What are Flat-Plane Cranks?

A flat-plane crankshaft is a typical type of crankshaft structure operated in some high-performance engines, specifically in certain V8s and V6s. Unlike a cross-plane crankshaft that arranges its crankpins at 90-degree intervals, a flat-plane cranks has its crankpins positioned 180 degrees apart, resulting in a flat pattern when viewed from the end.
This design results in a firing order where the cylinders fire in an alternating sequence between banks, creating an even firing order. It’s called “flat-plane” because the crankshaft’s counterweights are on the same plane, which allows for a more balanced rotation.

Flat-plane crankshafts offer several advantages:

High RPM Capability:

Reduced rotating mass and improved airflow characteristics typically allow flat-plane engines to rev higher compared to their cross-plane counterparts. This is beneficial in high-performance applications like racing or sports cars.

Exotic Exhaust Note

The even firing order creates a unique exhaust note that’s often high-pitched and exotic, giving these engines a distinctive sound that’s different from the deeper rumble of engines with cross-plane crankshafts.

Improved Engine Balance:

The flat-plane design allows for better primary and secondary balance, contributing to smoother high-speed operation.

However, there are trade-offs as well:

Reduced Low-End Torque:

Flat-plane crankshaft engines may sacrifice some low-end torque compared to engines with cross-plane crankshafts, which can impact their performance at lower RPMs.

Increased Vibration:

The design can generate more vibrations compared to cross-plane crankshafts, requiring additional engineering to manage these vibrations effectively.
Flat-plane cranks are often favored in applications where high-performance at high RPMs is a priority, such as in certain sports cars or racing vehicles, due to their ability to rev higher and produce a distinctive exhaust note.

What are Cross-Plane cranks

A cross-plane crankshaft is a type of crankshaft design commonly used in V8 engines. It’s named “cross-plane” because when viewed from the end, the crankshaft displays a cross-like shape due to the arrangement of the crankpins.

Key features of a cross-plane crankshaft:

90-Degree Arrangement:

The crankpins (the points where the connecting rods attach) are positioned at 90-degree intervals around the crankshaft. This layout results in a firing order that alternates between the two banks of cylinders, producing an uneven firing sequence.

Firing Order:
The firing order in a cross-plane V8 engine typically creates a distinctive rumbling or burbling exhaust note. The uneven firing intervals contribute to this sound, which is a characteristic feature associated with many V8 engines.

Torque Characteristics:

Cross-plane crankshafts often provide good low-end torque. The design inherently balances the engine well, leading to smoother operation and good torque delivery, particularly at lower RPMs.

Vibration Dampening:

Due to the even firing intervals provided by the 90-degree arrangement, cross-plane crankshafts tend to offer better vibration dampening compared to other configurations.
These crankshafts are commonly used in a variety of vehicles, from performance cars to trucks, where a strong torque curve and a more traditional V8 exhaust note are desired. They’re favored for their good low-end torque characteristics and smooth operation across a wide RPM range.

Difference Between Cross-Plane and Flat-Plane Cranks

Cross-Plane Crankshaft:

• The cross-plane crankshaft is commonly found in V8 engines. It has its crankpins positioned at 90-degree intervals around the crankshaft. This creates a cross pattern when viewed from the end, hence the name “cross-plane.”
• In a cross-plane crankshaft, the firing order of the cylinders is typically uneven. The uneven firing order leads to a distinctive rumbling or burbling exhaust note that’s often associated with V8 engines.
• This design offers good low-end torque and provides better vibration dampening due to the even firing intervals.

Flat-Plane Crankshaft:

• The flat-plane cranks is often used in high-performance engines like some V8s in sports cars or certain V6 engines. It has its crankpins positioned 180 degrees apart, resulting in a flat pattern when viewed from the end.
• In a flat-plane crankshaft, the firing order of the cylinders is usually even. This generates a smoother, high-pitched, and exotic-sounding exhaust note compared to the cross-plane crankshaft.
• Flat-plane crankshafts tend to allow for higher engine speeds due to reduced rotating mass and improved airflow characteristics. However, they can produce more vibrations compared to cross-plane crankshafts.

Cross-Plane Crankshaft:

Pros:
• Provides good low-end torque due to the firing order and design.
• Offers better vibration dampening, leading to smoother engine operation.
• Often favored in applications where a broad torque curve and smoother operation are desired, such as in many street cars and trucks.

Cons:

• Usually limited in maximum RPM compared to flat-plane crankshafts.
• Typically results in a different exhaust note that might be less exotic or high-pitched.

Flat-Plane Crankshaft:

Pros:

• Allows for higher engine speeds due to reduced rotating mass and better airflow characteristics.
• Generates a high-pitched and exotic exhaust note that’s often associated with high-performance cars.
• Ideal for applications where high RPM power and performance are priorities, such as in many sports cars and racing engines.

Cons:

• Can produce more vibrations compared to cross-plane crankshafts, requiring additional engineering to manage these vibrations.

• May sacrifice low-end torque in favor of higher RPM power.