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Author Topic: 3 inch vs 3.5 inch Downpipe  (Read 249 times)

Offline ttelracs

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3 inch vs 3.5 inch Downpipe
« on: October 04, 2021, 07:49:29 pm »
I need some advice please.

I have a 2005 AXX which is now running so much better than when I first bought it, but one thing was really annoying me and that was the blow from the flexi pipe on the exhaust.  I've had it since June and done two oil changes, filter, gearbox oil, polybushed one of the front wishbones (other to be done after exhaust), swapped out the 18 inch very rough Monzas for 17 inch ones and replaced the suspension with Gabriel Gas ones and Mk6 lowering springs.

I ordered 3 inch eBay downpipe as the one on the car didn't look very standard but once I removed the old one I noticed the inlet looked larger than the new one and once measured that confirmed it's 3.5 inch in size.  It's a homebrew job but it still has the cat and silencer although the bend after the cat isn't 3 or 3.5 inches it's approximately 2.5 at best and not mandrel bent.  I don't know whether I have a KO3, hybrid or KO4 turbo fitted as the chap I bought it from simply drove it and had is 'serviced' at a garage but it wasn't well looked after.  I had to smash the old oil filter off and replace the housing which is why I gave it two oil changes as it clearly hadn't been changed in years and I'll do another in about three months time.

My question is, do I get the flexi cut off and a new one welded in or would it be better to fit the new 3 inch and order two more 02 sensors?  Personally I'd prefer to keep the 3.5 inch as it pulls well from well under 2k rpm and really flies after 4k but if there is an advantage to the 3 inch downpipe I'll fit that.  I've attached a couple of images of them as well as a pic of the old and new gaskets next to each other.

Thanks in advance.







Offline LC5F

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Re: 3 inch vs 3.5 inch Downpipe
« Reply #1 on: October 04, 2021, 08:20:55 pm »
To me that looks like a standard downpipe - to be fair I have only intimately known the one I took off mine...that was blowing at the flexi too.
That larger section next to the turbo flange is the pre-cat - I am guessing that it has been knocked out already, this is meant to be a halfway house to downpipe - but reported to be noisey.

Offline ttelracs

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Re: 3 inch vs 3.5 inch Downpipe
« Reply #2 on: October 04, 2021, 10:19:21 pm »
Thanks for coming back to me and you may well be right but there is still the honeycomb part there of the cat and when I've knocked cats out before, nothing is left.

Offline Pudding

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Re: 3 inch vs 3.5 inch Downpipe
« Reply #3 on: October 07, 2021, 02:39:33 pm »
Those cheap 3" downpipes aren't great.  As you can see, there's a huge step in the flow.  Not the end of the world but the smoother the exit path for the turbo, the better, otherwise it can restrict flow a bit with high boost remaps.  Ideally you want a downpipe that matches the turbo outlet, or bigger.

That isn't an OEM downpipe btw, it's probably an aftermarket pattern replacement.

For reference, this is what the OEM one looks like:



2007 ED30 | Ohlins Coilovers | Revo Map | Plus some other stuff | Is all a GTI needs :-)

Offline ttelracs

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Re: 3 inch vs 3.5 inch Downpipe
« Reply #4 on: October 13, 2021, 11:59:10 am »
Thanks @Pudding, much appreciated.

I've now fitted the cheap eBay jobbie although the hangers don't line up precisely it'll do for now.  It does appear to allow the boost to spool quicker as it doesn't have the restrictive neck the previous one had, plus it appears to be 3 inch until the step down to 2.5 inches where it mates to the OE pipe and back box.  There isn't a mid silencer as that has been deleted at some point.

It's not too loud, it doesn't drone on the motorway and allows the turbo to be clearly heard when pressing on.  Economy has also improved on longer runs too. Time will tell as to it's longevity but it was one of the more straightforward downpipe swaps I've done so no real complaints and loathe to spend too much money on it either. Thanks again for your input.

Offline Pudding

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Re: 3 inch vs 3.5 inch Downpipe
« Reply #5 on: October 13, 2021, 02:28:24 pm »
Good stuff!

You won't notice the restriction until you get into bigger hp remaps. As you say, the larger bore and deletion of the first cat balances out any restriction that step may cause  :happy2:

I'm not surprised by the fitment woes.  Not many aftermarket ones fit as well as the OEM pipe  :grin:


2007 ED30 | Ohlins Coilovers | Revo Map | Plus some other stuff | Is all a GTI needs :-)