How much of that is on purpose (to sound cool) vs necessary (to provide 400hp) ?
They have a massive big block engine, but new cars are also big (5.0L = 300CI)
How much relates to emissions issues in 1974 ?
Catalytic converter reduces carbon monoxide, but makes airflow slower.
So, older cars has less restriction, which means you can have more air/gas mixture combust and faster exhaust flow ?
And how do annoying modern day “Honda ricer” exhaust mods relate to all this ?
How much of that is on purpose (to sound cool) vs necessary (to provide 400hp) ?
What exactly is cool? Is it the rumbling sound of the big block1960’s car or is it a Honda ricer?
Cool in in the ear of the listener (intentional or forced listening).
Personally I really liked the sound of my RX7 - Smooth quiet understated power.
Well, with a standard exhaust system, the V8’s had a much more mellow rumble compared to a straight 6. A V8 will still give you that sound but not a V6 as much, and certainly not a 4 cyl. Why I’m not sure but you need to start with an 8, then tune with the pipes. The little ricer 4 cylinders just sound like buzz boxes no matter what they do to the exhaust.
You get that unique rumble because of the firing order of a V8 with dual exhausts. You don’t get it with single exhausts because the exhaust pulses hit the muffler at evenly spaced intervals, but in a V8 with dual exhausts you get from each side a boom pause boom boom pause boom. And the patterns alternate but dovetail into each other.
Make that a boom-pause-boom-boom-pause-boom-pause-pause.
This is similar to the unique exhaust note of a Harley Davidson motorcycle. It has a 45 degree V twin so the exhaust booms are not evenly spaced.
Oh yeah, one more thing, glass pac mufflers.
American V-8s have 90º crank throws, which creates a smoother running firing order, but the exhaust pulses on each side hit at uneven intervals. Compare this to Ferrari V-8s, which have flat-plane crankshafts. They have a distinctive howl that almost grows to a shriek when they approach redline, which is usually around 8,500-9,000 RPM.
While I’m at it, the new 2012-2013 5.0 Mustangs sound glorious!
Almost any V8 will sound good with the right exhaust system. V12 motors sound pretty good too. V10s have kind of an odd sound that isn’t very pleasing in my opinion. A V6 isn’t too bad sounding really, but there isn’t much point. IMHO, there’s not much you can do for a 4-banger, no matter how powerful it is or how good the exhaust. Maybe it’s an acquired taste. The last ricer with a coffee-can muffler I heard was more reminiscent of a leaf blower than anything pleasing to the ears. I can’t imagine hearing that for hours on a highway trip. I also laugh when I see dual exhausts on any 4 cylinder motor that isn’t on a motorcycle.
There’s no reason you have to have loud to be powerful. With a correctly designed exhaust system, you can have a very nice sound when you step on it, but still have it whisper quiet most of the time. Ferraris are in a class of their own. They will make the hairs stand up on the back of your neck at full throttle.
Double Clutch has it correct. Early V8s including WW1 aircraft V8s of which there were many designs had flat plane crankshafts as did Cadillac cars from 1915 to 1922 after which they came with a 90 degree crank. The exhaust note from a flat plane V8 is like a straight 8 which has a bumblebee buzzing kind of sound, not the loping sound with which we are familiar. The flat plane crank design made more vibration which was later minimized with the 90 degree crankshaft. I have not seen or heard it said but it is my belief that manufacturing methods were not developed or pushed along enough at first make a 90 degree crank or else engineers were not inclined to spend more money for a 90 degree crank.
I recall during the late 1950s and early 1960s that occasionally an older car such as a Pontiac straight 8 made before 1955 when they converted to V8s could be heard with the exhaust manifold welded shut in the middle with an additional exhaust port added to make a dual exhaust system. These had an exhaust note that sounded exactly like a V8.
Fairlane Thunderbolt…
Guess I’d never paid attention to the cranks. By flat plane do you mean the journals and counter weights are not offset? Versus the counter weights on a 90 degree would be offset from center? If that makes any sense. Never really realized there was a difference.
I’ve owned a few old sports cars and muscle cars along with knowing quite a few people who are of the same persuasion. None of them were annoyingly loud at all so my feeling is that if noise is a problem then it was because someone stuck a couple of Cherry Bombs on them.
A flat plane crankshaft is like the crankshaft on an inline 4. The two crankpins on the ends are 180º from the two crankpins in the middle. Put it in a V-8 with a 90º angle between the cylinder banks and you have, basically, two 4 cylinder engines on a common crankshaft, each crankpin shared by two pistons, with the power strokes on one bank happening between the power strokes of the other, and the exhaust pulses evenly spaced on each bank.
Hooker Headers and glass-packs, or maybe even a cutout to bypass them was what I knew.
Also consider valve overlap. This has a dramatic effect on the type of exhaust note.
BTW- 300 ci is dinky for the muscle car era. I wouldn’t consider anything smaller than a 427 and prefer vintage 454s. The only people using glasspacks were small block people trying to get the sound a big block makes naturally 
“Turbo Mufflers” were also popular…These provided low back-pressure and a pleasant exhaust note but did not attract police attention as the “straight-thru” glass-pacs and steel-pacs sometimes could…
Doubleclutch “nailed” the answer.
If all other factors were exactly the, the difference in cranks (90 degree vs. 180 degree), the formen would still rumble and the latter whine. Valve overlap, high duration cams, headers, glasspacks, etc. all contribute to the tone, but it’s the crank that makes the big difference.
keith June 30
This is similar to the unique exhaust note of a Harley Davidson motorcycle. It has a 45 degree V twin so the exhaust booms are not evenly spaced.
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I cannot believe any company would build an engine that didn’t fire evenly spaced.
I’ve read that Harley does this intentionally now to get the flub-a-dub tune that they’re known for. Personally, I’m always amazed when I see someone on a Harley at a stoplight and the bike is shaking noticably. I always wonder why anyone would want that. But apparently some do.