I just wanted to eliminate a common misconception. Bigger is not better, stiffer is not firmer, price does not equal performance.
A KYB Gas-A-Just shock is a monotube shock. There are very few companies that make an equivalent shock. They are in a complete different ballgame as compared to anything Monroe or Gabriel makes. Almost every shock made by Monroe or Gabriel are twin-tube shocks.
A twin tube shock, like the name implies, has two tubes inside it. The outer tube acts as the housing as well as a container for the oil and gas inside the shock. The outer tube can be quite large, but the only affect this has is to increase the amount of gas and oil in the shock. The inner tube is the main part of the shock and it is the guide for the shock shaft and the piston. These are far smaller in diameter than the outer tube. There are holes or other outlets along the inner tube to allow the gas and oil to flow out and bypass the piston on the shaft these holes are varied in size and number to produce resistance in a given shock for a given application. Most twin tubes also have a valve in the piston that allows oil and gas to flow through it as well and is pressure activated (i.e. a sudden sharp jolt will open the valve and allow the shock to absorb the impact). The main problem with twin tube shock is that the oil and gas are mixed together. When a shock has been sitting for a long time they will separate. After a few minutes of driving they will be mixed again. After a longer period of driving, or if you hit some rough road, the oil and gas mixture will start to foam. When foaming occurs, the shock becomes unstable as the piston can travel through the foamed oil with much less resistance than normal, losing ride quality and handling.
A MonoTube shock has 1 tube (yeah). It performs the same functions as both tubes in the twin tube shock, it is the housing and the guide for the piston and shaft. It is a larger diameter tube than found in twintube shocks. This, obviously (law of physics) provides much greater stability and control, over a smaller piston and tube. The gas and oil in a monotube are separated. There is a follower puck inside the bottom of the tube that seals the gas to one end of the shock.This puck can move as the shock shaft and pressure dictate allowing the gas to expand and contract. The piston in a monotube also has valving to allow the oil to pass through the piston creating the dampening effect. Firmness and dampening on a monotube are controlled by the pressure of the gas and the valving in the piston. KYB varies these in every shock to try and approximate a ride similar to factory, but with a much improved firmness and handling, typically 15-20% "firmer".
All Gabriel and 90% of all Monroe shocks are twintube as are almost all other brands. Besides KYB, Bilstein and Edelbrock are pretty much the only other aftermarket monotube shock makers. The only monotubes Monroe and Gabriel make are for a few select truck applications. In all fairness, the differences between a monotube and twin tube will rarely be noticed unless you are a road racer, drive long trips, you have a top heavy vehicle (i.e. durangos, tahoes, suburbans, etc.) or do towing/hauling.
Aside from all this, KYB does a few things that very few other manufacturers do. First is the shock bushing and mounting loop. KYB makes them from a solid steel tube. Everyone else tends to take a flat piece of metal and roll it (you will see the seam) then weld it to the shaft or the housing. KYB goes one step further, by welding a pin to the loop on the bottom, then pressing this into the housing and welding it again, providing an extra degree of strength on the lower mounts. They also taper the bushings and the loop to lock the bushing and sleeves in place. Pretty much the only way to get a sleeve or bushing out of a KYB shock is to completely destroy them. KYBs also have hardened chrome shafts to prevent rust, pits, scratches and any other imperfections that could ruin the main seal on the shaft.
One thing to keep in mind, KYB has 3 grades of shocks, the GR-2, Gas-A-Just, and the MonoMax.
The GR-2 is a Twin-Tube shock, but made with the KYB quality I mentioned above. They are designed with O.E. ride in mind and usually 5-10% firmer than original. They are grey or black in color. Usually $20 to low $30s in price.
The Gas-A-Just is the original monotube, $30-60 in price.
The Monomax are a monotube with a larger bore than the Gas-A-Just but usually with less aggressive valving. These are limited to only trucks and suvs and provide a good all around performance, will still retaining some of that Cadillac plushness. Priced $40-$80.
Gabriel and Monroe are all twin tube designs and they generally fall into 3-4 categories. The cheap shock is a generic low quality shock that has no vehicle specific valving, basically cheap junk, usually less than $20, I would never buy these for any reason. Next is the Guardian (Gabriel) or Sensatrac (Monroe) which are an O.E. like twintube shock, usually $20-$40 in price. Last is the Gabriel MaxControl, Monroe Reflex, or Monroe Magnum. The only difference between these and the lower grade shocks is the valving and levels of gas/oil. Price is $30-$70+
