Car Tuning Engine Performance Principles 4 — Questions and Answers
Question 1: What is 'lambda' (λ) in engine tuning, and what does λ=1.0 represent?
- Ignition advance in degrees; λ=1.0 means MBT timing
- The ratio of actual air-fuel mixture to stoichiometric; λ=1.0 means a perfect stoichiometric mixture (Correct answer)
- Boost pressure ratio; λ=1.0 means no boost (naturally aspirated)
- Exhaust backpressure ratio; λ=1.0 means equal intake and exhaust pressure
Correct answer: The ratio of actual air-fuel mixture to stoichiometric; λ=1.0 means a perfect stoichiometric mixture
Lambda is the normalized air-fuel ratio where 1.0 represents exactly stoichiometric combustion (14.7:1 for gasoline), lean is above 1.0, and rich is below 1.0.
Question 2: What is the purpose of 'lobe separation angle' (LSA) in camshaft design?
- It determines the distance between the cam lobe and the valve stem
- It sets the angular difference between intake and exhaust lobe centerlines, affecting idle quality and powerband width (Correct answer)
- It controls how fast the cam lobe opens the valve
- It measures the total rotation of the camshaft per engine cycle
Correct answer: It sets the angular difference between intake and exhaust lobe centerlines, affecting idle quality and powerband width
LSA is the angle between intake and exhaust lobe centerlines; a tighter LSA increases overlap and top-end power while a wider LSA broadens the powerband and improves idle.
Question 3: In a naturally aspirated engine, what is the typical effect of adding a cold air intake (CAI)?
- Gains of 20-40 horsepower by cooling the combustion gases
- Modest power gains of 5-15 hp by reducing intake restriction and lowering intake air temperature (Correct answer)
- Significant fuel economy losses due to increased airflow
- No measurable power gain because the stock throttle body is the restriction
Correct answer: Modest power gains of 5-15 hp by reducing intake restriction and lowering intake air temperature
A cold air intake typically yields 5-15 hp by reducing intake restriction and drawing cooler, denser air from outside the hot engine bay.
Question 4: What does 'MBT timing' stand for and why is it significant?
- Maximum Boost Timing — the advance needed to maximize turbo spool speed
- Minimum advance for Best Torque — the spark timing that produces peak torque without causing knock (Correct answer)
- Motor Brake Timing — timing retard used during engine braking
- Manifold Boost Timing — timing adjustment based on manifold pressure
Correct answer: Minimum advance for Best Torque — the spark timing that produces peak torque without causing knock
MBT timing is the minimum ignition advance angle that produces maximum torque; advancing beyond MBT risks detonation without additional power gain.
Question 5: What is the effect of using a larger throttle body on a naturally aspirated engine?
- Always produces significant power gains regardless of other modifications
- Can improve high-RPM airflow but may hurt throttle response if oversized for the engine (Correct answer)
- Eliminates the need for a mass airflow sensor
- Increases idle speed by allowing more air past the throttle plate at rest
Correct answer: Can improve high-RPM airflow but may hurt throttle response if oversized for the engine
A larger throttle body helps high-RPM airflow but an oversized unit makes precise throttle control difficult at low speeds and doesn't guarantee big power gains.
Question 6: What is 'thermal efficiency' in relation to engine performance?
- How well the cooling system maintains optimal engine temperature
- The percentage of fuel energy that is converted into useful mechanical work (Correct answer)
- The efficiency of the exhaust heat recovery system
- How quickly the engine reaches operating temperature from a cold start
Correct answer: The percentage of fuel energy that is converted into useful mechanical work
Thermal efficiency measures what fraction of the chemical energy in the fuel is successfully converted to crankshaft power, with the rest lost as heat.
Question 7: What is the purpose of 'piston squish' or 'quench area' in combustion chamber design?
- To reduce piston weight by removing material from the outer edge
- To force air-fuel mixture toward the spark plug at TDC, improving combustion speed and efficiency (Correct answer)
- To create turbulence in the intake port for better fuel atomization
- To allow thermal expansion of the piston at operating temperature
Correct answer: To force air-fuel mixture toward the spark plug at TDC, improving combustion speed and efficiency
The squish zone forces the air-fuel mixture rapidly toward the combustion chamber center and spark plug at TDC, creating turbulence that speeds up combustion and reduces knock tendency.
Car Tuning Engine Performance Principles 4
What is 'lambda' (λ) in engine tuning, and what does λ=1.0 represent?