The catalyst (also referred to as a pollution or smog control) resides inside the exhaust. Its job is to create cleaner emissions by causing a reaction which reduces the CO2 and NO2 in the exhaust. Because they are constructed of a matrix they impede the flow of exhaust gases and have been blamed by many as power stealers. What options are out there that regain this robbed power?
Primarily there are 2 options to get faster flowing gases through the exhaust:-
The first and easiest is referred to as a de-cat. It is quite simply a pipe that sits where the catalyst should go and effectively the entire catalyst is removed.
What are the drawbacks of a de-cat? It is illegal in most states and countries unless your car can still meet its stringent emissions requirements without one. Many people swap back the catalyst at the time of the annual test to obtain a roadworthyness certificate but the Police are increasingly using spot checks at the road side to catch people out.
Some engines will also have a probe further up the exhaust from the catalyst and if this reports ‘dirty’ exhaust fumes it will adjust the ignition timing typically running the engine lean. This will result in a loss of power and could even cause the ecu to go into a limp home mode.
The second and most professional option is a sports catalyst. These are manufactured to higher tolerances than the OEM ones and have much better flow characteristics. The drawbacks of the sports cats are usually the cost but in real terms they cost little more than a standard original brand catalyst.
When you put a car on a rolling road what sort of results can you expect to see? Car Tuning Tips took a Mitsubishi Evo IV onto a rolling road and the baseline power with the original catalyst was 295bhp.
With the decat pipe this increased to 305 bhp and then when the sports catalyst was introduced power reached 304 bhp. So we have to conclude that there is very little perfromance difference between a sports cat and a decat. The standard EVO catalyst is actually very good which is why we see little gain between them but on more humble engines power gains reach around 5-10% higher.
If your catalyst needs replacing we would strongly recommend that you source a suitable sports catalyst rather than going for an illegal decat option. If you are using the car off road then the decat would be the sensible and most cost effective option open to you.
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8 responses so far ↓
1 robin // Dec 3, 2009 at 5:19 pm
i have a decat pipe, also with a sports cat on my janspeed twin exit exhuast.. so i got best of both worlds and power because my decat is racing rep adding 12 bhp (dyno tested) with the sports jansped cat also adding more power ( Dyno tested again)
so if you can afford both its so worth it
2 hilldawg // Jun 15, 2010 at 3:51 pm
lol your clueless mate, why lie? you cant have a decat and a sports cat at the same time they replace the same piece of pipe haha….
3 Moe // Oct 13, 2010 at 7:14 pm
Im pretty sure what robin meant was having bought both but not on at the same time e.g sports cat for mot purposes and decat for the rest of the year or for the race track.
4 scooby // Dec 23, 2010 at 4:19 pm
A decat pipe and a sports cat …. WOW – how did you manage to fit them together ?!
Does this mean that the end of your tail pipe sticks out further than it should
Numpty!
5 simon // Jan 21, 2011 at 12:22 am
can somebody help? i bought this toyota celica impo, the problem is that the exhaust system is too loud, any ways to bring the exhaust sound down? it has a hks induction kit.
6 Brett // Feb 28, 2011 at 9:04 pm
Best to visit an exhaust centre and let them take a look at it, also the induction kit won’t help on the noise front.
7 josh // Mar 6, 2011 at 5:38 pm
wanting to fit decat to my astra vxr, it will go into limp mode becasue its new and ecu is the ultimate boss. someone said about o2 extender to stop it doing this, any ideas what they mean?
8 Andy // Oct 13, 2011 at 11:43 pm
Why is he clueless? You can run a decat 2nd cat pipe on an impreza, while keeping the cat in the downpipe, or having a sport cat downpipe in place.
The second cat can be removed for road use also if a sportcat downpipe or original downpipe cat is used, as the prodrive ppp pack cars have this without being illegal or failing mot’s.
Thanks
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