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im thinking of doing this myself. Aside from the 340 headers and cats, what do I need? Does it make sense to add the HSV centre pipe as well?
Hey mate, I used the stock standard HSV cats that came with the headers. The 2.5 mid section was from planet parts, bolts in and out very worth it. Correct mate straight to the stock bimodal mufflers. I also got walkinshaw 3.5" tips welded on to keep that oem look but without the price tag!Hey Nico, just catching up on all these posts, great result, which cats did you use, an also what 2.5 mid section did you use, and im guessing that was into the standard bimodal cans
‘Bullet cats’ by themselves are $125-$150 each x2 so yeah around $550 the pair ready-made is OK.
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Dimensionally if you have the 340 set with 2 1/2” outlet at the merge then may as well stick to a 6100+ top end tuning target for max HP. 3” downpipe is just too big for this engine.
Otherwise the 2.25” outlet of R8 is better TQ/HP balance for everyday, matched with 2” downpipe gets you more torque 95% of the driving time.
The main thing is space restriction longitudinally to the stock catback flange location of course. Ideally you want the length of downpipe 9” measured along the centreline before entry to cat. The HM (left), HSV (right) are easy cos bolt-on ready-made but downpipe is way short:
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So grafting the required mandrel bend moves HSV cat all the way rear, then just flange your cat’s butt
As you’re taking a hacksaw to it could do same with Holden VF cats (below), doesn’t matter.
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Can hardly see down there, stainless won’t stay shiny at cat temps, silly to waste money on shiny branded stuff that is technically incorrect.
Deng Xiaoping famously said that it doesn't matter if a cat is black or white so long as it catches mice, here doesn’t matter which (bullet) cat as long as it has a 200+ cpi steel substrate.
Short fat barrel cat is better than long skinny for fit and theory, possibly not emissions.
The 340's and 2.5" will make a bit more peak torque and power. But the difference will be negligible and both are great for daily driving.If you are still monitoring this thread, are you saying that HSV 2¼ headers would be better for most daily driving? and stock midsection vs 2.5?
Thank you.