Custom DECAT - Milltek Murder!

[Hurdy]

DECAT your car....Why?

There are quite a few reasons why, but the main one for me was to see if it is true that decatting the car will free up some extra ponies and help the car spool the turbo faster.

Pro's

i. To free up extra power through the rev range
ii. Gives a more aural experience over a catted system
iii. Reduce that big torque spike a little

Con's

i. May not pass an emissions test to be legal on the road
ii. Some do not like the extra sound it would make
iii. Can be a pain to convert back to a catted system

The path forward

With both the pro's and con's in mind I decided to go down the route of having a system made where I could simply swap over from one to the other myself in under 15 minutes. This meant making a flanged system around 15 inches long that was fully accessible below the car and without having to resort to taking the downpipe section off.

The parts

Decatting requires 6 x 3" diameter flanges
One piece of 3" diameter stainless pipe (2 feet/ 60cm is enough)
4 x Copper nuts and 4 x galvanised bolts (6mm iso thread)

The process

Now you know I'm going to say this, but things are never as simple as they seem. Initially the entire exhaust had to come off to get access to the cat to cut it up  Cutting a Milltek system up is never a clear decision and something you take a sharp intake of breath for just before the first cut is made.

Milltek before surgery.



After surgery



The next job was to replace the remaining parts of the exhaust back on to the car and start clamping up the cat section so that the flanges are mated up correctly. This is harder than it looks as the orientation of the flanges will dictate where the bolts will finally be for removal afterwards. Also, if you look at the "after surgery" pic you can clearly see that the two ends are offset. This means that the decat section will need to be angled correctly and yet still have the flanges in the same orientation as the cat section.

Clamping up



Whilst the flanges were clamped together, they were tack welded in place to ensure that they didn't move once located correctly. Once tacked, the entire exhaust system had to come off again and be fully seam welded on both the inside and outside to ensure that the welds would be strong and airtight. Many of Craig's fingerprints were sacrificed at this stage

The system was then replaced and both the decat and cat sections tried out to see if the flanges were all aligned correctly.

The decat section had to be welded at an offset to ensure that the flanges mated up correctly. You can see in the pic below that the decat section is way off the vertical.



Finally, once all the parts had been welded and put back in place the decat section was silicon sealed and tested with the engine on for blowing. The pics below show the decat section in place with the sealant roughly put on to make the seal.



In the picture below you can also see the difference in the orientation of the flanges to ensure that my ham-fisted spanner access is in the best position possible.



Conclusions and some thoughts.

I have yet to test out what improvements the decat section has over the cat section on a dyno, but my first impressions are good.

i. The bottom dyno says more urge and an eagerness to rev, which could be more power throughout the rev range.
ii. My ears like what they hear when the decat section is on, a more visceral tone at the top of the rev range and under full load, but still accepably quiet when simply cruising
iii. Pricewise I'd say it was a success and a good experience along the way
iv. The option to swap over sections easily is a big bonus and one I will be doing periodically
v. The decat section, with all flanges is still 1kg lighter than the catted section on it's own, without flanges. Not much, but all adds up.
vi. Hasn't thrown up any error codes so far.

I'd just like to thank Craig and Steve Jr at Statllers again for being patient with me and for Craig's fingerprint sacrifices.

Cost was £225 + VAT and so it makes it a choice of Milltek decat downpipe from the turbo, or a custom decat which means you can swap over when you like and will still have the ability to sell on at a later date if you wanted to.

Would I do it again now I know what was involved?

Definitely

[Hurdy]

The car has just been on Noble's dyno and last time out made 378.6bhp and 353.6lbft before the decat

This time out the car made 383.1 bhp and 358.1lbft

So an increase of a whopping    .....4.5bhp and 4.5lbft

Doesn't seem like a lot for the work done, but the "devil is in the detail" as they say.

Torque is over 340lbft all the way from 2950rpm to 5750rpm

Before torque was over 340lbft at 3400rpm to 5100rpm

Power now hits 200bhp at 2850rpm and before power was 110bhp at the same rpm

The only downside is that I need to revise the fuelling setting as there is now a dip at 5750rpm, recovering at 6250rpm.

Here's the decat v's catted dyno plots

[Hurdy]

reserved for video sound clips.

[Janner_Sy]

christ you wernt joking about the hike in the lower range.
4.5hp peak difference sounds like feck all.

but when you consider

before 200 cell cat@3000rpm
125hp/220Ibft

after de-cat@3000rpm
200hp/345Ibft

there must be absolutely no feeling of lag if your pulling figures like that so low in the RPM range.

well impresed with that. Did it feel that much different low down.

proabably a good job you have the LSD now, cos a little bit of lag goes a long way in the traction department.

I think this is on my list of future mods now. Damn you

[Hurdy]

The pick-up feels instantaneous. 

Definitely needed the Diff.

[Janner_Sy]

i wander how it would be in a k03 though. supposably less lag than a k04 to start with it might also  free up a bit more power at the top end as well as thats where they are most restrictive

[Hurdy]

i wander how it would be in a k03 though. supposably less lag than a k04 to start with it might also  free up a bit more power at the top end as well as thats where they are most restrictive

You'll definitely free up some horsies.

The engine will breathe more freely too

[Janner_Sy]

need to find myself a reputable place who can do that now though. Its pretty daunting to give someone your nice expensive DP to chop up.

could you maybe get hold of a gasket to save using and reapplying silicone sealent each time

[Hurdy]

could you maybe get hold of a gasket to save using and reapplying silicone sealent each time

Yup, I'm on it already. The sealant was used as being my normal dumb self I forgot to ask for the matching gaskets when I bought the flanges

[Janner_Sy]

thats better,  scraping off silicone and reapplying itn everytime you need to change could be done without, plus the sealwould be better full stop i think

[WhiteGTI]

May I be the first to say thank you for a great review John! I bet you're happy, the figures speak for themselves! Looking forward to hearing the sound clips! Does it pop and spit more now??? 

[tony_danza]

I wish I had a before and after graph with mine, but as I went stage1 to stage2 & decat in one go it's impossible to compare - the 'feel' tells me it's done the same as yours though.

i wander how it would be in a k03 though. supposably less lag than a k04 to start with it might also  free up a bit more power at the top end as well as thats where they are most restrictive

I've a KO3.

I find power delivery actually better with the lower spool, makes it almost feel like an N/A with a flat torque curve, there's power everywhere. It does indeed help it breathe much better up top too.

Traction - the only time I've missed a diff was at Anglesey, spinning up the inside wheel when I was on the road tyres in the morning (didn't on the Cups come afternoon). I don't have any issues at Oulton or the 'Ring for example, as they're higher speed corners.

I put that down to a good set up by Votex, before they played with it, even on stage 1 it was like trying to keep a cat in a bath tub on a tight track.

[Janner_Sy]

what power are you making on yours tony. i dont have revo so i cant adjust settings. all i ca do is switch between standard, stage 2 and stage 3(2+) maps

[tony_danza]

Gah, in dyno-lottery style I can't confirm if these are 100% - but I'll post up the graph at some point so you can see the curves instead, which are more meaningful.

270BHP/330Tq on road settings (i.e. no aggressive boost & timing) with an OEM pump.

[Janner_Sy]

im interetsed in the low down side anyway