We invite the reader to review the following links to understand the differences between twin-tubes and monotubes so our comments are not taken to merely be our own 'hype.'

HKS USA - useful diagrams comparing twin to monotubes
Bilstein shocks vs. Twin Tubes
Monroe Shock Absorbers
Mobil Oil, 'In for a Shock'
Glossary of shock types and terminology

First, without adequate internal pressure for the intended vehicle usage, any shock will foam due to the extreme pressure differences generated during operation. These pressure differences cause air to 'boil' out of the oil, which is called cavitation or foaming. Professional racers invariably use monotubes and even vacuum-bleed the shock oil (to remove as much air from the oil as possible). Adding a nitrogen charge increases the point at which this cavitation starts and creates a more consistent, reliable, longer-lived product. For shocks that see road use, the internal pressures are higher than those found in the majority of competition vehicle (excepting off-road/baja). Bilstein uses about 350 psi on their OEM monotube shocks. We use between 100-200 psi depending upon the application.

Another important factor that prevents cavitation is piston diameter. From physics, F = P * A or Force = Pressure * Area. To achieve a certain damper force, as the piston moves a pressure drop is created with higher pressure below the piston and less pressure above it. This pressure works along the annulus (ring) area which is the piston area minus the rod area. The larger the piston diameter, the larger the ring area and the lower a pressure drop needed to achieve the desired force. Hysteresis is essentially a time-delay between input and response, like getting the punch-line 10 seconds after hearing a joke. It means not connected to the instantaneous, changing road conditions - the suspension is lagging behind. Because a twin-tube has less area, it must stroke farther to develop a similar pressure (and force) to a mono-tube. Combined with the fact that a twin-tube must be designed to provide higher pressures to achieve the same force, pressure drop is higher in a twin-tube. The ultimate result is that a twin-tube is less precise, provides less damping when the suspension requires it, and has a time-delay in its operation (which is called hysteresis). This is sometimes touted as a positive ('The twin tube is less sensitive and provides better ride than a monotube') but think about it logically. A monotube can be made as soft as desired for low-frequency operation, yet provide sufficient damping when higher frequencies are present. meaning it cannot react as quickly to road inputs. This delayed reaction causes a vagueness in the handling. This makes a larger piston diameter monotube (like a Bilstein 46mm piston) able to produce the necessary forces with less pressure drop (and the inherent hysteresis) than a twin tube shock (with smaller piston diameter). Cavitation can occur much more easily in a twin tube because the oil and fluid are not physically separated. Even with 'air bags' introduced into the shock, oil under pressure will slowly work away at the bag and degrade it, eventually causing the air and oil to emulsify, which means instant cavitation. A twin-tube is destined to become ineffective over time. Because of the separator piston in a monotube, gas charge remains reliably isolated from the oil column. Bilstein shocks are made with the highest quality materials and personal experience has seen 200k+ miles on factory shocks with practically zero damping loss. The virtue of having higher piston area means all internal components have to work less hard to generate the pressures (and hence forces) needed to control the suspension. The long life of Bilsteins and other quality monotube is no surprise - it is designed in!

Second, let's compare a known twin-tube to a known monotube and put the gas spring rate FM mentions into perspective. On the twin-tube Koni Sports (considered low-pressure shocks), it takes about 40 lb to move the piston to 50% of stroke. An FCM Bilstein (a moderate to high pressure shock monotube) requires about 70 lb to reach the same position - this is a preload, about 1/4" of additional height with a 300 lb/in spring that can be adjusted by ride height changes. However, once at this position (where the shock rests inside the vehicle), it only takes about 5 lb to begin moving the Koni and 15 lbs to begin moving the Bilstein! This is a trivial amount considering the car's weight and the magnitude of weight transferred during any handling manuever or road disturbance. The FCM driver certainly does not notice a delay between when they turn the wheel and when the car responds! Quite the opposite - the suspension is sometimes too responsive and you have to train yourself to use less input, or be going faster for the same input.

It's critical to understand the reason for pressurization. The above links discuss it as well. The analogy is having a pressurized radiator cap that keep the fluid from 'boiling over.' Foaming/cavitation degrade damping, shock oil life, and internal valve/piston life.

This essentially increases your spring rate and makes it harder for the shock to deal with pavement imperfections.

Again, this is misleading - an independent user confirmed our assessment that the FCM Bilstein coilover would have superior ride quality to the FM AFCO. The gas spring rate has two effects - one is a static effect of a slight increase in preload, the other is a 5-15 lb/in spring rate. The preload is compensated for by adjusting the perch. As the choice of main spring goes up, the contribution from the shock spring rate is smaller. The fact is that OPTIMAL damping can ONLY be generated by a monotube shock.

Don't take our word for it, here is a review, in a Miataforum thread titled 'FCM revalve == smooth as butter', where the author found better ride quality in an FCM Bilstein coilover than the AFCO.

BACK-TO-BACK TEST! After increasing his spring rates to 450/300 (same as used on the AFCO suspension), the same user re-confirmed his experience - the FCM custom coil-overs had more grip, rode better and was more comfortable than the AFCO suspension!

With our twin-tube design, the springs do the work of springs and let the shocks do the work of shocks.

The shock's job is to dissipate the energy stored by the spring, through a range of operating conditions. The faster the car goes or the bumpier the road, the greater the need for a functioning damper that can consistently generate the required internal force, then dissipate the resulting heat produced. A twin tube cannot perform either function as well as a monotube. Bilstein monotube shocks are original equipment on many luxury and performance cars, along with truck suspensions which require durability.

The AFCO has less surface area than a Bilstein, which means the AFCO has to move farther to generate the same force as a Bilstein. To compensate, a twin-tube designer will increase internal pressure but that means more time-lag (hysteresis) and less precise damping. We have found that higher compression damping (than typical shock manufacturers use) increases grip and ride quality. No twin tube can develop the necessary force to meet these requirements.

They also have a wider adjustment range than monotubes ...

Wider adjusting range than which monotubes? There is absolutely nothing inherent in the twin tube design that makes this true!

and are more tolerant to damage to the shock body.

In theory, yes. However there's a good reason Bilsteins are the most widely used off-road shock - they are incredibly stout and consistent over a race. Two disadvantage of the double-wall design are poor heat dissipation and higher internal temperatures leading to shock fade (reduced damping force).