Car Tuning Engine Performance Principles Questions and Answers — Questions and Answers
Question 1: Advancing ignition timing typically results in which of the following outcomes?
- Increased risk of engine knock at high loads (Correct answer)
- Decreased peak cylinder pressure
- Improved engine torque at very low RPM
- Reduced engine heat generation
Correct answer: Increased risk of engine knock at high loads
Advancing ignition timing means the spark plug fires earlier in the compression stroke. While this can increase horsepower, advancing it too far, especially under high load, increases cylinder pressure and temperature, raising the risk of engine knock or detonation.
Question 2: A tuner is aiming for maximum power on a turbocharged gasoline engine. Which air-fuel ratio (AFR) is most appropriate to target under full boost?
- 14.7:1
- 11.5:1 (Correct answer)
- 15.5:1
- 13.0:1
Correct answer: 11.5:1
Forced induction engines require a richer air-fuel mixture (e.g., around 11.5:1 for gasoline) under boost to help cool the combustion chamber and prevent detonation (knock) caused by the higher cylinder pressures and temperatures. Stoichiometric (14.7:1) is ideal for emissions and cruise, while leaner mixtures are for fuel economy and richer mixtures (to a point) are for power and safety under boost.
Question 3: What is the primary function of a camshaft in determining an engine's performance characteristics?
- To regulate the engine's operating temperature.
- To control the timing of the ignition spark.
- To determine the engine's redline.
- To control the timing, duration, and lift of the intake and exhaust valves. (Correct answer)
Correct answer: To control the timing, duration, and lift of the intake and exhaust valves.
The camshaft's lobes dictate the precise moment the intake and exhaust valves open, how long they stay open (duration), and how far they open (lift). This control over the engine's 'breathing' is fundamental to its powerband, torque curve, and overall performance.
Question 4: A customer complains of sluggish turbocharger response and a noticeable loss of power after installing a new, highly restrictive 'quiet' muffler. What is the most likely cause?
- Increased volumetric efficiency
- Reduced ignition timing advance
- Excessive exhaust backpressure (Correct answer)
- A richer air-fuel ratio from the ECU
Correct answer: Excessive exhaust backpressure
Excessive exhaust backpressure, caused by a restrictive exhaust system, creates resistance that the turbocharger's turbine must overcome. This slows the turbine's speed, reducing its ability to spin the compressor and build boost, leading to turbo lag and a loss of power.
Question 5: Which of the following best defines Volumetric Efficiency (VE)?
- The ratio of an engine's horsepower to its displacement.
- The measure of how efficiently fuel is converted into power.
- The ratio of the actual amount of air entering the cylinder to the theoretical maximum amount it could hold. (Correct answer)
- The speed at which exhaust gases exit the engine.
Correct answer: The ratio of the actual amount of air entering the cylinder to the theoretical maximum amount it could hold.
Volumetric Efficiency (VE) is a percentage that represents how effectively an engine is 'breathing.' It compares the actual mass of air drawn into a cylinder during the intake stroke to the theoretical volume of the cylinder. Factors like intake manifold design, camshaft profile, and RPM all affect VE.
Question 6: During a high-load tuning session on a dynamometer, the knock sensor detects detonation. How will a modern Engine Control Unit (ECU) typically respond?
- Increase fuel pressure
- Advance the ignition timing
- Open the EGR valve
- Retard the ignition timing (Correct answer)
Correct answer: Retard the ignition timing
The knock sensor detects the specific frequency of vibration caused by detonation. When this occurs, the ECU's primary protective response is to retard (delay) the ignition timing, which lowers cylinder pressure and temperature, thereby stopping the knock.
Car Tuning Engine Performance Principles Questions and Answers
Advancing ignition timing typically results in which of the following outcomes?