I have a 2.5 inch exhaust with them behing my 400. I wouldn't have anyother muffler. Good sound at idle, not too loud. Cruising you don't have the interior resonance like flowmaster. Wide open they sound great. Mean. IMHO best muffler you can buy for the money. Oh yeah, I did notice a performance gain also. Good Luck.
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1969 Firebird "400". 408 Pontiac, #62 heads, 274 XE cam, Eddy RPM intake, Quick Fuel Slayer 750, Carter Super Street fuel pump, Mallory Unilite, Turbo 400 with shift kit, Continental "Jim Hand Special" 3200 stall, 12 bolt 4.10 posi. 1 5/8 inch headers, 2.5 exhaust with Dynomax Ultraflos.
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