Cat Back Exhausts

Shikkaka · 1 Mar 2006 at 16:41

Hi everyone,

These cat back exhausts? Don't they fail your mot because of emissions? If so whats the point in getting one if you gonna fail every year?

Cheers

L0rdMike · 1 Mar 2006 at 16:43

Diff cars have diff tests. My 1994 car doesnt need a cat.

But thats cause its an import and the engine was never released in the UK.

needmorespeed · 1 Mar 2006 at 16:52

cat back exhausts normaly leave the cat in place so you shouldn't fail the MOT. De-cat pipes remove the cat which could cause you to fail the MOT

Lowe · 1 Mar 2006 at 16:59

I rekon this should do the trick:

merlin · 1 Mar 2006 at 17:00

Hawk Tuah · 1 Mar 2006 at 17:06

Cat back exhuasts just replace the bit of pipe after cataletic converter. The name kind of gives it away.

Therefore it makes absolutely no difference to your emissions. You could still get done for noise though, but I dont think db's are a part of the MOT test.

nutcase · 1 Mar 2006 at 17:29

I've never really understood the point of leaving what is usually one of the most restrictive parts of the car in place - the downpipe and cat. Do they really make that much difference?

Dogbreath · 1 Mar 2006 at 17:35

The backboxes on some cars can be quite restrictive. Downpipes usualy are not, though they are rarely designed for peak power. The point of a cat back system is to get a sportier exhaust note, maybe a small power gain and all without worrying about emissions tests.

Clarkey · 1 Mar 2006 at 18:12

its great having a car which isnt in the emmissions database

The only test done is a standard non-cat one done at idle, so you can have the wordls most polluting 300zx and still pass the test

Hawk Tuah · 1 Mar 2006 at 18:36

nutcase_1uk said:
I've never really understood the point of leaving what is usually one of the most restrictive parts of the car in place - the downpipe and cat. Do they really make that much difference?

Most people leave the cat in because otherwise your car will fail an MOT. Ok, you can put it back in every year come MOT time, but to be honest, most people arent that obsessed with a few HP to bother. The alternative is a race cat, but they are megabucks.

I got a stainless cat back exhaust because my old exhaust was knackered and a good looking, deep sounding custom stainless exhaust with a 10yr guarantee was was only £20 more than a stock OEM one.

To be honest, you'd be mad not to.

Dogbreath · 1 Mar 2006 at 18:58

Clarkey said:
its great having a car which isnt in the emmissions database The only test done is a standard non-cat one done at idle, so you can have the wordls most polluting 300zx and still pass the test

MR2 is the same, but hasn't stopped me having a huge argument with two diferent MOT stations at test time! Last one I had to go home and print off the relevant section of the manual as he refused to belive me!

L0rdMike · 1 Mar 2006 at 19:00

Yeah MR2 Turbos upto Aug 95? dont have to have a cat I think.

Bear · 1 Mar 2006 at 19:51

Bug One said:
The alternative is a race cat, but they are megabucks.

£200 isnt exactly megabucks, although it isnt cheap.

ramirez · 1 Mar 2006 at 20:02

I think it's 1994 Mike but yeah it is handy having an old import isn't it

My Soarer TT doesn't need a test for cat either.

Meridian · 1 Mar 2006 at 20:38

Dogbreath said:
The backboxes on some cars can be quite restrictive. Downpipes usualy are not, though they are rarely designed for peak power. The point of a cat back system is to get a sportier exhaust note, maybe a small power gain and all without worrying about emissions tests.

Actually it's usually the other way around, especially on forced asperation cars (on NA cars the exhaust is seldom the restriction at all). Look at it this way: on its way out of the engine the exhaust gas is at its hottest (about 8-900oC) and thus occupies the most volume for a given weight of gas. It therefore needs the maximum diameter pipe to escape through. As it passes through the exhaust it cools considerably, and thus occupies less and less volume - thus needing a lower diameter to achieve the same flow-rate as higher upstream. Bug One probably has it best: catback systems are really just about noise (and some bragging rights).

Bear

You can get a sports cat for £200? Jammy goit - mine is costing me £425!

M

Zip · 1 Mar 2006 at 22:43

Isnt the new Subaru Impreza terribly restricted by its cat?

Fett · 1 Mar 2006 at 23:03

Zip said:
Isnt the new Subaru Impreza terribly restricted by its cat?

Great article here. If you can't be bothered to read the whole thing and want a quick answer, just check Test 1 against Test 2 results, the only change being the exhaust.

Meridian · 1 Mar 2006 at 23:06

Zip said:
Isnt the new Subaru Impreza terribly restricted by its cat?

Yes - which follows from what I've said above. Assuming things haven't changed since my 02 then the scoob has two cats, both fairly restrictive. Some go for full decat, but all you really need is to replace both cats with a single sports cat. The centre section and backbox don't need changing as by that stage the gases have cooled a fair bit.

M

Zip · 1 Mar 2006 at 23:12

Fett said:
Great article here. If you can't be bothered to read the whole thing and want a quick answer, just check Test 1 against Test 2 results, the only change being the exhaust.

so about 30-45 bhp difference :eek:

Also was that on the new scoob or the older one because ive heard the new one is far worse then the older one and has no power after about 5k or 6kRPM when the limiter goes up to about 8k or 9k RPM?

The new impreza im thinking of:

Siliconslave · 1 Mar 2006 at 23:16

+ a decat is pretty wise if your running rich / overfueling on lift off as the excess fuel tends to ignite, killing the poor kitty

(decat & induction opened up ~30 bhp on my sx. Made 220 on the RR2 when its supposed to be 190 stock)