Fair warning, this is a long post.
If you’ve spent any amount of time around cars, you’ve heard people talk about ‘sleepers’—cars that exasperate and confuse some while they entertain and are enjoyed by others.
So, what exactly is a ‘sleeper’?
A good working definition of a sleeper is:
A car that, while it looks relatively standard on the outside, is actually quite fast, handles well, and is more powerful than it should be. They’re built from cars that are not normally fast, or even fast-looking.
These cars are often sedans, station wagons, economy compacts, or other unassuming cars. The difference, though, is they’ve been modified—with forced induction (either turbo or supercharger), better suspension, an entire engine and drive-train swap—often to a V8 in a car that was never equipped with one, or just some expert knowledge.
The purpose of a sleeper is to have a fast, fun car to drive that otherwise looks like your average boring car—and that nobody can really identify, until you hit the gas or open the hood.
There are some people, though (as always), that don’t quite get what a ‘sleeper’ is or why you would want one. And there are, all too often, people who build cars that they call ‘sleepers’, but are most definitely not.
Where people go wrong
The main place where people go wrong when they’re building a sleeper is they make it too conspicuous.
There are some modifications made that you cant help but make visible—brake upgrades, wheel upgrades, moderate suspension lowering, exhaust upgrades—and that is perfectly fine. These are things you cant get around when improving the power ans handling of your car, and often most people won’t notice right away.
Its the modifications that are generally aesthetic-only that give too much away and call more attention to your car than you should want if you’re building a sleeper.
It’s not uncommon for someone modifying their car to want people to notice the work they’ve done. Whether they’re adding wings, “slamming” the car, putting oversized, flashy wheels on, or covering the car in “sponsor” stickers, all these things do is say “look at me!”.
The other thing that they do is tell people that their car is a sleeper. Sure, you didn’t cover your car in stickers, but telling everyone that you have a “sleeper” takes away from the fact that you built one in the first place. They should be surprised when you open your hood at the local meet, or post a 10 second run at the drag strip with you grocery-getter.
The rules for building a sleeper
So, there aren’t really any written rules for how you should build a sleeper. There are some guidelines that apply to most, though.
Start with a plan
I can’t stress this enough. You need to have an idea of what you want out of your sleeper before you start. There’s nothing worse than getting part of the way through a project and deciding that you need to replace parts you’ve already bought and installed.
This doesn’t mean that you can’t upgrade later, you just don’t want to buy parts and decide that you want to do something else entirely.
Plus, a plan will help you think about what you’re doing and budget accordingly. (It’ll also help you decide what tools you need to buy, which can ruin any budget you’ve prepared.)
Keep it low key
There’s nothing worse for a sleeper than it grabbing people’s attention.
The goal with any good sleeper is for it to surprise anyone who happens to notice it—usually when it pulls away from a Mustang at a light, leaving the Mustang’s driver to pick their jaw up off the ground and play catch-up to the next light.
Sleeper’s aren’t built by accident: they’re intentionally built so that no one would want to pay attention to it. From the beginning it should be a car that looks boring. No wings. No side skirts. Stock wheels. Boring paint.
If at all possible, and you have the option, it should be a base model—or at least not the sport model—of whatever you’ve chosen. Also, four doors helps a lot, too. It gives off that ‘family car’ vibe that people tend to ignore immediately.
If you’re going to do any suspension work—and you should, since you’re going to be increasing the power—limit any lowering to less than 2 inches. Any more than that and it’ll give away your secrets.
Concentrate on power and handling
With the design under control (because you shouldn’t be changing much to begin with) it’s time to look at the important bits—your engine, drivetrain, and suspension. They’re the keys to going fast and keeping the car under control.
The first thing anyone even thinking about building a sleeper should do is maintenance.
I know this might sound silly, but you can’t evaluate a car if its not in its best condition. Start with proper engine and suspension maintenance: give your engine a tune up, lash the valves, replace old bushings, make sure your springs and shocks are in good working order. Then go for a drive. Or two. Or three.
It doesn’t always make sense to replace some parts with stock if you’re just going to be replacing them again with high performance parts, but this does give you a baseline from which you can gauge your upgrades. This is up to personal preference and budget, but I still stand by it as a good starting point. If your budget doesn’t allow for it, then at least try to test drive a well-maintained stock version of your car if you can.
