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Old 12-06-2017, 11:47 PM
lifespeed lifespeed is offline
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Emissions-legal catalytic converter upgrade

Not sure how many will find this interesting, but there is plenty of evidence restriction after the exhaust manifold (pipes and cats) are a source of horsepower loss. Short of installing long-tube headers, the best you can do is open up the exhaust including the cats. We have 2" from the factory, although it does choke down to 1-3/4" after the rear cats. Even stock engines benefit from less restriction here. Although I can't find the article where somebody did a before/after dyno with exactly this experiment, it was worth about 20 HP on a bolt-on only engine. Modified would be worth more.

As more states adopt the restrictive CARB rules, knowing how to live within the rules without re-configuring your car every few years could be useful information.

If you look at the CARB catalytic converter database you find a list of aftermarket converters that have been officially blessed by application. There are bolt-ons assembled with pipes, but what we want are larger cats with an EO number for our cars. Turns out AP emissions makes #761015 which is approved under CARB EO D-754-1. This is a 2" cat, and the same part number is approved for front and rear locations. However, they make the same part in 2-1/4" and 2-1/2" under the same EO number! these are part numbers 761016 and 761017 respectively. I need to confirm, but I believe the numbers stamped on these that are checked at inspection (if your cats have been replaced) are the EO numbers, not the manufacturer part number - the same for all sizes.

So buy some stainless mandrel U-bends from Burns stainless (press bends are verboten for horsepower), make sure you hire a skilled exhaust guy, and go to town!

2004 Silver Birch Marauder 130K miles
Wilwood Aero6 F, NMDP R brakes, Penske 7500DA shocks, Hypercoil 600lbs F and Grand Marquis R air springs, Addco tubular sway bars, Metco control & Watts, 31 sp axles, Stainless Works cat-back exhaust, Lidio tune, American Racing AR883 9.3" 50mm F 10" 59mm R 20" wheels, Pirelli P-Zero 265/35-20, 275/40-20 tires

Coming soon: Livernois 5.0L stroker, ADTR/Vortech V2 intercooled SC, Silverfox trans and Circle D converter
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