power loss after decat???

[keithnewton]

i have recently cut the cat and mid box out of my hdi and welded a bit of old exhaust in and added a 5" cherry bomb rear box and seems to have lost abit of power is this normal or as anyone herd or done this?

[Mr Whippy]

Has it really lost power?

Before/after dyno runs or acceleration runs?


It's really tough to go purely on subjective feelings and things, empirical measurement really is the only way to go... assuming your exhaust isn't really bad then chances are you have added 5bhp but simply think that it's slower because the torque comes in lower down the rpm range and thus the 'kick' at 1750rpm as it comes onto boost has gone... that kick is what makes the car 'feel' fast.

Chances are you are quicker now, it just doesn't feel it

Dave

[cully]

mine stayed the same removing the cat !
the biggest differance was made once i removed the standard backbox and replaced it for a mid box

[procta]

mine was a little more responsive off the mark and top end was better too.

[Mr Whippy]

I found that mine felt flatter with a de-cat, but it was mainly because that area around 1250-1750rpm was more linear in response... rather than surgey which made it feel faster.

I agree though Cully, the back box is what kills these.

However, you notice the impact of a cat a lot more if you have a modded back box but a cat still in place! It's almost like standard still!

The combo of de-cat and free flow back box has to add a good 5bhp even to a standard car imo... and a load more engine reponse.

Remove the EGR and it's responding twice as fast


Dave

[kentiiboii]

Mine felt more responsive under 2k after decat and I still have standard back box. I only go over 2k when I fancy a bit of boost now

[Midnightclub]

I left my de-cat till last on mine after i'd gone stage 2.. made a MASSIVE difference in response and certainly makes the car feel free'er higher up the revs IMO

[Rebellious]

de-midbox'd mine, now the rearbox has perished its joint to the midbox so im running round with no backbox atm and its made a big difference, it feels loads better, even with the cat in place so, il second the backbox therory :dance:


as for EGR, can i blank mine with a simple plate like the older stuff?

[Mr Whippy]

The back-box is the main exhaust restriction as it's not a straight through type.

However, it does also reduce sound levels a great deal.

The cat on the other hand just blocks the exhaust and doesn't done anywhere near as much as a silencer so it's dead weight really... more so if you consider the cost to efficiency, power and response.

The centre sections are the BEST parts of the standard system. Replace the back box with a spare centre section in it's place and combined with a de-cat it's the best and cheapest system all-round in my view.



The Estate cars have a different exhaust though. My brothers HDi took really well to a remap with the standard exhaust and he said it drove more like a 306 HDi with a sporty exhaust as standard in the way it responded to the throttle and reduced turbo lag etc.



There is no real need to blank off the EGR Rebellious, just make sure it can't activate (usually unplug the wire to the vacuum solenoid)
Blanking it off looks elegant but I've seen no proof it does anything except make the car hard to revert if required, and costs you time and money better spent on other mods

Dave