The heights of automotive technology always arrives in sports cars first. Historically, that new doodad that increases power while improving efficiency always hits Formula 1 first, then NASCAR, and then it filters its way down to the world's premier sports car brands. And soon enough, wealthy celebs will be parading their newest world-beating hypercar in tabloid magazines and on social media while the rest of the universe is left thinking, "Yah, well at least my Camry gets 33 MPG." But wouldn't it be nice to pull up next to Floyd Mayweather behind the wheel of an old, beat-up commuter and the moment the lights turns green, knock him out with one blip of the throttle? Talk about revenge on behalf of Pacquiao. And so, while the world's wealthiest gearheads enjoy their Bugattis, Ferraris, and Lamborghinis, the regular folk are left dreaming of the concept known as the 'sleeper' in response.
Updated Februaru 2022: If you want to build a sick sleeper car, you'll be happy to know that we've updated this article with more details on the dos and don'ts of sleepers so you can build an inconspicuous vehicle that's capable of slaying supercars.
The sleeper class of builds differentiates itself from high-end sports cars and custom tune-jobs by keeping the vehicle's exterior completely low-key. But under the innocuous skin lurks a secret: usually in the form of some maniacally genius conception of how to pack serious power into a tiny engine bay. The key is to keep all cues towards potential performance under wraps; minivans, beaters, and tiny cars make excellent platforms for a sleeper build—and even keeping the engine from rumbling too loud as a giveaway comes into play. Interested in building a sleeper in your backyard? Keep scrolling for 10 first steps and 10 things to avoid because they ruin any potential for sleepiness.
20 Perfectly Sleepy: Drop In A V8
Paul Newman loved a Volvo with a V8 under the hood, which means the rest of the world should, too. When it comes to goood, old-fashioned beef, not much beats a V8—whether you're a Detroit fanboy, a Mad Max: Fury Road disciple, or just a regular driver who occasionally notices the badges on the back of (fewer and fewer) cars in the next lane.
The V8 engine optimizes balance, power, and weight provided that it will fit into the engine bay of a sleeper build. Some wild builders have even just dropped a V8 into the backseat of their sleeper to make it a mid-engined monster. Either way, today's crate small-blocks are a great building block to plan a sleeper around.
19 Total Waste: Huge Exhaust
Yes, bigger exhaust can allow a car to have more power. But a huge exhaust system is one of the surest ways to ruin a sleeper build from the get-go. Not only is it readily apparent that something special is going on under the skin just from looking at the car, but huge exhaust also creates way too much noise to maintain a sleepy vibe.
If the neighbors wake up every time the car starts up in the morning, it's definitely not a sleeper. If everyone at Starbucks turns to stare in annoyance at a stoplight, it's definitely not a sleeper.
18 Perfectly Sleepy: Turbocharge It
Not every car can handle the naturally aspirated potential of a V8 engine (or bigger). It's abundantly clear from the wide range of race cars and sports cars these days, however, that forced induction is becoming a more popular way of improving power output and efficiency. And with more and more aftermarket turbo manufacturers with inventories available online, finding a turbo to drop onto a modern engine has never been easier.
Of course, this route requires quite a bit of know-how; upgraded internals become a necessity when pushing higher levels of boost. (A turbo isn't a bad idea to bolt onto a V8, either, it turns out.)
17 Total Waste: Giant Wheels
So many sports cars leave the factory with wheels measuring up to 19 or 20 inches these days that it seems like huge rims are a must-have for any performance build. But going overboard can actually ruin a sleeper's performance—while also ruining the sleepiness.
Donks with 28-inchers are silly, but equally silly are Subarus running 22s. Take a look at any rally car that's seen actual competition and it becomes clear that huge wheels and low-profile tires aren't even beneficial to real-life performance. They'll ruin comfort, lead to more flat tires, and worst of all, warn everyone that the car pulling up might have something going on under the hood.
16 Perfectly Sleepy: Supercharge It
Turbochargers suffer from the famous syndrome known as 'turbo lag' because they utilize exhaust gases to spool up before they begin cranking out max PSI. For drivers who dream of a sleeper with instantaneously available torque, a supercharger might be a better bet.
