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.
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:
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.
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.
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.
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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.