Twenty years ago, the "All-New" 1989 Carrera 4 was about eighty-five percent new. The bulk of the changes were mechanical. The visible differences included the bumpers, side skirts, and spoilers. The 993, which arrived in 1995, appeared to have been a larger design change. However, the 993 was only thirty percent new. The 993's makeover was the more visually exoteric of the two. Since the 964 looked a lot like the outgoing Carrera 3.2, yet cost $8K more, Porsche had to make sure prospective buyers knew this was "all new".
Technologically the 964 was a breakthrough car. The 3.2 liter engine was increased to 3.6 liters. This increased horsepower to 250, and gave the car more bottom end. In addition, the car had new underpinnings. Out back, a new multi-link suspension was placed in a subframe. This made the car a lot quieter and improved both the ride and handling. Torsion bars were exiled and coil-overs were used at all four corners. The front still used a MacPherson design, and the rear still used semi-trailing arms.
Like many 911s, there were some problems. Many of the problems were serious, but none were insurmountable. If you have a 964, you already know about these issues. If you're in the market, this will tell you what to look for. The good news is, being a 20 year old car, most of the problems should have been resolved long ago. Once the initial issues have been corrected, the 964 can be a beautiful car.
The mechanical issues began with the first models. Some 1989 through 1991 3.6 engines leaked between the head and the cylinder. Ordinarily the problem would be described as a blown head gasket. However, the initial cars didn't have head gaskets. That made it more of a "cylinder to head" sealing issue. While the vehicle was under warranty, if the 'cylinder to head joint' was wet to the touch, the dealer would fix it. The factory fix was to machine the cylinders and install head gaskets. Those that weren't fixed under warrantee have probably been fixed by now. If it hasn't been fixed yet, it may not be a problem.
Initially, this may sound odd. That's because some view this as a blown head gasket. It's not. The original factory seal was an aluminum head bolted to an aluminum cylinder. The reason dealers fixed it when it was "wet to the touch" was because when the engine was turned off, oftentimes there was some fuel in the combustion chamber. As the engine cooled, that fuel pooled, and ran downward. If the aluminum to aluminum seal wasn't perfect, the fuel could find it's way out. When the engine was at operating temperature, there wasn't a seal issue. On the other hand, if the seal is a problem and has to be fixed, it can get expensive.
People have repaired the leakage in a number of ways. Everything from backyard band-aids, to 'for sale' signs, and professional repairs. Personally, I'd call a good machine shop like Competition Engineering in Lake Isabella and ask what they recommend. They'll probably want to see the heads, pistons, and cylinders, charge for an inspection, and then make a recommendation.
Aside from the cylinder to head leak, some 964's suffered from premature valve guide wear and/or worn valve stems. The fix for that is a valve job.
The 964 had a dual-plug ignition system. The second set of plugs was fired by a second rotor in the dual distributor system. That second rotor was driven by a drive belt from the first rotor. The belt was known to fail. When that happens, the rotor stops spinning. Unfortunately, the ignition will continue to fire. Since the rotor is in the wrong position, the wrong sparkplug fires. The misfire can cause detonation. Unchecked, detonation can lead to a premature engine rebuild. Porsche later released an updated distributor and belt. This is a simple fix.
The all-wheel drive system used in the C4 was a different story. These early systems were incredibly complex and repair prices reflect it. When working, this system is fantastic. When not, it can be a money pit. The thousand dollar sensors are only eclipsed by the 964 specific expertise that's required to diagnose which sensor has failed. For all-wheel drive, you may want to consider the 993. The 993 was introduced in 1995, and it's all wheel drive system was completely revamped. The resulting system was lighter, much less complex, and more pocket book friendly.
Another problem was the dual mass flywheel. Those were introduced for the 1990 model year and discontinued in May of 1992. In a nutshell, the flywheels came apart.
Once past these problems, the 964 was a great car. The new suspension gave the car a more neutral feel. Coupling this with the new powerplant, made the 964 a faster car than the previous model. As was the case with all previous 911's, the fastest way through a corner was "slow in" and "fast out". The new suspension allowed a faster entry speed, while the new engine's additional torque produced a faster exit speed.
The 964 may be viewed by some to be undesirable. The later 993 was more visually appealing, and less maintenance intensive (hydraulic lifters). The 3.2 Carrera was supposedly a more dependable model. For someone in the market for a 911, that makes the 964 an interesting option.
I'd personally go with a later model 964. Aside from normal production improvements, they had head gasket o-rings and plastic intake manifolds. One shortcoming all the 964s had was a restrictive exhaust system. The 993 exhaust was much freer flowing and produces close to 20 horsepower on the 964 based 3.6 engines.
This isn't a 993 page, but I'll close by covering some of the differences between the 964 based 3.6, and it's 993 based successor. As everyone knows, the 993 is a faster car. The 993 based 3.6 made 270 horsepower. I believe much of the 993's improved acceleration is due to the lighter crank and pulley assembly. Lightening the rotating assembly usually doesn't change peak horsepower, but the ability to rev faster will widen the power band and make dramatic driving improvements. Aside from the lighter rotating assembly, the 993 has the aforementioned improved exhaust. The 993 also has larger intake ports and matching runners. The new ports are 43 mm, whereas the 964 has 41.5 mm ports. On the exhaust side, the 993 has larger exhaust valves, 42.5 mm versus 41.5 mm. 993's have reduced maintenance due to the use of hydraulic lifters, and a more precise engine management system. In 1996, the VarioRam system made it's debut. VarioRam is Porsche's version of a variable length intake manifold. Variable length intake manifolds allow the intake stream to follow a longer path at low RPMs. This longer path improves low and mid-range torque. At higher RPMs, the longer paths are closed and shorter paths are used. This increases power in the upper rev range.