Andrew Pugh wrote:...a little restriction in the exhaust isn't going to hurt.
Except significant intentional restriction isn't necessary.
Exhaust gases have a lot of energy in the form of heat and exit from the cylinder at high velocity, producing the noise. There are two ways to dissipate that energy without making a lot of noise: restriction or expansion. Expansion is better but the silencer has to be quite large in order to for it to be effective. If expansion is confined to a series of chambers whose structure is such that the expansion shock waves are not conducted to the environment, the apparent noise level is reduced, but without significant restriction. Furthermore, the heat in the gases can be retained so that when they do finally exit to the environment they dissipate via convection. That is the principle upon which the above exhaust silencer works.
Below is an illustration of its innards.
This is what is referred to as a reverse flow silencer, since the exhaust gases travel to the rear of the unit and then start expanding toward the front (front is on the right). There are four expansion chambers, the largest one's volume being approximately 10 times the engine's displacement. The three baffles that separate the chambers are pierced with 143 equal-size holes. The hole size was calculated to produce enough pressure differential to promote expansion, but not enough to generate significant back pressure. The total area of the holes is approximately 1-1/2 times the area of the tubing that acts as inlet and outlet.
Here's what the baffle looks like.
About half of the total expansion takes place in the first and second chambers. The third and fourth chambers are responsible for killing the high frequency noise component, which is what is usually objectionable to most people. By the time the gases exit the silencer their pressure is only a couple of atmospheres, so the noise level is low and it's all low frequencies.
In order to prevent the silencer from itself acting as a resonator it is constructed from 3/16 inch wall rectangular tubing. The baffles are plug-welded top and bottom so they do not resonate—also, the welds keep them from drifting out of position. The inlet and outlet tubing is 1-1/2 inch exhaust pipe, nothing special. In order to retain as much heat in the exhaust gases as possible, the silencer is wrapped in an insulating blanket, which also keeps it from heating up the interior of the carbody.
This last picture is of the exhaust pipe, which exits out of the engineer's side of the loco down by the false fuel tank.
The guy for whom I made this thing is quite happy with the exhaust tone. The mechanical noises from the engine almost drown out the exhaust, which is a low rumble. As a bonus, the locomotive got heavier.