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I have glasspacks on my 77 F150 with a 400. I purchased the dual kit from LMC and I love the sound. I like to drive around in first and get the revs up to about 4K then let off the gas and listen to it pop and bang, makes people jump . I think for overall sound though I'd go wiht the flowmasters. I had a single flowmaster that exited before the rear wheel on the passenger side on the same motor before the glasspacks and i thought it sounded better than the glaspacks do. There is no mistaking the tin can flowmaster sound.
I have glasspacks on my 74' true duals straight out back. No way I would even consider blowing the $$$ on FMs when my truck sounds so good with cheap glasspacks.
My 79 currently has a single exit Flowmaster coming off the 400, stock manifolds. I really don't like the sound at all. Too much of the 'tinny' sound, not enough engine noise. I only paid $50 for it, including installation, so no loss. My roommate's Chebby with a 350 has headers and dual Flows, and it sounds great though.
My 73 has duals off of the 390 with glasspacks, and I LOVE the sound of it. Before I got a muffler on the 79, it had no muffler, and while it sounded really good, it was WAY too loud to drive around a small town and not get pulled over.
I just got my headers for the 400 in the mail yesterday, and I'm going with duals and glasspacks when I get the exhaust done. Well, unless I find another Flowmaster for cheap.
By far the Flow Masters sound the best if you get the right ones. But because of the cost, Ill prob never have any. GlassPacks are GOOD Cheap sound. Flows are GREAT Expensive sound. Its all in how much money is the diff worth to you?
Local muffler shop around here swears that Flowmasters only sound good on Mustang GTs. He recommended Magnaflows. I still only have 1 old Flowmaster on mine running out to duals. Too high-pitched for me, but I have to fix the things that don't work first.
Glasspacks are white trash Flowmasters!! But they sound good. I have a set on my Bronco and on my 77. They are quiet at idle and they make noise when you want it.
I've found that Flowmasters on Ford engines have kind of a hollow, tinny sound that resonates in the cab at cruise speeds. I don't know if it is the firing order or what the cause is. But I have a set on a 65 Chevy that don't have this problem.
I had 40'' glasspacks on my 1976 F-100 after a 390 engine swap. I didn't think it was loud enough so I went with Magnaflow and love the rumble they produce when you put your foot in it.