I don't think I noticed any better mileage when the CAT got the axe. Makes sense as the flow thru the system just isn't that great while running 'normal'. I'm sure that while running under BOMBed fuel & boost conditions the reduced backpressure gives better numbers.
I'd go for the 4" system from the CAT test pipe back, with a straight thru 4" muffler with perforations into the baffling. You'll like the sound better. The old glass pack types had those ridges in the flow that were more restrictive than a turbo muffler.
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The body of my 4" muffler is 12" long & sounds good.
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Money Sink - 96, 5 speed, TST #11/280 slid to 340 position + Injectors.
4" CAT back & CATless, 4" chrome turn down straight out back.
Stand back & listen to the turbo whine :-}~
ISSPRO Pillar, Delvac 1 & Amsiol, K&N, Monicas silencer ring removed, Exhaust brake, Mag-Hytec, 3:54, 235s on Alcoas, Rancho 9000's, Valentine One.
HX35s days are numbered, HX40-16 is here.
Un-Bombed = Boring