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Author Subject: The truth about decatting your exhaust
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

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Post #1
Now guys and gals I'm looking to get a definitive answer about whether removing the cat from the exhaust will result in power loss, power gain or make no difference in the world. I'm not looking for theory or ideas but hard evidence if possible. Ideally I'm looking for a rolling road of a 6 with and without cat fitted with the rest of the spec being the same or very similar. I know I'm asking a lot there but let's see what people have. I'm just wondering why large companies like maniflow, spoox, pug1off etc make decats if they result in power loss?

Cheers

Dave
Posted 21st Dec 2014 at 19:06
Rich E Forum Admin

Location: Hertfordshire

Registered: 27 Apr 2005

Posts: 7,757

Status: Offline

Post #2
Different engines respond in different ways.

On the GTi-6 engine you'll lose a little torque low down in the rev range as the engine is tuned to use the exhaust back pressure. Higher up in the rev range it will be freer-flowing, and it will pull to the rev limiter more easily.

Ideally you'd want an engine map designed for use with a decat as the standard map has extra enrichment higher in the rev range to keep the exhaust gas temperature from getting too high and damaging the cat. With the cat removed this enrichment can be mapped out to get more power at the top end.

________________________________________

Rich
Posted 21st Dec 2014 at 19:12
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

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Post #3
Cheers Rich, so unless I'm going to use an aftermarket management system that has good mapping ability there is no point in me removing my cat yes?

Dave
Posted 21st Dec 2014 at 19:23
pug_306

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Location: Great Hockham

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Post #4
I find the genuine cats seem to provide more power than the aftermarket pattern part ones.

________________________________________

1998 Supercharged White Rallye
1992 205 GTi6
205 Turbo 340bhp
Posted 21st Dec 2014 at 19:32
buzzbrightyear

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Location: hiding all receipts for car

Registered: 09 Jul 2008

Posts: 11,894

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Post #5
I thought I lost power when I fitted one back in my standard days.

________________________________________

andrew315rawson@live.co.uk
Moonstone phase 7 gti6
My Project thread: http://www.306gti6.com/forum/showthread.php?id=119992&page=1
Posted 21st Dec 2014 at 19:38
prism7guy

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Location: DoncastAAAAARGH

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Post #6
What Rich says.

I had my daily driver gti6 with the oe cat fitted, tried a de-cat then went back to oe cat, nothing else changed on the car and there is very noticably more low end torque with an oe cat fitted. There does seem to be more top end power with the de-cat but imo not really worth it on a road car unless you love pops, bangs and flames.

________________________________________

Goldie the track car.
Posted 21st Dec 2014 at 19:57
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #7
Well I've currently got a 200 cell sports cat fitted and was on the vehicle when I bought it. However, I have read people commenting saying the sports cats are crap and that you should only use or cats? Probs very pricey from pug though
Posted 21st Dec 2014 at 20:10
cully

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Location: Cullompton

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Post #8
bought my 6 with a decat and stainless long-life single box system,
I changed it back to a std system when I drove it to Germany it gained power in the bottom end enough to notice a difference and it also stop making my ears bleed so its stayed on the car ever since
Posted 21st Dec 2014 at 21:44
adam b

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Location: The Nam

Registered: 24 Jan 2006

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Post #9
Had both, now run a decat due to car living at high revs and weight is less. Get nice bangs from the PS magnex with it too

________________________________________

Nothing to see here
Posted 21st Dec 2014 at 21:56
walker_texasranger

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Location: Sunny Retford

Registered: 26 Jan 2008

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Post #10
Last time I checked with Pug a cat was around 590 quid including VAT Dave Smile

________________________________________

Almost standard 1999 Bianca Rallye
Hilux InvincibleX daily

306 No.13 Doh

Carp and specialist Anglers Click here
Posted 21st Dec 2014 at 22:06
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #11
walker_texasranger wrote:
Last time I checked with Pug a cat was around 590 quid including VAT Dave Smile


What!!!!!!!!!!! I'd rather buy a set of b8s with top mounts Wink
Posted 21st Dec 2014 at 22:41
walker_texasranger

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Post #12
Mossy1985 wrote:
walker_texasranger wrote:
Last time I checked with Pug a cat was around 590 quid including VAT Dave Smile


What!!!!!!!!!!! I'd rather buy a set of b8s with top mounts Wink


Thumbs up

________________________________________

Almost standard 1999 Bianca Rallye
Hilux InvincibleX daily

306 No.13 Doh

Carp and specialist Anglers Click here
Posted 22nd Dec 2014 at 10:23
ash

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Location: Chelmsford

Registered: 25 Mar 2003

Posts: 7,520

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Post #13
I think I've got an original cat in good nick, I'll have a look.
Posted 22nd Dec 2014 at 10:37
Chr1sch

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Location: Worcester

Registered: 02 Aug 2014

Posts: 1,025

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Post #14
I recently decatted mine, its has been mapped but dont know if it was mapped for decat or not (it made 174bhp @ fly) (it came with a decat that had clearly been used...)

I have also stripped out the rear of the car, and it runs lighter wheels etc.

In all honesty its too loud, but as a track day car that only ever gets thrashed I much prefer the pull towards the top end. For example it pulls much harder from 4-7k rpm, which sees it comfortably pull past 120mph on the speedo where as before it eeked up to that kind of speed.

I'd love to know what has actually been done to my car mapping wise, one thing I would say is that my tickover is smoother and it doesnt pop or bang at all regardless of how hot it is. Based on comments above that sounds like it may have been mapped 'for' a decat.

