Car Tuning Engine Performance Principles 3 — Questions and Answers
Question 1: What is 'brake specific fuel consumption' (BSFC) used to measure?
- How much fuel the brake system uses during hard stops
- The mass of fuel consumed per unit of power output per unit of time (Correct answer)
- The ratio of fuel economy at highway versus city speeds
- How efficiently the braking system converts kinetic energy to heat
Correct answer: The mass of fuel consumed per unit of power output per unit of time
BSFC measures engine efficiency by quantifying fuel mass consumed per horsepower-hour (or kW-hour), allowing comparison across different engine sizes.
Question 2: What happens to an engine's power output when intake air temperature increases?
- Power increases because warmer air expands and fills the cylinder better
- Power decreases because hot air is less dense and contains less oxygen per volume (Correct answer)
- Power is unaffected because the ECU compensates automatically
- Power increases due to improved fuel atomization in warm air
Correct answer: Power decreases because hot air is less dense and contains less oxygen per volume
Hot air is less dense, meaning fewer oxygen molecules per cubic foot, which reduces the amount of fuel that can be burned and lowers power output.
Question 3: What does a 'stroker kit' do to engine displacement?
- Increases displacement by using a longer crankshaft stroke (Correct answer)
- Reduces displacement to increase compression ratio
- Increases displacement by boring the cylinders to a larger diameter only
- Replaces stock pistons with lighter forged units without changing displacement
Correct answer: Increases displacement by using a longer crankshaft stroke
A stroker kit uses a crankshaft with increased stroke, moving the pistons farther up and down to displace more volume each cycle.
Question 4: In a turbocharged engine, what is 'compressor surge' and when does it typically occur?
- Excess boost causing the wastegate to open prematurely
- Reverse airflow through the compressor when throttle closes suddenly under boost (Correct answer)
- Oil starvation in the turbo bearing housing at high RPM
- Excessive exhaust backpressure overwhelming the turbine wheel
Correct answer: Reverse airflow through the compressor when throttle closes suddenly under boost
Compressor surge occurs when boosted air has nowhere to go after throttle closure, reversing flow through the compressor and causing a characteristic flutter.
Question 5: What is the function of a Variable Valve Timing (VVT) system?
- To adjust ignition timing based on engine load and RPM
- To alter camshaft phasing to optimize valve timing across different RPM ranges (Correct answer)
- To vary the compression ratio electronically during operation
- To control fuel injector pulse width based on throttle position
Correct answer: To alter camshaft phasing to optimize valve timing across different RPM ranges
VVT systems advance or retard camshaft position to optimize valve opening and closing events for both low-RPM torque and high-RPM power.
Question 6: What is the primary advantage of a dry-sump oiling system in a performance engine?
- It eliminates the need for an oil filter
- It prevents oil starvation during hard cornering and allows a lower engine mounting position (Correct answer)
- It increases oil pressure at idle for better cold-start protection
- It reduces oil temperature by using a smaller, more efficient oil pan
Correct answer: It prevents oil starvation during hard cornering and allows a lower engine mounting position
A dry-sump system uses a separate reservoir and scavenge pumps to keep oil away from the crank and prevent starvation during high-g cornering, while also enabling a lower center of gravity.
Question 7: How does 'port matching' improve engine performance?
- It aligns intake and exhaust port diameters to minimize turbulence at the head-manifold junction (Correct answer)
- It programs the ECU to match fueling to the intake manifold volume
- It calibrates the throttle body opening to match the engine's airflow needs
- It synchronizes multiple carburetors to flow equal amounts of fuel
Correct answer: It aligns intake and exhaust port diameters to minimize turbulence at the head-manifold junction
Port matching removes steps and mismatches between the cylinder head ports and intake/exhaust manifolds, reducing turbulence and improving airflow.
Car Tuning Engine Performance Principles 3
What is 'brake specific fuel consumption' (BSFC) used to measure?