Subaru: Decatted Downpipe and ECU ReMap


This entry is part 6 of 19 in the series Subaru Legacy
Hey. This page is more than 5 years old! The content here is probably outdated, so bear that in mind. If this post is part of a series, there may be a more recent post that supersedes this one.

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

DeCatted DownPipe

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 suit.

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. I regret this now.

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 bit 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 (in the downpipe& mid-pipe) warm-up. Looking at the diagram below, I am not sure whether I have had the downpipe and mid-pipe taken out (and the two cats) or just the downpipe one.
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), decatting gives you more power by reducing the restrictions to flow through the exhaust system. Whatever the physics, the result is more turbo power and an earlier turbo response….but there are also downsides (see below).

I have read of numbers up to a 30HP power boost being bandied around online (for various Subaru turbo models). Seems a lot? This post puts boost to nearer to +10HP for the up-pipe mod and says the turbo comes in 500rpm earlier.

The Consensus (and Downsides)

I have been running with it for about a fortnight now. Things feel a 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-thousands developing an exhaust 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 could smell it and there was not car in front. 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.
  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).
  5. The world Just not really cool or the done thing is it? These days? The catalytic convertors are there for a reason.
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 research

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 upgraded to a sports exhaust which might not actually have cost much more and I would not be asphyxiating wildlife as I drove along leaving a plume of death smoke. And it would be way cooler.

Series navigation

<< Subaru Legacy (to) STi LegacySubaru: Sound system upgrade >>