Subaru: Decatted Downpipe and ECU ReMap


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TLDR: Don’t do it. Save up and get a proper exhaust system.

DeCatted DownPipe

So, to try and get a little more oomph from my car, I took my car to SubTech and got the OEM down-pipe swapped out for an un-catted one, and got the ECU re-mapped.

To be honest, I did not do a great deal of research on what I was getting done – I think I just watched the vid below and thought “Well, that seems kind of obvious” and went for it. The problem with writing this is that I have learnt what I got done…and kind of regret not being a little more clued up.

Reading around a little to find what actually got down the first penny that dropped was that the video above was talking about de-catting the up-pipe (that leads into the turbo), not the downpipe. I have since actually learnt a great deal more of what makes up the car exhaust system.  From what I have read there are three catalytic converters in my car normally: one upstream of the turbo and downstream from the manifolds (up-pipe) which keeps cold start emissions down until the two downstream cats (down-pipe) warm-up. Looking at the diagram below, I am not sure whether I have had the downpipe and mid-pipe taken out (and two cats) or just the downpipe.

The OEM exhaust system for a 2.5L turbo. I’d guess mine was identical.

Up-pipe, down-pipe, mid-pipe (or all three, I suppose), I believe it works by reducing the restrictions to flow through the exhaust system and so more energy is there for the turbo to do its bit. Anyhow, whatever the physics, the upshot is more turbo power and an earlier turbo response….but there are also downsides (see below). I read numbers up to a 30HP power boost being bandied around online (for various Subaru turbo models). Seems a lot, but I don’t know. This post puts boost to nearer to +10HP for the up-pipe mod and says the turbo comes in 500rpm earlier. I’d imagine my mod is closer to this, but I have no idea really, nor will I ever really. While 10HP isn’t nothing I suspect the turbo kicking in earlier is really what I am noticing as ‘performance’.

The Consensus (and Downsides)

I have been running with it for about a fortnight now. I think that things feel a little more spritely for sure: But there have been downsides.

The downsides, in order of least to most annoying…

  1. The car seems to sound a little ‘off’. I suppose the harmonics have changed with the new pipe geometry. Subaru spends hundreds-of-thoasands developing and I chucked it in the bin and put in something some guy welded together. Go figure.
  2. The exhaust smell: I thought it was the car in front until I accepted attributing it to all the cars in front was unfair. I googled it, and it is a ‘thing’. I am kind of hoping it’ll just go away, but it might not.
  3. The lower ride height of the flange to the bolted connection of the new pipe to the rest of the old exhaust: The flange is is bigger/ wider than whatever was there before. Unfortunately, this means that every time I go over the hump into our driveway the thing scrapes on the ground is annoying and concerning. I would be pretty keen to see how the current connection differs from the OEM one, and perhaps modify what I currently have (might need some welding and wot-not).
  4. Will it pass the Warrant of Fitness? I am not sure, my reading of the rules is that it might not, except there is still a cat in the up-pipe I think, and maybe one in the mid pipe (if it still there).
Emissions requirements from NZTA. Point two sounds ominous…and perhaps even point one. Daaaaaah!

What I Should have Done

So, the jury is very much out on this modification, and I regret to be honest – I also regret not hanging on to the catted downpipe to put it back in.

I should probably have done a little more presearch (yep, I just made that up…).

Rather than watching the guy in the vid at the start of this post, I should have trusted the gal in the video below…I could have actually upgraded to a sports exhaust which might not actually have cost much more in the long run and I would not be asphyxiating wildlife as I drove along leaving a plume of death smoke. And it would be way cooler.