Car Tuning Forced Induction Systems 4 — Questions and Answers
Question 1: What is 'detonation' (knock) in a forced induction engine and what makes it dangerous?
- Premature valve float that causes valves to contact pistons at high RPM
- Uncontrolled auto-ignition of the air-fuel mixture that creates extreme pressure spikes damaging pistons and bearings (Correct answer)
- Excessive fuel entering the cylinder causing a hydraulic lock event
- Spark plug misfiring due to overly rich air-fuel ratio
Correct answer: Uncontrolled auto-ignition of the air-fuel mixture that creates extreme pressure spikes damaging pistons and bearings
Detonation is spontaneous combustion of the end-gas before the flame front arrives, creating violent pressure spikes that can destroy pistons, rings, and bearings rapidly.
Question 2: What upgrade is typically required to the fuel system when significantly increasing boost pressure on a turbocharged engine?
- Upgrading to a mechanical fuel pump only
- Installing larger fuel injectors and a higher-capacity fuel pump to supply additional fuel demand (Correct answer)
- Switching from port injection to carburetion for better atomization
- Reducing fuel pressure to prevent injector cavitation at high boost
Correct answer: Installing larger fuel injectors and a higher-capacity fuel pump to supply additional fuel demand
Higher boost levels demand proportionally more fuel to maintain a safe air-fuel ratio, requiring larger injectors and a pump capable of higher flow rates.
Question 3: How does a Variable Geometry Turbocharger (VGT) eliminate turbo lag?
- It uses an electric motor to pre-spin the compressor wheel before exhaust gases arrive
- Adjustable vanes change the turbine's effective A/R ratio to maintain high exhaust velocity at low RPM (Correct answer)
- It uses two turbos of different sizes that alternate based on engine load
- It eliminates the wastegate and controls boost entirely through vane angle
Correct answer: Adjustable vanes change the turbine's effective A/R ratio to maintain high exhaust velocity at low RPM
VGT vanes close at low RPM to accelerate exhaust gas velocity, spooling the turbine quickly, then open at high RPM to allow greater flow without excessive backpressure.
Question 4: What is the role of forged pistons in a high-boost forced induction build?
- Forged pistons are lighter and allow higher rev limits than cast pistons
- Forged pistons are stronger and better able to withstand the higher cylinder pressures generated by boost (Correct answer)
- Forged pistons expand less under heat, allowing tighter oil ring tolerances
- Forged pistons are required because superchargers run hotter combustion temperatures than turbos
Correct answer: Forged pistons are stronger and better able to withstand the higher cylinder pressures generated by boost
Forged pistons have a denser grain structure than cast pistons, providing greater strength to withstand the elevated cylinder pressures that come with increased boost.
Question 5: What is 'boost by gear' (BBG) tuning and why is it used?
- Limiting boost to lower levels in lower gears to prevent wheelspin and drivetrain damage (Correct answer)
- Increasing boost in higher gears to maximize top-speed power output
- Adjusting boost based on coolant temperature as the gear selector position is sensed
- A method of controlling supercharger boost through the automatic transmission's gear ratio
Correct answer: Limiting boost to lower levels in lower gears to prevent wheelspin and drivetrain damage
BBG reduces boost in lower gears where high torque is most likely to break traction or damage the drivetrain, then allows full boost in upper gears where the car is more stable.
Question 6: Which type of fuel is most commonly recommended for high-boost street/strip forced induction builds due to its knock resistance?
- Standard 87-octane pump gasoline
- E85 (85% ethanol blend) (Correct answer)
- Aviation fuel (100LL)
- Diesel fuel
Correct answer: E85 (85% ethanol blend)
E85 has an octane equivalent of approximately 105 and a high latent heat of vaporization, dramatically cooling the intake charge and resisting detonation under high boost.
Question 7: What is a 'top-mount' intercooler configuration and what is its main trade-off versus a front-mount (FMIC)?
- Top-mount is mounted above the engine for shorter piping but suffers from increased heat soak (Correct answer)
- Top-mount is mounted at the front of the car for better cooling but requires longer boost piping
- Top-mount intercoolers are water-to-air only and cannot use ambient airflow for cooling
- Top-mount configurations only work with centrifugal superchargers, not turbochargers
Correct answer: Top-mount is mounted above the engine for shorter piping but suffers from increased heat soak
Top-mount intercoolers sit directly above the engine with minimal piping length for quick boost response but are prone to heat soak from the hot engine below and limited airflow.
Car Tuning Forced Induction Systems 4
What is 'detonation' (knock) in a forced induction engine and what makes it dangerous?