Muffler & tail pipe

Exhaust muffler, Silencer

How much will the muffler sound / dampen?
It is never possible to say exactly whether a muffler will sound more or less. One would have to compare size, shape, diameter of inlet and outlet, type of damping material, type of perforated pipe, drawing of inner pipe, etc., etc.

How do you know what type of damper to use to get the same sound?
If you have an existing exhaust system, the simplest starting point is the size and shape of the current silencer. As well as whether it is a full flow damper or not.

How to choose a silencer for a new exhaust system?
If a new exhaust system is to be built, it is recommended to use two dampers as large as will fit. If you want to race or have a car with high power output, you should choose a full-flow damper.

What is a full flow damper?
A muffler called a "full-flow" muffler has the internal tube mounted straight through without bends. This is to get the best flow through the muffler. This does not dampen the sound as well as a "regular" muffler where the internal pipe can be routed back and forth in the muffler.

Dampen more = Less sound = More back pressure

Larger muffler
More mufflers
Larger silencer body 
Irregular / Oval silencer shape
Larger hole on perforated pipe in muffler
U-tube / bent tube in muffler
Smaller pipe diameter

Dampen less = More sound = Less back pressure

Smaller silencer
Fewer silencers
Smaller silencer body
Small round silencer shape
Smaller holes on perforated pipe in muffler
Straight pipe / Straight through the muffler (full flow)
Larger pipe diameter