Once you know how the car handles when its been maintained, you can move on to upgrading things.
To begin, the best modifications you can do are to the suspension. By improving the weight transfer and handling of any car you can go faster without having to increase the power one bit.
Springs, shocks, and struts are probably the first thing you’ll change.
When you replace these, you need to make sure that the shocks/struts and springs work together. You also need to decide what sort of drop in ride height you’re looking for. Since you’re building a sleeper, you don’t want to make things too low. Usually a drop of one to two inches is enough to lower the center of gravity for you car without making it too obvious. You’ll also want to decide on the stiffness of the ride that you’re looking for. Sleepers generally function as daily drivers while at the same time having some performance behind them, so you’ll want to make sure that you don’t make the ride too harsh, otherwise you’ll quickly wish that you had bought something else.
Coil-overs are a wonderful investment, though they can be expensive. They also are meant for more performance-centered driving, allowing you to dial in suspension stiffness and ride height (center of gravity) for the terrain you’ll be driving on, but this doesn’t mean that its right for everyone—sometimes lowering springs paired with appropriate shocks and struts will get the job done just fine.
With the ride height, springs, and shocks/struts under control, you’ll notice an improvement in the handling of your car, but there are always more things you can do to help dial the suspension in the way you want it. These options include bushings and sway bars.
Sway bars (or anti-roll bars) tie the left and right sides of your suspension together, helping to hold your tires on the ground while cornering. These are what control body roll and tire contact. The larger the sway bar (or adding an additional bar in cars that only have one) can reduce body roll and increase tire contact through a corner.
You should be careful, though, as too large of a sway bar can produce oversteer (most cars that qualify as candidates for sleepers are generally engineered with some level of understeer as a safety feature). You’ll want to be careful to increase the diameter of sway bars in a balanced way so that the car handles in a predictable way—though this also means that in order to achieve a specific goal you may need to adjust your driving style or habits to accommodate the altered handling characteristics of the car.
In addition to sway bars, there is also the option of adding a strut tower bar (or bars) to the top end of the front suspension and chassis braces underneath. These will add more stiffening to both the suspension and the body by reducing the amount that the strut towers can flex while cornering.
Another option for stiffening up suspension is to replace the rubber bushings with polyurethane. Polyurethane tends to be stiffer than rubber and also has a longer working life compared the the stock rubber bushings that your car came with.
Replacing your bushings with polyurethane, as with all of the above suspension modifications will increase the harshness of your ride, so replace them with caution.
Now that your car has more power, and can control it through the corners, you need to start thinking about being able to stop the car.
There are a variety of options for improving the braking system of your car. Some include using higher quality brake disks and pads, replacing drum brakes with disc brakes, and replacing the stock braking system with high performance or larger braking systems.
What changes you make to your braking system are dependent on your goals and the modifications you’re making to your car. Regardless of your goals, though, being able to stop your car is the most important thing you can do.
With better suspension to keep your wheels planted and better brakes to help stop the car, one other piece of the puzzle that needs to be considered is the tires.
Whether you’re keeping the stock wheel size or upgrading to a larger wheel, having good tires underneath your car will help with cornering, acceleration, and stopping. When choosing a tire, keeping the wheel it will be mounted to in mind is important. If you’re looking for better traction, having a wheel and tire combination that is wider will help with this. Selecting a wheel and tire combo that reduces the sidewall height will also help with transferring weight while cornering by reducing the amount of flexing the tire can do.
Often, a better wheel and tire combination can greatly improve your traction and cornering, even without a stiffer suspension. It can also help with breaking by using a higher performance tire. At the same time, tires should not be a replacement for better braking, especially if you plan on driving your car at higher speeds.
Now that your car is handling like a champ (and is most definitely faster through the corners) its time to look at increasing the power.
There are a few different ways to go about this: tuning the engine with performance parts and knowledge, forced induction, an engine and drivetrain swap, or adding combustibles like nitrous.
Engine tuning is the hallmark of creating power. It’s also arguably the most difficult way to increase power. It takes a combination of specially made performance parts and knowledge about how different engine settings affect power output to increase the amount of air and fuel that can be put into an engine and the amount of power produced by this.
