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Exhaust Catalyst removal/knock through.


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Has anyone knocked out the cat that sits in the first section of the original exhaust? or would you?( CrD 2008 ) Are there any benefits to exhaust flow if you did? The cat is there to burn any unburnt fuel etc but i think it would offer some resitance to the exhaust flow.


When i removed the first section (to weld in a 1/4 BSP socket EGT) it dawned on me that you could knock out the cat and possibly improve exhaust flow. Is it worth it or not?


If all the after market large bore exhausts are fitted with a cat then i am obligated to leave it be.


Would be nice to hear your thoughts





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Firstly, the "cat" in ZD30 is diesel oxidation catalyst It is there to convert leftover oxygen in exhaust gases from CO and HC to CO2 and H2O.

They are not used for NOx control thats a job of EGR in this case.


That device is/can be a great source of problems/restrictions if it starts getting damaged/trashed it restricts the exhaust, in any case it is unwanted restriction.


To knock out the internals .... environmental vandal or not, sure to free up flow of exhaust gases and consequent benefits of that are there but at increased polution levels.


Aftermarket exhaust are usually fitted with less restrictive cats thus making both the EPA and the engine performance happier. Tampering with vehicle emission control system can land you in a lot of trouble.

This whole thing is a grey area especially in Diesels as not all vehicles have the DOC (Diesel Oxidation Catalyst). ZD30 was one of the cleanest Diesels around when it came out thanks to this device as well. You will find that EPA and various other gubermint bodies are going to disagree with your view too


I read a "white paper" on Diesel emissions/polution in light vehicles from 2001 and there authors were advocating legislated compulsory retrofit of DOC and DPF to every light Diesel vehicle to lower the emmisions, with compulsory testing vehicle emmisions annually and forcing owners of aging poluting vehicles to have all these emmision reducing technologies fitted at their expense. That paper has not been forgotten, it is waiting to be implemented once there is a right time for it.

This is huge and is coming whether we like it or not, the days of Diesel engines are numbered. It will be so expensive to operate that in the end noone in their right mind will be purchasing such vehicles.


To "f" around with all these addons like SCR, EGR, DPF, DOC is gonna put such financial pressure on operating Diesel powered vehicles that it'll be simply cast aside in cars especially. Just look what Nissan is doing with the latest Y62 and up and coming Pathfinder models.

Today a majority of engine failures/breakdowns on commercial vehicles are due to emmisions tech shackles, its a major disaster for the industry.


Nissan has droped DPF from this year Navara STX with the Renault sourced Diesel V6 line up, I wonder why.....


Here is an anecdote for you: when I visited World's second largest commercial vehicles manufacturer powertrain facility I had the chat with a few engineers about latest development in Diesel emissions tech. A truck engine with EGR, SCR, DOC and DPF will have cleaner gas/air coming out of the exhaust pipe than the air in and around NY City. It has been measured by these nutters. When I saw the size and complexity of that stuff hanging of the engine I just shook my head. Its time for electric vehicles me think.


Back on the topic; I'd leave the OEM cat alone and fit aftermarket exhaust with high flow cat instead.



Edited by Rumcajs
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Thanks for your insightful and informative advice.

At the moment the cat does not appear to be causing more than its designed restriction on exhaust flow.


Fitting an aftermarket exhaust goes on the list.


As a footnote to removing the first section of the exhaust (post Turbo). By removing the VNT vacuum actuator and the bracket that holds the actuator in situ (bolted to turbo) the heat shield can be readily removed and replaced. (CRD 2008).


Thanks once again.


Great reply.

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