Why NASCAR Engines are Not Spec Engines

NASCAR mandates a 750-hp package and a 550-hp package. That’s led a lot of people to suggest that NASCAR runs spec engines, or that all the engines are identical. Here’s why that’s far from true.

Spec Engines

The point of Spec Engines is to keep costs down — something important to all racers. The Saturday-night racer wants to test her driving ability, not compete to see who can spend the most money on their car. Spec Engines have spec (specified) parts, usually from valves and springs all the way down. In some series, engines are sealed. In others, you’re allowed to take the engine apart, but you can’t change out any of the pieces.

With all the talk about the 550-hp package and the 750-hp package, I thought it might be useful to talk about what that actually means — and why NASCAR Cup Series engines are far from spec.

Engine Basics Work

Motor Show motor omotor GIF

Whether spec engine or custom down to the last valve spring, internal combustion engines work on the Otto cycle. Each cylinder in an engine goes through the same process:

  • Fuel and air come into the cylinder
  • The piston compresses the fuel/air mixture
  • The spark plug ignites the fuel/air mixture
  • The resulting combustion pushes the piston back downward
  • The combustion gases leave the cylinder

Rinse and Repeat, as shown to the right.

Combustion changes the energy contained in the fuel and the air into kinetic energy — the energy of motion. The piston drives the crankshaft, and the transmission eventually gets the energy to the wheels.

But we don’t measure an engine in terms of how much energy it produces. What we care about is power.

Power and Energy

Power is how much energy the engine produces in a certain amount of time: In other worse, how fast the engine produces energy.



Horsepower is a uni of power, but it was developed by way of a marketing strategy. When James Watt invented the steam engine in the 1770s, he needed to convince people that his engine could do everything horses could. So he estimated how powerful a horse was and expressed the power of his engine in terms of horses. (Also: Engines don’t need hay or brushing and don’t excrete horse dung.)

One horsepower is about 745 Watts. That’s approximately the power it takes to run twelve 60-W light bulbs and one 30-W light bulb. (And, of course, the metric system named its unit of power after Mr. Watt.)

A healthy human can produce about 1.2 hp for short bursts and 0.1 hp for sustained periods of time. (In comparison, an elite athlete can produce 2.5-3 hp for short periods of time and 0.35 hp for periods of several hours.

And what about the horse? Watt wasn’t too far off. If you average a horse’s energy production over the course of a workday, they do produce around a horsepower. But to be fair to horses, they can actually produce up to 14 horsepower for short periods of time.

And this brings us to the main point:

Even a Horse Doesn’t Always Make One Horsepower

A walking horse puts out much less power than a galloping horse. Similarly, an idling engine puts out much less horsepower than one about to redline.

When someone cites an engine’s horsepower, they’re telling you the engine’s maximum horsepower. The actual horsepower depends on how fast the engine turns. (RPM is the engine equivalent of a horse’s gait.)

The horsepower of an internal combustion engine as a function of rpm looks something like this.

A line plot of the horsepower of an internal combustion engine as a function of rpm

At low rpm, each cylinder makes a smaller number of combustions every minute. As the engine speeds up, it produces more combustions every minute, so the horsepower goes up.

This is why racecars run at high rpms: It’s the only way to get more power. My 2010 Mustang reaches its maximum horsepower at 5500 rpm, whereas the target rpm for NASCAR Cup Series racecars is 8500 rpm.

Unfortunately, you can’t make a racecar go faster just by pushing the motor to higher rpms. One problem is that parts can break because they can’t withstand the forces that go with those high rpms. But it’s also actually possible for the engine to turn too fast. The combustion process takes time. If you don’t give it the full time, you don’t get out all the energy. There’s also the problem that a high-revving engine cylinder doesn’t have time to cool down between compressions, which makes combustion less efficient. And our old friend fiction increases as rpm increases.

A Tale of Two Engines

While having high horsepower is critical to getting down the straightaways fast, the secret to engine engineering is in the shape of the horsepower curve. Let’s compare two hypothetical engines:

  • Engine 1 has 900 hp
  • Engine 2 has 875 hp

Here are their horsepower vs. rpm curves

A line plot comparing the horsepower of two engines with different peak hp values, but differently shaped peaks

Engine 1 reaches its peak around 9100 rpm. You’ll note that it’s a fairly narrow peak. That maximum horsepower is only accessible within a narrow range of rpm. Compare that to Engine 2, which has less peak horsepower, but a much broader peak. At lower rpm, Engine 2 actually has more horsepower than Engine 1. Depending on the type of racing, Engine 2 might actually be the faster engine.

Engine builders are interested in maximum horsepower, of course, but they spend far more time on tweaking the entire horsepower curve, especially at road races. NASCAR constrains the engine’s physical dimensions: The cylinder size, the number of valves, etc. But even before EFI, NASCAR engine builders tuned every one of the eight cylinders differently to optimize the power output. With EFI, engine builders and tuners have a much larger range of variables to control.

And while we’re talking horsepower, we should talk about one more thing…


Horsepower determines your top speed, which is why land-speed record racers need 1000+ hp engines. Stock car racing requires speed, but you spend a very small fraction of a lap at top speed. At race tracks like Bristol, up to 60% of a lap time is spent turning. Even at Bristol, no one turns at maximum speed.

Torque is what gets you up to speed. That feeling you get when you step on the accelerator and you’re pushed back in your seat? That’s torque. Torque is what lets you come out of the corner faster.

Horsepower sells cars, torque wins races.

Carroll Shelby

Torque and power are related to each other. When power is in horsepower and torque is in pound-feet:

Power=\frac{Torque * rpm}{5252}

We can plot the torque and power curves on the same chart, as shown below.

A line graph showing the power and torque vs rpm

The torque and power curves cross when the engine speed is 5252 rpm. Always.

What this relationship tells us, more importantly, is that tailoring the shape of the power curve also changes the shape of the torque curve. That’s what will determine whether you have the necessary speed to pass coming out of a turn.

So saying that the NASCAR Cup Series runs spec engines is just wrong, whether you use the strict definition, or even the looser definition as is used in the Truck series.

NextGen Engines?

Although NASCAR will use the same engine next year in the NextGen car, they’re already talking about electrification. Even if you hate the idea of electric cars, you should give some serious thought to the advantages of electric motors.

Why? Because the torque in electric motors doesn’t depend on how fast the motor is turning. You have full torque the minute they kick in.

A line graph comparing the torque for an internal combustion engine to an electric motor

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