Superchargers are belt driven by the engine, meaning their power delivery comes in a smoother, more predictable fashion. They may lack the jerk that slams everyone back into their seats when a turbo finally reaches full pressure, but they've got a bit more to offer at lower RPMs. Plus, superchargers are typically considered more reliable because they (usually) max out at slightly lower levels of compression.
15 Total Waste: CAI
Cold Air Intakes are increasingly popular among amateur tuners looking to get a little more horsepower. But all they really do is increase the 'whoosh' noise of an engine's intake system—and they're typically ruining performance, in reality.
Much like the Clean Air Act reduced greenhouse gas restrictions despite its name, most CAIs are actually causing hot air to be sucked into the engine because they're installed by removing the air box. Throw in the fact that aftermarket air filters tend to prioritize airflow rather than filtration, and a CAI could be ruining the engine while simultaneously reducing its real-life output.
14 Perfectly Sleepy: Bigger Intercooler
Whether choosing to bolt a turbocharger or a supercharger onto a sleeper built, everyone utilizing forced induction should look into upgrading their intercooler. Unfortunately, one of the downsides that physics wreaks on forced induction via turbochargers and superchargers is the fact that compressing air heats it up. And hot air isn't good for engines—in fact, a major goal of engine management is keeping the entire system relatively cool to increase efficiency and longevity.
An intercooler installed between the turbine and the engine cools that air in the same manner as a radiator, leading to improved combustion in the cylinder by the time it reaches the engine.
13 Total Waste: Nitrous
Nitrous oxide systems became popular after The Fast and the Furious and Gone In 60 Seconds popularized the idea of pushing a button and gaining a couple of hundred extra horsepower. But anyone thinking that nitrous is a good addition to their sleeper build should go the tried-and-true routes of turbo- and supercharging well before they actually install nitrous on their cars.
Not only does using nitrous oxide have the potential to blow an engine after one hard pull, but it's also illegal in most regions because of exactly how it improves engine output.
12 Perfectly Sleepy: ECU Tune
Any car can benefit from an ECU tune. Ever since engine management software proliferated around the automotive industry, carmakers have been using computers to keep their engines in what they deem as they optimal operational parameters. But their concept and the concept of a speed freak don't always mesh, so plenty of aftermarket tuning companies have popped up and offer consumers a way to optimize their ECU programming.
A tune is especially effective for cars with turbos, and can easily improve low-end torque, top-end horsepower, and overall efficiency simultaneously. Sound too good to be true? Chalk it up to conservative carmakers who are more interested in avoiding scandal than pleasing customers.
11 Total Waste: Huge Wing
A surefire way to let everyone know that a car is packing heat is to throw a massive wing on the rear end. In fact, it's a surefire way to let everyone know that the car's owner is delusional—after all, plenty of people bolt wings onto their otherwise unmodified Civics in the hopes of just looking cool (which is literally the opposite of what a sleeper should do).
But even on heavily modded cars, that rear wing is unlikely to actually help with any kind of performance because it's almost certainly causing more drag than it is any kind of downforce to help the tail stay grounded.
10 Perfectly Sleepy: Tires
Tires are the single best investment that any car owner can make to improve their vehicle's performance—whether they're interested in drag-strip acceleration, nimble cornering, all-terrain traction, or vehicle longevity. Almost everything that a car does has to be translated to the tarmac through the tires, so a few extra bucks on higher quality tires goes a long way.
Just dropping in a V8 or bolting on a turbo will be useless if the tires start spinning with the slightest blip of the throttle and even though they are on the vehicle's exterior, not many people can tell the difference between high-end and low-end tires at first glance (or at all).
9 Total Waste: Limo Tint
Getting a limo-level limo tint might seem like it keeps things super low-key. And in some ways, a limo tint does seem sleepy. But in reality, anyone driving around with a tint so dark they can barely see the cars around them isn't about to be slamming their car into overdrive without rolling all their windows down.
Instead of having an actual sleeper, they're more likely to be falling asleep because it seems like it's night out at 12 o'clock noon. Plus, limo tints are a favorite among celebrities (and their sports cars) because of the added anonymity—and being like celebrities is the opposite of how a sleeper build should be conceived.
8 Perfectly Sleepy: Rear Diffuser
Part of the sleeper aesthetic and ethos is about flying below the radar. Because of that, not many improvements can be made to the vehicle's exterior without giving away the performance that's lurking below the shell. Luckily, there's one solid method for improving aerodynamics that doesn't show nearly as much as the others: a rear diffuser.
