Turbo mufflers can sound great.
I ran a set on my '72 Chevelle when I was younger as they were dirt cheap.
They are super simple as seen below.
They usually have an internal "S" shape with the inlet, center, and outlet pipe that are perforated and wrapped in a steel wool material to absorb the soundwaves.
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Flowmasters are different as they are a fully chambered muffler with no internal packing to blow out. They use the soundwaves against each other to cancel some of the sound waves.
Here is a cut away of a Flowmaster 40 series muffler.
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From Flowmaster you can get all kinds of different mufflers with different tones.
From loud as you want to have a conversation in the car simply.
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To go even deeper, you can concern yourself with the CFM of the muffler.
I have read for simplicity you want to have double the CFM of the induction for the muffler selection flow. So if you have a 600CFM carb or injection, you should in theory have (2) 600CFM mufflers in the exhaust.
Of course muffler CFM is not widely advertised, but has been tested by many sources and published on the web for us to peruse like the chart below.
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This is the same as the chart above, just easier to read.
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For way more information than I can remember, look here:
any automotive engine will run best with little or no back pressure on the exhaust system , back pressure ALWAYS tends to reduce the cylinder fill and scavenging efficiency, this is not a guessing game. you can simply measure back pressure in the exhaust every500 rpm from about 3000rpm to where...
garage.grumpysperformance.com
This fine feller has been collecting information and posting it for years....