Performance wise it is identical to a Clio 172 with an exhaust...
Posted 22nd Dec 2014 at 10:59
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #15
Cheers Chr1sch that's good to know, I'm just umming and arrring whether to revert back to a standard cat, leave my sports cat on or decat. Decat seems too much noise and then there is the emissions too. So it's probs a toss up between sports cat and one cat. However if I were to fit a 4-2-1 manifold I would have to decat............... f**k sake lol. Are you running standard ecu? As I thought mapping the standard ecu was either very limited or basically not doable?
Posted 22nd Dec 2014 at 13:32
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #16
ash wrote:
I think I've got an original cat in good nick, I'll have a look.


If you could ash please
Posted 22nd Dec 2014 at 13:33
Chr1sch

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Location: Worcester

Registered: 02 Aug 2014

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Post #17
Mossy1985 wrote:
Cheers Chr1sch that's good to know, I'm just umming and arrring whether to revert back to a standard cat, leave my sports cat on or decat. Decat seems too much noise and then there is the emissions too. So it's probs a toss up between sports cat and one cat. However if I were to fit a 4-2-1 manifold I would have to decat............... f**k sake lol. Are you running standard ecu? As I thought mapping the standard ecu was either very limited or basically not doable?


Great question, and i have no idea! - Where is the ECU?!

All I know is that i have a before and after rolling road printout/booklet in the receipts that came with the car, with a 11hp gain - i've seen ecu's on ebay discribed as 'unlocked'?

It may just be rose tinted glasses, but with it stripped out as a track car, the decat suits it IMO
Posted 22nd Dec 2014 at 15:08
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #18
Ecu is behind the battery in a separate tray. So you have a rolling road of the car without a cat and then another rolling road of the car without a cat after getting mapped? And you saw 11 bhp from the map? That correct?
Posted 22nd Dec 2014 at 15:53
ian7675

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Location: Gloucestershire

Registered: 12 Oct 2007

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Post #19
Mossy1985 wrote:
walker_texasranger wrote:
Last time I checked with Pug a cat was around 590 quid including VAT Dave Smile


What!!!!!!!!!!! I'd rather buy a set of b8s with top mounts Wink


If you think that's a lot, try getting one for a Civic TypeR.

________________________________________

Less than 24 hours to go!
Posted 22nd Dec 2014 at 16:02
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #20
ian7675 wrote:
Mossy1985 wrote:
walker_texasranger wrote:
Last time I checked with Pug a cat was around 590 quid including VAT Dave Smile


What!!!!!!!!!!! I'd rather buy a set of b8s with top mounts Wink


If you think that's a lot, try getting one for a Civic TypeR.


Thankfully my toda manifold did away with it Smile
Posted 22nd Dec 2014 at 19:54
Chr1sch

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Location: Worcester

Registered: 02 Aug 2014

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Post #21
Mossy1985 wrote:
Ecu is behind the battery in a separate tray. So you have a rolling road of the car without a cat and then another rolling road of the car without a cat after getting mapped? And you saw 11 bhp from the map? That correct?


Sorry not quite, the car came with the decat in the boot and it had been on the car until recently. The car had a huge wad of receipts within it including the rolling road, but I have no evidence at all what was or wasn't on the car at that time. All I know is I've refitted the decat and it (in my opinion pulls much harder and has a smoother tickover)

I'd love to know what's been done to the car, it has quite a few trick bits on it the previous owner clearly wasn't aware of (eg lightweight crank pulley etc )
Posted 22nd Dec 2014 at 22:59
Mossy1985

Senior User

Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #22
Ok I'm with you, you fitted decat back on and you think it pulls better and harder but only at the top end? Ie after 4k? I'm really wanting to improve low end grunt if possible which would mean a 4-2-1 as apposed to the 4-1 that is standard. Question is can I find a 4-2-1 that allows me to kee the cat? Most unlikely I would say
Posted 22nd Dec 2014 at 23:03
adam b

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Post #23
You are wasting your time and money. Map won't make any more power without cams, stick with the standard manifold as again won't help you.

Improve handling and brakes. Make it lighter. Amen. Or charge / itb it if you think it's worth spending that much cash on.

Should annihilate a 172 tbh

________________________________________

Nothing to see here
Posted 22nd Dec 2014 at 23:26
Chr1sch

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Location: Worcester

Registered: 02 Aug 2014

Posts: 1,025

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Post #24
I don't mean to be contentious (and I agree with you wholeheartedly about weight loss/brakes etc) however without a significant power increase how could a heavier and less powerful 306 ever annihilate a 172?

I haven't gone silly but have no rear seats/carpets/trim etc, fitted lighter buckets and wheels and im still heavier than a standard 172. Yes I am quicker through the corners bits certainly not an annihilation

If you're looking for low down torque I wouldn't decat based on a million comments I've seen across multiple forums, however it does suit the nature of a track car IMO
Posted 23rd Dec 2014 at 07:51
Mossy1985

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Location: Darlington

Registered: 04 Nov 2014

Posts: 545

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Post #25
adam b wrote:
You are wasting your time and money. Map won't make any more power without cams, stick with the standard manifold as again won't help you.

Improve handling and brakes. Make it lighter. Amen. Or charge / itb it if you think it's worth spending that much cash on.

Should annihilate a 172 tbh



I've bought a brand new full b8 set up, a referb jord axle with 21mm torsion bars, baker b m rear beam mounts and baker b m adjustable top mounts, phase 1 xsara vts steering rack and some c5 steelie so I think I'm about there on the suspension/handling set up bar an lsd as I wanted a car that would handle before I looked at increasing power big time

Posted 23rd Dec 2014 at 08:39

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