Not many people are liable to notice a subtle rear diffuser popping out below the rear bumper cover, but they're a cheap and subtle addition and have the ability to increase downforce at the tail end (which helps keep the rear wheels planted during acceleration, braking, and cornering).
7 Total Waste: Stance
Stance enthusiasts don't want to be about sleepers and, truly, can't be about sleepers. In fact, cars that have been stanced are just about the opposite of what a sleeper can be: impractical, useless, and all about style over substance.
Not only does the stance style ruin a sleeper build's below-the-radar exterior, but it also renders the vehicle unable to outperform a shopping cart, much less than corporate exec in his Maserati that's always flapping into his cell phone and running stop signs like he's the most important person this side of Elon Musk. Stanced cars are ruined cars while sleepers should be rescued cars.
6 Perfectly Sleepy: Sway Bars
As any fan of the Fast and Furious franchise can readily attest, the driver is more important than the car. But part of a driver's ability does come from being able to understand the feedback their car is providing. And while anyone can mash on the gas pedal and hope to burnout going in a straight line, it takes more skill to feel a car's subtle weight changes during hard cornering.
One way to improve weight transfer from side to side is to invest in an upgraded set of sway bars (front and rear), which will help make a vehicle's weight shifts feel more predictable. After all, sleepers need to be able to take on a Porsche in the twisties, not just a Hellcat in a straightaway.
5 Total Waste: Fake Scoops
A quintessential sleeper should have all the style of grandma's grocery-getter and the opposite level of performance. On the exterior, every penny should be saved rather than invested in a new paint job or anything flashy. Under the hood, no expense should be spared to crank out every last additional horsepower and all the torque humanly possible.
Fake air intakes like hood scoops or side inlets ruin a sleeper not just because they make it clear the car is trying to be something it's not, but also because they reveal that the car's owner hasn't really thought out what they're up to.
4 Perfectly Sleepy: Exhaust Cutout
While most sleepers focus on keeping their visual cues to a minimum, plenty can be learned about a car just from listening to it. And there's nothing subtle about pulling up to a stoplight with a big V8 engine roaring, snorting, and burbling through race-style, straight exhaust pipes. But straight pipes to help the engine exhale better and create more power.
One way to have the best of both worlds is to install an exhaust cutout that opens up at the push of a button or at a certain RPM, thereby improving airflow at the perfect moment to blast by an unsuspecting Lamborghini owner.
3 Total Waste: Stickers
This one should seem pretty obvious but somehow, more and more tuners keep adding stickers to their cars to show off the brands of aftermarket parts they've used. The grocery list above satirizes the practice, which has developed as a (slightly) more subtle way of mimicking the branding on actual competition race cars.
But those stickers on real racers are meant to signify monetary investment by the brands, rather than the fact that the racing companies paid the likes of Bilstein, Cobb, or Red Bull. Paying to turn a car into a rolling billboard should be the last thing any sleeper's owner wants to do.
2 Perfectly Sleepy: LSD
No, LSD stands for Limited-Slip Differential. The differentials that most cars leave the factory with—other than hardcore models—is typically an open differential. This sacrifices performance at the edge of traction in favor of improved efficiency while cornering since the outside wheel is allowed to travel further when the car is turning.
During spirited driving, this means that only one wheel is actually sending power to the ground (and on 4x4s without LSDs or even locking differentials, only two wheels are). Finding the right limited-slip differential to swap in may be a bit of a challenge, but the investment is worth it for the greater predictability of handling.
1 Total Waste: Shift Stick
With the increasing popularity of JDM culture, drifting, and gymkhana has come a focus on features that are cartoonishly out of proportion, like enormous shift levers and emergency brake handles. No, you're not Ken Block. And no, you don't need a shift stick that's the hilt of a Samurai sword. And no, you shouldn't use your E-brake to pull into McDonald's for a Big Mac (and no, you shouldn't be eating Big Macs).
Rather than a shift stick, a short shifter is a more worthwhile investment—though in reality, unless a shifter rebuild becomes necessary, neither is particularly necessary for a sleeper build.
Sources: ECS Tuning, Pelican Parts, and VW Vortex.