Why do people de-cat

I have no cats, but now its MOT time and it seems difficult as the car won't pass with no cats.

So, why do people de-cat instead of putting a hi-flow/sports cat on that allows BPU boost levels and will pass an MOT?

Basically I am thinking of putting a sports cat on so I can go anywhere for my MOT with no issues on emissions and am wondering if there is anything that I am missing about why decat could be better than sports cat.

Basically decatting nets you anywhere between 40 and 60 BHP! Supras are generally pretty good on emissions , mine passed the test year after year with no problem, because of there age the emissions data is classed higher anyway.

Cats are restrictive. No cat means easier exhaust flow. For supras, the big gain comes from the fact that boost increases and therefore power also does. Maybe less lag too?

Mine's decatted, and would have a sports cat fitted, but my local friendly MOT centre doesn't test the high idle, so it passes every time.

some wont even pass on idle either, had probs before with other menbers on here... and your freindly mot station needs to be careful as alot are being closed down for not testing a vehicle correctly.

I'll be going with a high flow cat on my current project. It's just a PITA having to either swap the cat/s back on or having to make sure your 'friendly' station is able to do it.

Decatting gets the extra power, but hi-flow cats will still give the same power increase won't they? but pass an MOT.

Basically I am looking for reasons why people don't fit a sports cat, I know why people decat from standard. Think my title was a bit confusing.

Decatting gets the extra power, but hi-flow cats will still give the same power increase won't they? but pass an MOT.

 

Basically I am looking for reasons why people don't fit a sports cat, I know why people decat from standard. Think my title was a bit confusing.

I would fit a sports cat, but there's about a 5-10% decrease in power, and they cost around £300 IIRC. For that reason, I will wait until I really have to get one, but I probably will, eventually.

How much is a 'sports cat' ?

Decatting gets the extra power, but hi-flow cats will still give the same power increase won't they? but pass an MOT.

 

Basically I am looking for reasons why people don't fit a sports cat, I know why people decat from standard. Think my title was a bit confusing.

High flow Cats are still more restrictive than a hollow pipe. Like OW says, don't get one unless it's neccessary for you.

Decatting gets the extra power, but hi-flow cats will still give the same power increase won't they? but pass an MOT.

 

Basically I am looking for reasons why people don't fit a sports cat, I know why people decat from standard. Think my title was a bit confusing.

Any cat will cause a restriction in flow, even the hi-flow variety, and any restriction will rob power from a turbocharged engine, just for example i decatted my Supra when i first got it, it already had an aftermarket 5zigen exhaust, i found it a bit loud so i fitted a baffle to the tail pipe.

Put it on its first RR and found it was only making 330 BHP, took out the baffle and gained 50BHP;)

I am considering buying a double decat very soon but I need to get the car MOTd again (Jap TT). When they last did the test it came up as engine number not recognised "tested as 2 gas" unsure what 2 gas is or how high the emissions would be if it had the double decat on.

Can you pretty much switch the first cat without issue and just swap in the second cat for MOT? seems to be a bit of a grey area on what will pass and what won't.

How much is a 'sports cat' ?

A decent one that fits the Supra is expensive. SARD do 2 options, from memory it was £600+ just for the 1st cat.

They are expensive because Platinum is usually used inside as the catalyst.

Cheapest option would probably to get a generic 'sports cat' and get it welded into the middle of a 2nd decat pipe ie. further away from the engine.

They are expensive because Platinum is usually used inside as the catalyst.

 

Cheapest option would probably to get a generic 'sports cat' and get it welded into the middle of a 2nd decat pipe ie. further away from the engine.

That is what I am thinking of doing. Are they very restrictive?

That is what I am thinking of doing. Are they very restrictive?

They will be more restrictive than a straight through pipe, just how restrictive they are will vary from make to make. Ideally you want one welded into the 2nd decat pipe that would allow you to remove the restrictor ring and safely run at 1.2bar without the exhaust gas temps getting too high and that would reduce emissions to pass the MOT.

There are cheap universal 'sport cats' on Ebay for less than £100 what effect they will have on emissions who knows, they could have b*gger all effect, but just add a restriction. I know most branded products will be more expensive, but £100 vs £600+ makes me wonder how effective they'll actually be.

Might be worth chatting to a 'trusted' UK exhaust fabricator/fitter to get their view on what would be the best catalytic convertor to use.

Ideally you want one welded into the 2nd decat pipe that would allow you to remove the restrictor ring and safely run at 1.2bar without the exhaust gas temps getting too high and that would reduce emissions to pass the MOT.

I really like the sound of that, and if anyone can offer a solution, I'd be interested to hear about it.

I am considering buying a double decat very soon but I need to get the car MOTd again (Jap TT). When they last did the test it came up as engine number not recognised "tested as 2 gas" unsure what 2 gas is or how high the emissions would be if it had the double decat on.

Can you pretty much switch the first cat without issue and just swap in the second cat for MOT? seems to be a bit of a grey area on what will pass and what won't.

I have posted this before but wont hurt to post again(cant find original link on this new site!)

yes, and from that its just straight across the top row for a TT supra, as it goes on engine number which is the same as the UK spec TT.

Full test with Cats.

They can't carry out a specific test on an import Supra as there is no data, its not the same as a UK spec, so they go through

on the non cat test, and most Supras will pass it.

Looking at my MOT again "no engine no found tested as 2 gas", where is the engine number located? I am fairly sure I dont have a dodgy motor

FOr the CO level it shows the max limit as 3.5 which ties up with that diagram. Question is though...if I take it to this garage again with double decat would it still likely be under this upper limit of 3.5? Quite handy if it is as a personal view

They can't carry out a specific test on an import Supra as there is no data, its not the same as a UK spec, so they go through

on the non cat test, and most Supras will pass it.

That is not correct.

Every MOT tester I have asked in the last 2 weeks....and I have asked a lot! say it goes on engine number not car model, so if its a 2JZ-GTE there is an exact match and it needs a cat to pass. If it is an NA it will be a 2JZ-GT, no match, so it will pass.

I am sure there will be MOT testers backing me up on this very shortly

Looking at my MOT again "no engine no found tested as 2 gas", where is the engine number located? I am fairly sure I dont have a dodgy motor

FOr the CO level it shows the max limit as 3.5 which ties up with that diagram. Question is though...if I take it to this garage again with double decat would it still likely be under this upper limit of 3.5? Quite handy if it is as a personal view

No cats will still be well under 3.5

That is not correct.

Every MOT tester I have asked in the last 2 weeks....and I have asked a lot! say it goes on engine number not car model, so if its a 2JZ-GTE there is an exact match and it needs a cat to pass. If it is an NA it will be a 2JZ-GT, no match, so it will pass.

 

I am sure there will be MOT testers backing me up on this very shortly

Guess my MOT place either didn't know what they where doing, or where very friendly, they could not find a match on their database,

So all cars are only tested on there engine code, sounds a little off to me, as there are a lot of models that share engines but have different spec, especially Japanese cars.

I would think they would be listed under Toyota---> Supra---> 2JZ-GTE. I don't know for sure though, I am just going by what people tell me at the test centres.

The thing is if you get stopped by the police and spot check your emissions (which apparently they are starting to do) they check the MOT its not you that gets in trouble, its the place that gave you your MOT