What exactly are the disadvantages? I need Facts/Numbers not trying to put you on the spot, but you and Denny seem to be the guys who understand the geometry of these cars.
They add unsprung weight. They are not as efficient in handling applied loads, because the coilovers are mounted at an angle which translates forces in two planes and changes how the suspension reacts as the control arms move through their arc. Once the ride height has been set it's more problematic for the geometry to change it significantly. They apply forces to an areas of the chassis that were not in any way designed to carry it. They move the weight of the suspension higher on the car.
For numbers I'd need to have a suspension analysis program and the dimensions of the coilover systems. But adding a different type of suspension to a chassis that was not designed for it places some significant compromises into the design. Control arm length, mounting points etc are usually pretty limited, and that obviously has an effect on camber curves etc. Not to mention that the chassis of our cars was designed to car the suspension loads in the K frame and crossmember, not the rails and uprights.
Again, I'm not saying that torsion bars are a
superior system, or that coilovers are the scourge of the Earth. Each system has advantages and disadvantages. I think by far the biggest advantages of the coilovers is their ability to be tuned. There are by far more spring and shock combinations available with coilovers. They also provide a lot more space under the car, and overall tend to remove weight, even if they raise the location of some of that weight on the car. And of course the use of a steering rack instead of a box is a decided advantage. More options, no rebuilt boxes with worn gears, less weight, better feel, etc.
But, even though the tuning options are more limited, the stock suspension offers a very nice camber curve, especially if the ride height is adjusted so that the control arms are parallel to the ground. There's very little bump steer with the stock system at that ride height, so there's no real improvement to be gained there. The roll center is pretty good at that height as well. With tubular UCA's you have the adjustability to set alignments anywhere from mild street to aggressive road or autoX. Even just offset UCA bushings can get you decent negative camber and positive caster settings.
I think tomswheels said it best, the coilover conversions will make it
easier to drive fast. And they do make the car easier to set up. But that doesn't mean that the stock system can't be driven just as fast (or faster, as tom has demonstrated), or tuned properly. And I think that for the vast majority of drivers (myself included), the limiting factor in how competitive their cars are will not be the torsion bar suspension design, which makes spending thousands of extra dollars pretty unnecessary. And even if I
can't get the stock system tuned perfectly for my application I'm going to have just as much fun, even if I'm not as absolutely fast as possible. Still, I think cars like the Green Brick, the Hotchkis Challenger, and a few others have more than demonstrated that the torsion bar system
CAN be competitive.
It really comes down to what you want and how you're going to use the car. I see absolutely no reason whatsoever why I need to add a coilover system to any of my cars. None. Take a look at the Hotchkis '76 Dart Sport build. That car is changing EVERYTHING. Loads of custom fabrication so that the car can compete here and abroad at things like Pike's Peak and Targa Newfoundland. But yet, they're keeping the torsion bars. If the stock system was that awful, and budget and fabrication is basically a non-issue, why would you keep it?
http://www.forabodiesonly.com/mopar/showthread.php?t=270106
