Car Tuning Forced Induction Systems 3 — Questions and Answers
Question 1: What is the primary purpose of an oil return line on a turbocharger?
- Supply pressurized oil to lubricate the turbo shaft bearings
- Drain oil from the turbo center housing back to the engine sump by gravity (Correct answer)
- Cool the turbocharger compressor housing with engine oil
- Regulate boost pressure through an oil-controlled actuator
Correct answer: Drain oil from the turbo center housing back to the engine sump by gravity
The oil return (drain) line relies on gravity to drain oil from the turbo center section back to the oil pan, so it must never be kinked or run uphill.
Question 2: Which forced induction method produces peak boost at lower RPM without turbo lag?
- Ball-bearing turbocharger
- Positive displacement supercharger (Correct answer)
- Centrifugal supercharger
- Variable geometry turbocharger at idle
Correct answer: Positive displacement supercharger
Positive displacement superchargers (Roots or twin-screw) are belt-driven and produce boost proportional to engine speed, delivering strong low-RPM response.
Question 3: What does 'intercooler efficiency' measure?
- The volume of air the intercooler can hold at maximum boost
- How effectively the intercooler reduces charge air temperature relative to maximum possible cooling (Correct answer)
- The pressure drop across the intercooler core at peak flow
- The intercooler's ability to withstand detonation events
Correct answer: How effectively the intercooler reduces charge air temperature relative to maximum possible cooling
Intercooler efficiency is the ratio of actual temperature reduction to the maximum possible reduction (inlet air temp minus ambient temp), expressed as a percentage.
Question 4: Why do many high-boost setups use a water-methanol injection system?
- To increase fuel octane rating and reduce intake charge temperature simultaneously (Correct answer)
- To replace the need for a wastegate at boost levels above 20 psi
- To lubricate the turbocharger shaft at elevated boost pressure
- To prevent the intercooler from freezing in cold climates
Correct answer: To increase fuel octane rating and reduce intake charge temperature simultaneously
Water-methanol injection cools the intake charge through evaporation and raises the effective octane rating, allowing more timing advance and boost without detonation.
Question 5: On a turbocharged engine, what does a higher A/R (area-to-radius) ratio turbine housing produce?
- Faster spool-up and better low-RPM torque
- Higher peak power at elevated RPM with increased top-end flow capacity (Correct answer)
- Lower exhaust backpressure at idle and light throttle
- More stable boost pressure control at partial throttle
Correct answer: Higher peak power at elevated RPM with increased top-end flow capacity
A higher turbine A/R housing reduces exhaust velocity and backpressure at high RPM, supporting greater airflow and peak power but at the cost of slower spool-up.
Question 6: What is 'heat soak' as it applies to forced induction systems?
- Coolant temperature rising after the turbo is shut down
- Intake charge temperature rising due to heat absorbed from the engine bay between runs (Correct answer)
- Turbocharger housing expanding due to differential thermal growth
- Intercooler core becoming saturated with condensed water vapor
Correct answer: Intake charge temperature rising due to heat absorbed from the engine bay between runs
Heat soak occurs when the intercooler and intake piping absorb engine bay heat between pulls, raising charge air temps and reducing power on subsequent runs.
Question 7: What is a 'recirculating' blowoff valve (also called a bypass valve) and how does it differ from an atmospheric BOV?
- It vents boost pressure directly to atmosphere with a louder sound
- It recirculates vented air back into the intake pre-turbo, preventing metered air from being lost (Correct answer)
- It recirculates exhaust gases into the intake manifold to reduce emissions
- It bypasses the intercooler and routes hot air directly to the throttle body
Correct answer: It recirculates vented air back into the intake pre-turbo, preventing metered air from being lost
A recirculating bypass valve returns vented air back into the intake tract before the turbo, preventing the lean spike that can occur on MAF-based systems when metered air is lost to atmosphere.
Car Tuning Forced Induction Systems 3
What is the primary purpose of an oil return line on a turbocharger?