ABA Engine Performance & Tuning 3 — Questions and Answers
Question 1: What is the primary advantage of a twin-scroll turbocharger over a single-scroll design?
- Lower manufacturing cost
- Reduced turbo lag and better scavenging (Correct answer)
- Higher maximum boost pressure
- Simpler installation requirements
Correct answer: Reduced turbo lag and better scavenging
Twin-scroll turbos separate exhaust pulses from cylinder pairs, improving exhaust scavenging and reducing turbo lag by delivering more consistent energy to the turbine.
Question 2: Which variable in valve timing has the greatest effect on engine torque at low RPM?
- Valve lift height
- Intake valve closing point (Correct answer)
- Exhaust valve opening point
- Valve stem diameter
Correct answer: Intake valve closing point
Retarding intake valve closing (late IVC) improves low-RPM cylinder filling efficiency by allowing the intake charge inertia to continue filling the cylinder after BDC.
Question 3: An intercooler primarily improves engine performance by:
- Increasing exhaust backpressure
- Reducing intake air temperature and increasing air density (Correct answer)
- Raising boost pressure limits
- Decreasing turbocharger RPM
Correct answer: Reducing intake air temperature and increasing air density
Intercoolers cool compressed intake air, reducing its temperature and increasing density, allowing more oxygen per stroke and reducing knock tendency.
Question 4: What phenomenon occurs when exhaust backpressure causes spent gases to re-enter the cylinder during valve overlap?
- Positive scavenging
- Internal EGR (residual gas trapping) (Correct answer)
- Knock suppression
- Volumetric efficiency gain
Correct answer: Internal EGR (residual gas trapping)
When exhaust backpressure exceeds intake pressure during overlap, hot residual exhaust gases flow back into the cylinder, acting as internal EGR and reducing effective volumetric efficiency.
Question 5: A wideband oxygen sensor differs from a narrowband sensor in that it:
- Only reads at stoichiometric lambda
- Provides accurate lambda readings across a wide AFR range (Correct answer)
- Is only used in diesel engines
- Measures exhaust temperature instead of oxygen content
Correct answer: Provides accurate lambda readings across a wide AFR range
Wideband sensors use a pumping cell to accurately measure lambda from approximately 0.65 to over 1.6, making them essential for tuning rich or lean mixtures.
Question 6: What is the primary role of a MAP (Manifold Absolute Pressure) sensor in engine management?
- Measure exhaust oxygen content
- Determine engine load by measuring intake manifold pressure (Correct answer)
- Control idle air bypass
- Monitor fuel rail pressure
Correct answer: Determine engine load by measuring intake manifold pressure
The MAP sensor measures absolute pressure in the intake manifold, allowing the ECU to calculate air density and engine load to determine appropriate fuel and ignition settings.
Question 7: Which condition most commonly triggers knock sensor-based ignition retard?
- Cold engine start below 40°F
- High engine load combined with low-octane fuel (Correct answer)
- Idle operation with EGR active
- Closed-throttle deceleration
Correct answer: High engine load combined with low-octane fuel
Knock occurs most readily under high cylinder pressure and temperature conditions, typically high load and high RPM situations, especially when fuel octane is insufficient for the timing map.
ABA Engine Performance & Tuning 3
What is the primary advantage of a twin-scroll turbocharger over a single-scroll design?