It wasn't long ago that true sleepers were available from the showroom floor. If none really interested you, building your own was as easy as calling a few aftermarket tuners to turn that sedate daily driver into a fire-breathing powerhouse. Today it seems the sleeper has all but disappeared; fewer and fewer enthusiasts are building them on modern platforms.
Defining a sleeper is difficult. What constitutes a sleeper will vary proportionally to the number of enthusiasts asked. The most general term refers to a car whose pedestrian appearance belies its performance. Performance, however, is also a somewhat subjective term. Do we consider sleeper performance to be based on straight-line ability, handling, braking? Maybe it's a combination of all three.
Manufacturers have moved away from building sleepers for a variety of reasons. From a marketing perspective, they want to advertise just how quick their brand-new machinery is. Vehicles like BMW M cars, Mercedes AMGs, and Audi S and RS lines are all hot-rod versions of more mainstream offerings. They still have the same basic shape of the original platform, but are festooned with more aggressive aerodynamic packages, larger wheels, gloriously detailed brake calipers and large, snarling, chrome exhaust tips that scare small children.
From a technical perspective, those traits-wheels, brakes and shiny orifices-are all necessary to get the desired performance. It's also necessary to consider how much faster they can (or even need to) make a true sleeper. Performance figures from most base European models are enough to send 15-year-old sports cars to run and hide. do you need to build a sleeper version of an E90 335i when it's equipped with over 300 lbft of torque from the factory? It'll already give the previousgeneration M3 a run for its money-and isn't that far behind the current model.
The same can be said for just about every category. The lowest powered S-Class comes with 391 lb-ft of torque and will run to 60 mph in six seconds. The base Volkswagen Rabbit now leaves the factory with 180 lb-ft. That's nearly 40 lb-ft more than the Mk II 16-valve GTI, still considered by many to be the ultimate example of the bloodline. Performance is relative, but at some point you have to try and determine how much is sufficient and at what point sacrifices in usability outweigh performance.
With the continual improvement of factory vehicles, tuners are having a tougher time creating real sleepers. Part of it lies with cars being optimized from the factory. It's becoming increasingly difficult to wring power out of factory setups with simple bolt-on upgrades. At one time, it was easy to add one or two components for a noticeable improvement without the tell-tale outward signs of modification. Engines now require a much broader view for real improvement. You can swap a part here and a part there, but true optimization requires a full tuning package. It's harder to hide extensive modification than one or two key components, making them more obvious to outside observers. More power may come from software tuning, but increasing boost on a forced induction engine too much will work past the limits of the fueling and intercooling systems. Increased boost can soon find the factory exhaust's shortcomings as well. Clearly, a complete solution is the answer.