Replaced my 5 cylinder Volvo V70 2.5l turbo engine with a 6 cylinder S80 2.9l twin turbo engine and I'm having trouble with what to do about the exhaust.

The original 5 cylinder had only 1 turbo, 1 pipe, leading to one big catalytic converter. It had the one O2 sensor before cat and one directly into top of cat.

The 6 cylinder has twin turbo therefore has 2 pipes, each with its own cat followed by the pipes (1+1=2) conjoining just before I'd say a medium cat. Now, I know both pipes have an O2 sensor before its respective cat.I want to say that the other two sit just after the med. cat.

Other than the original large cat, the other pipes I purchased at the junkyard. I actually found an S80 with the two cats I'm missing and they took them from me at checkout as it's illegal for them to sell them to me. You couldn't imagine how infuriated I was and look what I'm having to do now!

So to my question, should I use the single large cat only and weld in an extra O2 housing on top of the cat so there's two before and two in cat?

should i say to hell with the cat, straight pipe it with the 2 into 1 exhaust pipe i have acquired from a completely different S80 non-turbo? I can use a small piece of the 2 into 1 two conjoin the two exhaust pipes connected to the engine with the original catalytic converter pipe and then worry about O2 sensors in the cat, welding a second 02 sensor housing into it.

Now that sounds more reasonable than simply straight piping it and having to worry about check engine light. Or since my other option is altered (two missing small cats, and would make a second O2 sensor hole in cat) will I have to worry about a check engine light anyways?

I think that about sums it up. Thanks for any advice in advance, community!

  • Welcome to Motor Vehicle Maintenance & Repair! Feb 23, 2021 at 0:44
  • Don’t make a hole in any cat. Make one either before or after.
    – Solar Mike
    Feb 23, 2021 at 7:38

1 Answer 1


If the 6-cyl engine had two cats, you're going to need two cats for the engine to run correctly. The only way around it is to have someone tune it for you. If I were you, I'd just look online for new replacement cats and exhaust for the 6-cyl engine. I'd bet its not as costly as you might think. You could also try to get aftermarket cats of approximately the same size and use them, but you'd need the exhaust connection between the exhaust outlet and the cat to hook them up. You'd just need to tune up your welding skills to make it all come together.

On a personal note, I feel your frustration in having what you need only to have it taken away from you. Definitely sux.

  • What does tuning an engine to the exhaust consist of? Is it something I can possibly do myself? Feb 23, 2021 at 0:58
  • 1
    The tuning will probably be more expensive than the exhaust parts.
    – GdD
    Feb 23, 2021 at 8:54

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