I installed some
Magnaflow 59956 Spun Metallics a few years ago for exactly this reason. At the time, I didn't have to undergo emissions testing, I was just tired of the stink. They're so tiny it's hard to believe they do anything at all, but they worked like a charm, and withstood the abuse of road course hammering for several years too. When I moved to an emissions-testing county, where pre-OBD-II cars get a dyno sniff test, they passed pretty well. A few years more abuse, and they failed, but I'm going to replace them with identical units when the time comes for the next test.
Three years into them, I got these results:
test...allowed...result
HC......1.00......0.63
CO....12.00......4.16
NOx....2.50......0.82
At 4.5 years/33000 miles on them, I got these results:
Test...allowed....grams/mile
HC.......1.00........0.64
CO.....12.00........7.77
NOx.....2.50........0.38
With with 6.5 years and 40,000 miles on them (probably 10% of those on road courses), I got these results, and failed, even with new plugs/cap/rotor
HC.......1.00........1.35
CO.....12.00........26.92
NOx.....2.50........1.63
I put a stock H-pipe back on it, still no air pump attached, and got this. Notice how similar the stock H-pipe is to the original readings I got at the 3-year mark with the Magnaflows:
Gas.....Allowed....Test
HC.......1.00........0.63
CO.....12.00........3.86
NOx.....2.50........1.27