Engine Performance & Tuning Flashcards
7 cards from real ABA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Engine Performance & Tuning flashcards as text
In forced induction engines, why are lower static compression ratios typically used compared to naturally aspirated engines?
Answer: To compensate for increased cylinder pressure from boost
Boost pressure raises effective dynamic compression ratio, so static compression is lowered to keep peak cylinder pressure and knock tendency within safe limits.
What is volumetric efficiency (VE) and what does a VE above 100% indicate?
Answer: The ratio of actual to theoretical air charge; ram-tuning effects are boosting cylinder filling
VE compares actual air mass inducted per cycle to the theoretical maximum at ambient conditions; values above 100% occur when exhaust scavenging and intake ram effects fill cylinders beyond their static volume.
Which engine modification typically produces the greatest gains in high-RPM power output?
Answer: Ported and polished cylinder heads with larger valves
Ported and polished heads with optimized valve sizing improve airflow (VE) at high RPM where airflow restriction is the primary power-limiting factor.
What does a 'camshaft duration' specification describe?
Answer: The number of crankshaft degrees the valve stays open
Camshaft duration specifies how many crankshaft degrees elapse from when the valve opens to when it closes, typically measured at 0.050" of lift for consistency.
What is lobe separation angle (LSA) in camshaft design?
Answer: The angular distance between intake and exhaust lobe centerlines
LSA is the angle in camshaft degrees between the intake and exhaust lobe centerlines, influencing valve overlap, idle quality, and the RPM range of peak torque.
Detonation differs from pre-ignition in that detonation:
Answer: Occurs after spark ignition when unburned end-gas auto-ignites
Detonation is the spontaneous auto-ignition of the remaining unburned end-gas after normal spark-initiated flame front propagation has already begun.
Why does increasing exhaust backpressure generally reduce high-RPM power output?
Answer: Residual exhaust gases displace fresh intake charge, reducing VE
High exhaust backpressure makes it harder to expel combustion gases, leaving more residuals in the cylinder that dilute and displace the fresh air-fuel charge, reducing volumetric efficiency.