follow the same route as the stock system. Aftermarket exhausts including Milltek are only about 5mm wider internal bore which for everyday use is fukc all and not noticeable either on road or dyno. Unless you are going hybrid turbo or you think the CAT is blocked/cracked for some reason there is no real point in removing it. Replacing the stock backbox with a good ( not the cheap Halfolds shite ) free flow one is all that is needed to get the best power from your exhaust. If you are running a stock or remapped car i personally don`t see the point in spending hundreds on an exhaust.

 

[Clone]

thats pretty good to hear... is there a website that you guys use to buy your milket backboxes from? Or other well branded names? The ones i used to use don't do them for my car.

 

[devonmikeyboy]

Any custom exhaust place can sort you out with a back box and tail pipe of design you like for your car. Just ask around your local area for places people recommend. Don`t forget most places are franchises so a good powerflow/custom chrome etc garage in place A has nothing to do with how good powerflow garage in place B will be.

 

[devonmikeyboy]

I am in no way knocking Milltek as they do make very good exhausts with good quality metal that is very thick and should last the life of most cars, hell i buy them myself but you do pay for the previlage.

 

[Clone]

Got some good advice today on here, i'll look into that then, cheers! We should start a rep system and people ge rep'ed for useful info

 

[devonmikeyboy]

What i have posted is my personal opinion others on site or other sites might disagree. It is up to you to try and wade though all the crap and misinformation that gets posted on forums and try to make your own mind up as it is your money. There is no set in stone right or wrong way in doing things. What you must remember is petrol and diesel tuning is totally different.

 

[Fl@pper]

dont panic about back pressure its basically a myth power wise but none at all can lead to lean running motor depending on the engine setup

in fact the only worry with it is a melted cat/blocked exhaust makes too much - try breathing through a straw for an hour you'll see what i mean

more fuel+more air = big bang and get waste out as soon as possible

 

[Danny_FR]

Agree with Fl@pper, turbo'd cars aren't so much bothered about back pressure as the turbo produces enough torque (one of the only products of back pressure). No back pressure at all would be a problem but I can't remember why!

 

[daveyonthemove]

Agree with Fl@pper, turbo'd cars aren't so much bothered about back pressure as the turbo produces enough torque (one of the only products of back pressure). No back pressure at all would be a problem but I can't remember why!

Trust me mate, my car was a bag of shite without the cat and it had to be down to back pressure. I'm unsure why mine seems to be the only one that suffered with this, but i can promise you that it did. Good backbox is adequate on the TDI's IMO

 

[Danny_FR]

Trust me mate, my car was a bag of shite without the cat and it had to be down to back pressure. I'm unsure why mine seems to be the only one that suffered with this, but i can promise you that it did. Good backbox is adequate on the TDI's IMO

How odd is that? Even on N/A petrol cars I've never had a problem with de-cats. I'm still undecided as to what to do to mine, back box only or straight thru from the cat back... I'll hear yours soon enough anyway to make my mind up!