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Ours is a VX with the factory twin cat system but it still merges into a single 2.5" after the diff. I upgraded to a full twin V8 system which allowed the engine to rev out better and it wasn't pulling timing anymore either.

I done a back pressure test on a old Holden v8 I had many moons ago. Will let you guys know how I go over the next couple of weeks
Thanks for the info again

Small update cat was welded on chopped it of Today and was half collapsed inside.
Only gave car a couple of full throttle runs today looks like boost has dropped 2 psi or so and top end were it died over before has picked up heaps

'quipt4it wrote:That exhaust would have to be really restrictive to be almost doubling the boost pressure.
I would guess that the cat is either blocked or partially melted. Or the muffler has collapsed.

Not a bad guess eh...
Muffler next?

Lol
Il drive it for now probably wideband it just to see what the a/f is like on top end and worry about rest of exhaust down the track. Once i finish the 50 other projects i have going on at the moment

Follow up on this has been 2 weeks of daily driving now car has picked up heaps more all round.
I wideband it other day and has gone lean (fuel wise) on the top end but pulls way harder up top.
Just cruising for now will have to richen it up a bit more on the top end when i get time

For anyone else in similar situation.
On the weekend i done some more wideband tuning. Car required 3% more fuel and dropped boost from 11psi to 8.5 psi when i logged some full throttle runs
So win win