Let's face it...most stock Fox Cougar springs are made for mainly one thing: a cushy ride. That's not a bad thing, unless you want the car to handle and corner better. Fortunately you can fix that situation rather easily with a new set of handling springs. Keep in mind that when we're talking "handling", we are also meaning that the springs will lower the car a bit. How much is really up to you—it can be as little as 1/2", or as much as 2"-3". Also, you have to remember that as the spring rate increases, so will the harshness of the ride. And with any drop of 1-1/2" or more, offset steering rack bushings are pretty much a requirement.
You may be tempted to do things the cheap way (cutting coils from the stock springs, heating up the coils with a torch, using spring compressor-type lowering kits) but they will not be good, or safe, or permanent. In order to get a lowered car you're going to have to do things the correct way: installing new springs.
These are springs that will either keep the same ride height, or lower very modestly. For most people that want this look while using very good handling springs and maintaining a bit sportier ride, the 1987-88 XR7/Thunderbird Sport V8 springs, available through your Ford dealer or good parts stores, are probably for you. They have a revised rate over stock springs for better handling and give you a snappy ride without too much jounce. You can use these springs on all V8 cars from 1983-88. Ride is decent with great handling. They are the perfect blend of decent ride and decent performance for this chassis, already engineered by Ford for you. You can purchase new TRW/Moog V8 Sport springs under the following part numbers (thanks to Philippe , Troy and Bryan for the info):
You also have another choice: the 1987-88 Turbo Coupe springs. Yes, that car was a 4-cylinder, but the spring rates were really high. These springs make a good choice for both lowering your V8 Cat, but be warned: expect your car to drop at least 1.5" all around, perhaps 2" or more.
If you own a V6 Cougar, you already know that you shouldn't use a V8 spring. This is because they will actually raise up the car and make handling very dangerous. However, you can use stock 1994-98 Mustang V6 convertible springs just fine in your car. Dimensionally and weight-wise, SN95 Mustangs are a lot closer to the Cougar and Thunderbird than any other car. Several people have reported success using these springs in their Fox Cats and Birds. An additional benefit is that you can usually find them dirt cheap at the salvage yard, swap meets or online.
The same SN95 rule applies to V8 Cougars as well. A set of stock 1994-98 Mustang GT springs give a snappy ride. You can even use 1994-98 Cobra springs, which lowered that car about 1" from the factory.
Clay has some information concerning the 2001 Mustang Bullitt. As you may know, the Bullitt had its own suspension components unique to the car. Any Ford dealer should be able to get them for you. The individual part numbers are as follows:
You should probably use the matching shocks and struts with those springs:
The stock 1995-up Mustang GT spring rates are 450 lb/in linear front and 210 lb/in linear rear, which is almost identical to the M-5300-D springs (see below). The Bullitt springs are 600 lb/in front and 250 lb/in rear.
For years we've been asked, "Do these things exist?" Well, yes they do...you just have to dig quite a bit to find them. Eibach used to make specific lowering springs for 1983-88 Cougars/Thunderbirds. Some people have reported that the kit requires some sort of strange modification to the car; we cannot confirm this. These springs are no longer produced; last production was in 2000. JC Whitney also occasionally sells a type of lowering spring; how it works is unknown, and it's been out of stock for several years now.
It used to be that you could order lowering kits from Ford Racing. Two different kits were available: the M-5300-D (1983-86) and M-5300-E (1987-88). Ford Racing has stopped selling these springs as of 1999. You can really use either kit with any 1983-88 car; the spring rates were slightly different but not significantly so. The kit lowered V6 cars about 3/4"; for V8 cars, an easy inch or more. They were not recommended for turbo-4 cars though. Most people really loved these springs and they're almost legendary. If you can find them, buy them!
If you just want to say goodbye to the dreaded sag while keeping the stock front springs, then we'd recommend what's known as the cargo coil. It's basically a heavy duty variable rate spring intended for towing, but it works great at keeping your back end up to proper ride height. Plus, the more weight you add to the car, the more the spring will try to force the car body up to correct ride levels. If you have a stereo system and/or subwoofer box in the trunk, these springs work miracles for you. Highly recommended. TRW/Moog part number is CC823.
Now if you tow a lot, there are aftermarket kits available that allow you to place an adjustable inflatable bladder inside the rear springs. With the on-board air compressor mounted, you can adjust the air bags to give you a floaty yet controlled ride while keeping the back end of the car at normal height. They work pretty good, and the kits aren't all that much money either. Might be worth looking into even if you don't tow. This can have an added bonus of giving you the "low-rider" look.
Along similar lines are the stock air bag suspensions from the Lincoln Mark VII/Continental. These air bags replaced the traditional coil spring and were auto-leveling from the factory via a microprocessor. Especially early on (circa 1984), there were many problems with air leakage, so be aware of this. With enough patience, it is entirely possible to swap out your suspension for this type of air bag support. Be prepared to run lots of air lines and wiring though. With a Cougar, an air bag suspension does sort of fit into the car's image, so this would be a natural step for you should you decide to do it. You really won't gain any performance handling with air bags; in fact, you'll lose quite a bit. But you will get a silky-smooth luxury ride.
Last, the temptation may be there to use a 1979-93 Mustang spring. Physically they will fit and work. However, most people report very harsh ride quality, and some bounciness over road imperfections and bumps. These springs really cannot be recommended for the level of quality you're probably wanting. Any of the other alternatives mentioned above will work much better than these springs.