Since I like as many opinions as I can get when I make a decision like you are trying to, I'll throw my two bits in also. When I put the flowmasters on my car, I tried to listen to cars with them, but that was almost 7 years ago, when they were first catching on. A buddy of mine put the 2 chambers on his Edelborck package 350, in a chevelle, with headers and full exit at the bumper 2 1/2" exhaust. I thought it was a bit loud. That was a 9.5 :1 motor. The 350 I had in the car at the time had 10.5:1 so I knew the engine would be louder than my friend's, so I went 3 chamber with 2 1/2" pipes to the bumper, and absolutely love the sound, couldn't ask for it to be any nicer. Now with the 402 and about 9.5:1 compression, it sounds almost exactly like the old 350 did. Another thing to consider besides compression is how many RPM's will you be turning at highway speeds? The more RPM's the louder it will be, and more likelyhood of it resonating. If I did it again, any car I plan on driving for more than just an occasional weekend rip around town would get the three chambers.
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Steve
72 Chevelle SS402/4sp