What difference will a decat centre section make to noise?

[Lrchorne]

I have a Hayward and Scott backbox (which i am keen to sell as it is the non-turbo fitment I accidentally ended up with) and want to change back to my P1 embossed prodrive back box.

Would the hayward and Scott decat centre section improve the noise? The P1 has a loud engine and although the P1 genuine exhaust makes a nice noise it is too 'sensible' and quiet.

Any input on how the centre section will alter this appreciated.
Cheers
Lawrence

[DocJock]

Slightly deeper/louder, decatting the downpipe has far greater effect.

If noise is a concern, look out for a Prodrive resonator centre pipe such as was fitted as part of the PPP.

I run the Prodrive backbox with decatted downpipe and centre pipe and in all honesty it is too loud.

[Lrchorne]

But if you do the downpipe as well as the centre section don't you struggle with emissions tests?
I had a prodrive backbox and then fat downpipe on my old '94 turbo wagon and that gave it a slightly better noise and occasional mini pops on the overun when hot.

I don't want chav loud, and certainly no droning, just a little more burble so i can hear it over the engine and makes tunnels a bit more fun.

Hayward and Scott centre section will not affect MOT and only costs £100!

[markCSC]

Yeap you'll get a bit more noise with the centre section but little power gains.

I have an H&S back box and centre section, which I think is just the right balance between "nice" noise and OTT. And it does pop and burble when hot

A decat downpipe would give more power and noise but:

a) it is technically illegal to have no cats
b) it is a pain to refit the cat for the MOT
c) you might find it way too loud.


Mark

[DocJock]

markCSC said:

Yeap you'll get a bit more noise with the centre section but little power gains.

I have an H&S back box and centre section, which I think is just the right balance between "nice" noise and OTT. And it does pop and burble when hot

A decat downpipe would give more power and noise but:

a) it is technically illegal to have no cats
b) it is a pain to refit the cat for the MOT
c) you might find it way too loud.


Mark

Yep.
So is exceeding 60 on a SC NSL or 70 on a motorway...

I don't disagree with any of your points tbh.

[dnb]

markCSC said:

a) it is technically illegal to have no cats
b) it is a pain to refit the cat for the MOT
c) you might find it way too loud.

a) No it isn't. It never has been illegal in the UK, but this might change in the future. All the car has to do is to do is pass the emissions test without cats. (This is difficult, but not impossible)

b) See a). If you have to do this, then it IS illegal without cats.

c) Possibly. Mine's 100.2dB without any cats. And 100.0 with a centre cat only. Either way it passes the MOT, although it's much more difficult (ie it doesn't pass the test first time) without any cats.

[ScoobieWRX]

Fully decat you would have to run hotter spark plugs (e.g standard and not PFR7B) and have it mapped so that it ran very lean at low RPM's/engine loads specifically for the MOT test, and then have it mapped back to what you had prior to the MOT afterwards.

If your aftermarket system was designed to mate up to the std system then changing the downpipe or centre pipe back to a single 600cell OEM pipe isn't very hard to do and doesn't take very long at all. Or do what i did and get a downpipe with a 100cell sports cat installed as i currently have and you'll get through the MOT, although that's going soon in favour of a full decat downpipe.

Alternatively find yourself a friendly MOT centre and just enjoy the noise. Life's too short!!

Edited by ScoobieWRX on Saturday 10th October 17:55

[DocJock]

ScoobieWRX said:

Alternatively find yourself a friendly MOT centre and just enjoy the noise. Life's too short!!

Listen to this man, he speaks sense.

[Animal]

Interesting. Had my car MOT'd in March (standard save for a Green panel filter) and it failed emissions first time round. I had to run the engine for several minutes to get it to pass.

Have since had a decat centre (and a new backbox) and I get the feeling I'm going to struggle come MOT time. And there was me considering a 100-cell downpipe, remap and decat uppipe for Christmas!

[Lrchorne]

I only want the noise to be a bit better and with a gentle pop when warm from a de-cat centre I think I am sold.
Problem is my ill-fitting non-turbo H&S backbox is annoying me (you can hear a little knock over speed ramps as it moves around) so might put the P1 embossed prodrive upgrade box back on that I hyave but am concerned it will be too quiet again even with centre section.

Thoughts?

[jensen2000]

My experience with a Calibra V6 was that having a de-cat pipe made a massive difference in sound, cost just £30 and took ten minutes to swap in/out for long journeys to keep the OH happy.

I reckon the difference in SPL is commensurate with my swap from std back box to "jap-style" straight through custom made stainless jobby.

With yours being a turbo you may get even greater gains, you will also need to pay attention to what the experienced advise because your back pressure could significantly drop which will likely affect you turbo's behaviour (overboost!?) and such.

What's the worst that can happen if due precautions are taken? Most Jap turbo-nutters dive for the de-cat within seconds of ownership as a quick route to extra "bee-aitch-pee" and noise. (My GT-R33 is de-catted btw. But I've yet to run a comparison with the cat in place )

KR
Jensen2000

[JollyGrnMonster]

generally find the centre makes more noise in the car with a cat as the noise remains in the box and then into the car.. where as decat centre makes more noise out the back but less noise in the car..

downpipe cat is the biggest restriction..

Simon

[Animal]

Having done a bit of a snoop around it seems as though the best catted downpipe for an Impreza is the Prodrive-made one - are these an eBay-only find?

PS - Simon: if I did switch to a sports cat (I've already got a decat centre) would I need a remap ASAP? I'm trying to stagger the cost!

Apologies for the hijack!

Animal

[ScoobieWRX]

I've got a Kakumei 100CELL Sports catted 3" ceramic coated downpipe and i'm now ready for a 3" decat!! Swapsies anyone??