1991 Nissan 240sx Horsepower: A Look At The Specs

The 1991 Nissan 240SX, a beloved icon of drifting and tuning culture, often sparks conversations centered around its power output. While not a powerhouse from the factory, understanding its horsepower rating and the factors influencing it requires a deep dive into the specifics of its engine, the SR20DE (for later models) or the KA24DE, and the context of its era.

The KA24DE Engine: A Workhorse with Limitations

The early 1991 240SX models were primarily equipped with the KA24DE engine. This 2.4-liter, dual overhead cam (DOHC), 16-valve inline-four engine was designed for reliability and everyday drivability, rather than outright performance. Its iron block contributed to its robustness, capable of withstanding considerable abuse. However, its inherent design characteristics limited its ultimate horsepower potential.

Horsepower and Torque Figures

The official horsepower rating for the 1991 KA24DE in the 240SX was 140 horsepower at 5600 RPM. Torque was rated at 152 lb-ft at 4400 RPM. These figures are important because they represent the engine's peak performance under ideal conditions. It's crucial to remember that these are net horsepower figures, measured at the flywheel after accounting for parasitic losses from accessories like the alternator, power steering pump, and air conditioning compressor.

These figures might seem modest by today's standards, but consider the car's relatively light weight. The power-to-weight ratio contributed significantly to its nimble handling and perceived responsiveness.

Factors Affecting Horsepower in the KA24DE

Several factors influence the actual horsepower output of a KA24DE engine. Understanding these factors is crucial for diagnosing performance issues or planning modifications:

Engine Condition: The age and condition of the engine play a significant role. Wear and tear on components like piston rings, valve seals, and bearings can lead to compression loss, reducing efficiency and horsepower.
Fuel System: The fuel system, including the fuel pump, fuel injectors, and fuel pressure regulator, must deliver the correct amount of fuel to the engine. A clogged fuel filter, weak fuel pump, or leaking fuel injectors can starve the engine and reduce power.
Ignition System: The ignition system, including the spark plugs, ignition coil, distributor (if applicable), and ignition control module, must provide a strong and consistent spark to ignite the air-fuel mixture. Weak spark or incorrect timing can lead to misfires and power loss.
Air Intake System: A restricted air intake system can limit the amount of air entering the engine, reducing its ability to generate power. A dirty air filter or obstructions in the intake tract can have a noticeable impact.
Exhaust System: A restrictive exhaust system can create backpressure, hindering the engine's ability to expel exhaust gases efficiently. A clogged catalytic converter or a poorly designed exhaust system can significantly reduce horsepower.
Engine Management System (ECU): The ECU controls various engine parameters, including fuel injection, ignition timing, and idle speed. A malfunctioning ECU or incorrect tuning can negatively impact horsepower.
Ambient Conditions: Air temperature, humidity, and altitude can all affect engine performance. Hotter air is less dense, reducing the amount of oxygen available for combustion. Higher altitudes have lower air pressure, further reducing oxygen availability.

The SR20DE Option (Later Models): A Step Up in Performance

Later model 240SXs (specifically the SE trim in some years) offered the SR20DE engine as an option. This 2.0-liter DOHC inline-four was a significant upgrade in terms of performance potential. While still naturally aspirated, the SR20DE boasted a more advanced design and offered better breathing characteristics.

Horsepower and Torque Figures (SR20DE)

The SR20DE in the 240SX typically produced around 140 horsepower at 6400 RPM and 132 lb-ft of torque at 4800 RPM. While the horsepower figure might seem similar to the KA24DE, the SR20DE delivered its power at a higher RPM and offered a more responsive and rev-happy driving experience. The reduced torque figure, however, was noticeable in certain driving situations.

SR20DE Advantages

The SR20DE's advantages stemmed from several key design features:

DOHC Cylinder Head: The dual overhead cam design allowed for more precise valve timing and control, improving airflow and combustion efficiency.
Lightweight Components: The SR20DE utilized lighter materials for components like the pistons and connecting rods, reducing inertia and allowing for quicker engine response.
More Aggressive Camshaft Profiles: The SR20DE's camshafts featured more aggressive profiles, increasing valve lift and duration, which further enhanced airflow.

Dyno Testing and Real-World Horsepower

It's important to note that the factory horsepower ratings are measured under controlled laboratory conditions. Real-world horsepower figures can vary depending on factors such as engine condition, modifications, and dyno calibration. Dyno testing provides a more accurate assessment of an engine's actual power output.

Understanding Dyno Results

When interpreting dyno results, it's essential to consider the following:

Type of Dyno: Different types of dynos (e.g., chassis dynos, engine dynos) measure horsepower in different ways. Chassis dynos measure power at the wheels, while engine dynos measure power at the flywheel. Chassis dyno results will always be lower due to drivetrain losses.
Correction Factors: Dyno software uses correction factors to account for variations in ambient temperature, humidity, and altitude. These factors ensure that dyno results are comparable across different conditions.
Smoothing: Smoothing is a feature that removes noise from dyno graphs, making them easier to read. However, excessive smoothing can mask important details.

Modifications and Horsepower Gains

The 1991 Nissan 240SX is a popular platform for modifications, and many owners seek to increase horsepower. Common modifications include:

Intake and Exhaust Upgrades: Upgrading the air intake and exhaust system can improve airflow and reduce backpressure, resulting in modest horsepower gains.
Camshaft Upgrades: Installing aftermarket camshafts with more aggressive profiles can significantly increase horsepower, particularly at higher RPMs.
Forced Induction (Turbocharging or Supercharging): Adding a turbocharger or supercharger can dramatically increase horsepower, but it also requires extensive modifications to the engine and fuel system.
Engine Swaps: Swapping in a more powerful engine, such as the SR20DET (turbocharged version of the SR20DE) or even a V8 engine, is a popular way to achieve significant horsepower gains.

Regardless of the modifications chosen, it's crucial to ensure that all components are properly matched and tuned to avoid damaging the engine.

Conclusion

The 1991 Nissan 240SX, whether equipped with the KA24DE or the SR20DE, offered a respectable level of performance for its time. While not a horsepower king, its lightweight chassis and responsive engines made it a joy to drive. Understanding the factors affecting horsepower and the potential for modifications allows enthusiasts to appreciate the 240SX for its inherent capabilities and its adaptability to performance enhancements. Ultimately, the 240SX's legacy rests not solely on its raw horsepower figures, but on its balanced performance and its enduring appeal to car enthusiasts worldwide.