Car Tuning Forced Induction Systems 2 — Questions and Answers
Question 1: What is 'boost creep' in a turbocharged engine?
- Boost pressure rising uncontrollably beyond the target level (Correct answer)
- Boost pressure slowly decreasing under sustained load
- Turbo lag increasing as engine RPM rises
- Wastegate opening prematurely at low RPM
Correct answer: Boost pressure rising uncontrollably beyond the target level
Boost creep occurs when exhaust flow exceeds the wastegate's ability to bypass it, causing boost to rise beyond the set target.
Question 2: Which material is most commonly used for high-performance intercooler cores due to its thermal conductivity and weight?
- Stainless steel
- Aluminum (Correct answer)
- Copper
- Cast iron
Correct answer: Aluminum
Aluminum offers an excellent balance of thermal conductivity, light weight, and ease of fabrication for intercooler cores.
Question 3: What does a 'twin-scroll' turbocharger improve compared to a single-scroll design?
- Maximum boost pressure ceiling
- Spool response and reduced turbo lag by separating exhaust pulses (Correct answer)
- Intercooler efficiency at high boost levels
- Oil consumption and bearing longevity
Correct answer: Spool response and reduced turbo lag by separating exhaust pulses
Twin-scroll turbos separate exhaust pulses from paired cylinders, delivering more energy to the turbine wheel and improving spool-up response.
Question 4: When upgrading to a larger turbocharger, why might low-end torque decrease?
- Larger turbos require higher octane fuel to function
- A larger turbine wheel takes more exhaust energy to spool up, increasing lag (Correct answer)
- Bigger turbos reduce oil pressure to the engine
- Larger compressor wheels always reduce maximum boost levels
Correct answer: A larger turbine wheel takes more exhaust energy to spool up, increasing lag
A larger turbo's greater rotating mass and larger turbine wheel requires more exhaust gas energy before it reaches boost threshold, increasing lag.
Question 5: What is the function of a blowoff valve (BOV) in a turbocharged system?
- Increases boost pressure above the wastegate setpoint
- Vents pressurized intake air when the throttle closes to prevent compressor surge (Correct answer)
- Controls oil flow to the turbocharger bearing
- Regulates coolant temperature around the turbocharger housing
Correct answer: Vents pressurized intake air when the throttle closes to prevent compressor surge
The BOV releases pressurized air trapped between the turbo and closed throttle body, preventing compressor surge and protecting the turbo.
Question 6: A supercharger that uses two helical rotors to compress air is known as a:
- Centrifugal supercharger
- Roots-type supercharger
- Twin-screw (Lysholm) supercharger (Correct answer)
- Vane-type supercharger
Correct answer: Twin-screw (Lysholm) supercharger
The twin-screw or Lysholm supercharger uses two intermeshing helical rotors that trap and compress air internally before discharging it.
Question 7: What is 'compressor surge' in a turbo system and what causes it?
- Excessive oil pressure to the turbo shaft bearing
- Airflow reversal in the compressor wheel when operating outside its efficiency map (Correct answer)
- Overheating of the turbine housing due to insufficient coolant
- Boost pressure dropping suddenly due to a boost leak
Correct answer: Airflow reversal in the compressor wheel when operating outside its efficiency map
Compressor surge happens when the turbo operates outside its compressor map, causing airflow to reverse direction and creating a distinctive fluttering sound.
Car Tuning Forced Induction Systems 2
What is 'boost creep' in a turbocharged engine?