Engine Performance & Tuning Flashcards

How does altitude affect engine power output in a naturally aspirated engine?

Answer: Power decreases because lower air density reduces oxygen mass per cycle

At altitude, reduced atmospheric pressure means lower air density and therefore less oxygen mass per intake stroke, reducing maximum achievable combustion and power output.

What is the primary benefit of using E85 fuel in a performance-tuned engine compared to 93-octane gasoline?

Answer: Higher octane rating allowing more timing advance and boost pressure

E85 has an effective octane rating of approximately 105, allowing more aggressive ignition timing and higher boost pressure, which can significantly increase power output with appropriate tuning.

What issue commonly occurs when fuel injectors are sized too large for a given engine application?

Answer: Poor idle quality and difficulty maintaining stable low-duty-cycle fueling

Oversized injectors must operate at very short pulse widths at idle and light throttle, which is outside their linear operating range, causing poor fuel atomization and unstable combustion.

In an open-loop fueling condition, the ECU controls fuel delivery based on:

Answer: Pre-programmed fuel maps without closed-loop correction

Open-loop operation uses stored fuel maps based on load and RPM inputs without oxygen sensor feedback correction, typically used during cold start, wide-open throttle, or sensor failure.

What is the purpose of a fuel pressure regulator referenced to intake manifold pressure?

Answer: Maintain constant fuel pressure differential across injectors despite manifold pressure changes

A manifold-referenced regulator maintains a constant pressure drop across the injectors by rising and falling with manifold pressure, keeping injector flow rate consistent regardless of engine load.

Which tuning parameter most directly controls torque output at a given RPM and load point on a fuel-injected engine?

Answer: Ignition timing advance at that operating point

Ignition timing advance directly controls when peak cylinder pressure occurs relative to piston position, which determines how effectively combustion pressure is converted to crankshaft torque.

What does a 'power-to-weight ratio' improvement strategy prioritize over simply adding horsepower?

Answer: Reducing vehicle mass to increase acceleration without increasing power

Reducing vehicle weight improves acceleration, braking, and handling equally across all operating conditions, whereas adding power only improves straight-line acceleration and carries reliability trade-offs.