Increased power is often produced by removing restrictions in the air intake and exhaust, modifying the cylinder head, increasing the size of the intake and exhaust valves, using performance cam shafts, change the size and shape of the pistons… and many, many more tweaks, parts, and settings.
What this does is increase the compression ratio (CR) of the combustion chamber, increase the amount of air and fuel in the cylinder, change the amount of air and exhaust that are able to enter and exit the cylinder, and help the and exhaust flow in and out faster and more efficiently, all resulting in an increase in power output.
The difficulty in tuning for more power is that every change has an effect on other parts in the engine. Some modifications that seem like they would be beneficial might not be when the rest of the engine parts are taken into account. Engine tuning often requires a well thought-out plan since the parts required should be chosen to work together. It can also become an incredibly expensive endeavor.
Forced Induction (FI) is the “easy” way to increase power. Take an engine, add a supercharger or turbocharger (or two), and get more power.
Ok, it’s not quite that simple. Fuel maps need to be updated, the fuel delivery system needs to be upgraded, intercoolers need to be added and plumbed… I’d go through all the changes, but since that’s an article all by itself, I’ll leave that for another time.
There are a lot of things that can, and should, be done to run an engine with FI, unless you want your engine to become a ticking time-bomb (and it will).
FI, in a way, simplifies increasing the power produced by an engine by increasing the amount of air and fuel that is combusted in the engine by compressing air and then pumping this compressed air into the cylinders. More air + more fuel = more power.
The cost of FI varies depending on what car you’re starting with, however, it does provide a large amount of power for the money. You just need to make sure that your engine can handle the increased power, otherwise you might find yourself with a very large, very expensive paper weight.
While not exactly easy, an engine swap is the simplest way to create more power. Rather than modifying your existing engine, you’re replacing it with one that produces more power.
Common swaps for rear-wheel drive (RWD) cars are Chevrolet small blocks and Ford 4.6L and 5.0L V8s. These are generally readily available, they have a lot of aftermarket support, and so many people have swapped them into their cars that if you look hard enough, someone has swapped one into the model you’re currently working on and had probably put together a thorough walkthrough on how to perform the swap.
And if you’re modifying a relatively popular platform, there’s probably a kit for it!
Generally, though, an engine swap involves: a donor engine, a stand-alone engine management system (computer), modified or fabricated motor and transmission mounts, a modified drive shaft (or shafts), and a custom engine cooling system. Engine swaps aren’t something for the faint of heart. It takes skill (or a willingness to learn) fabrication, a small amount of engineering, and a moderate amount of mechanical know-how. It’s definitely not an easy task (unless there is a kit available).
If you’re willing to put in the effort, though, an engine swap can yield impressive power gains for your sleeper. Often, a swap can provide, at a minimum, double the power of the existing engine with little need to tune or modify the swapped power plant. This is why swaps are so popular, it’s a relatively quick way to make your car faster.
If you’re looking for a relatively cheap way to increase your power, nitrous is the way to go. As far as engine modifications go, for a small investment and some relatively minor engine modifications, you can add an incredible amount of power.
Nitrous adds extra oxygen to the cylinder and, when accompanied by additional fuel, functions similar to FI in producing more power.
It’s not without its drawbacks, though.
Nitrous introduces more combustible gasses to the combustion chamber. These additional gasses, combined with an appropriate addition of fuel, can produce a massive increase in power for a short period of time. The problems that come with these increases of power are similar to those from turbo- or super-charging: detonation, increased or accelerated wear, and destruction of the engine.
By introducing more combustible gasses to the combustion chamber, you’re increasing the amount of heat and pressure to the cylinders. If appropriate precautions aren’t taken (engine modifications), it’s only a matter of time before your engine can’t take the abuse anymore and fails (sometimes in impressive ways).
To sum it up
Sleepers are probably some of the coolest and most boring cars, all at the same time. They combine the knowledge of automotive modification with the art of camouflage. They’re not for everyone, though. Sleepers work best when you’re looking to have a little bit of fun with your car and make a big splash when other people see what car you’ve brought.