I ran Random Technology dual 3" cats on my LS7 swapped RX-7 and no more stinky smell from unleaded gas. It still made 520rwhp with a cam/headers only as upgrades and ran 111mph in the 1/8 and 138.3mph 1/4 best at around 2900lbs with driver. They barely sucked up any HP truly for the minimal mods I had.
I would definitely run long tube headers and then opt for dual 3" cats. Magnaflow as well as a number of other vendors make them. The less cell count the less restrictive they are.
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Project: 1969 Chevelle, alum 5.3, GTS76 turbo, Chiseled Perf A/W intercooler, Holley Dominator EFI, Davis TC, JRi coilovers, 14" brakes, T56 Magnum. CTS-V Recaro seats!
Coffey Fabrication Nashville, TN doing the fabwork! 800rwhp+/25mpg+/9's@150mph goal
https://lateral-g.net/forums/showthread.php4?t=52976
1993 Mazda FD RX-7, built LS7, T56 Magnum, Holley HP, Davis Tech TC, 8.8" 3.73 Cobra IRS diff, DSS axles, Ohlins coilovers, big brakes.
CCW 18x11 front 315/30/18's and 18x12.5 rear on 335/30/18 RA1's, 2850lbs
10.31@137mph 165mph Shift-Sector 1/2 mile speed
441ci AllPro headed motor and 200lb weigh reduction in the works....
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