Turbochargers are an engine modification that allow an engine to produce more power than it could with just natural aspiration. Turbochargers work by using exhaust gases to spin a turbine, which then spins an air pump that forces more air into the engine’s cylinders. This allows more fuel to be burned, increasing power output. But when exactly does that turbo kick in to start making boost?
What is a turbocharger?
A turbocharger is made up of two main components – a turbine and a compressor. The turbine is located in the exhaust stream of the engine. As the hot exhaust gases move through the turbine housing, they cause the turbine wheel to spin rapidly. This turbine wheel is connected via a shaft to a compressor wheel. So as the turbine wheel spins, it spins the compressor wheel in the intake tract of the engine. The compressor wheel draws in ambient air and compresses it before feeding it into the engine’s cylinders. The compressed air allows more fuel to be burned, increasing the engine’s power output.
When does boost start?
There are a few factors that determine when the turbo will start to produce boost or amplified intake pressure in an engine:
- Engine speed (RPMs) – There is a certain RPM range where the turbo will start to kick in. This is known as the boost threshold. On smaller engines, this threshold is typically around 2000 RPM. For larger engines, it may be higher around 2500-3000 RPM.
- Exhaust gas flow – The turbine in the turbocharger needs sufficient exhaust gas flow volume and velocity to spin and produce boost. So the engine needs to be at an RPM where exhaust flow is high enough.
- Turbo lag – All turbochargers have some amount of turbo lag which is the time it takes for boost to build after the engine RPMs rise. Larger turbos have more lag.
- Boost pressure setting – Modern turbocharged cars use wastegates and electronic boost controllers to control how much boost pressure is produced. There may be a pre-set boost threshold programmed in.
- Pedal position – The driver stepping hard on the accelerator will drop throttle plates open allowing more exhaust flow and reducing turbo lag.
Typical boost thresholds
While the exact RPM at which a turbo starts to produce boost can vary, here are some general RPM ranges:
- Smaller turbocharged 4-cylinder engines – 2000 to 3000 RPM range
- Larger turbocharged 6-cylinder engines – 2500 to 3500 RPM range
- High performance turbocharged engines – 3000 to 4000 RPM range
- Diesel turbo engines – 1500 to 2000 RPM range
- Twin-turbo V6/V8 engines – 2000 to 3000 RPM range
The size of the turbocharger, size of the engine, and engine power goals all affect what RPM the boost will come in at. A smaller turbo on a lower horsepower engine may start making boost around 2000 RPM. While a large turbo on a high-performance engine may not start making full boost until 4000 RPM.
Does higher RPM always mean more boost?
Not necessarily. Once a turbocharger starts to produce boost, it will generally increase proportional to engine RPM, but there are some caveats. The maximum boost level is affected by:
- Wastegate settings – A wastegate dumps excess exhaust around turbo at higher RPMs to control boost.
- Boost limiters – Electronic controllers will limit maximum boost pressure for durability.
- Turbocharger size – Bigger turbos can produce more boost overall but boost may taper off at very high RPMs.
Additionally, turbocharged cars often have a boost curve that is tailored for a certain power band. You may see 20 psi of boost from 3000 to 5000 RPM, then tapering off to 15 psi toward redline. This helps optimize power and torque in a certain RPM range.
Role of turbo lag
Turbo lag plays a big role in influencing boost onset and power delivery. Turbo lag refers to the delay between when engine RPMs rise and when the turbo starts to produce boost pressure. A few factors affect turbo lag:
- Turbo size – Larger turbos produce more peak boost but have more lag due to greater inertia.
- Twin scroll/twin turbo – Having two smaller turbos or divided exhaust housing reduces lag.
- Bearing design – Ball bearing center cartridges reduce friction for less lag.
- Location – Shorter and less restricted intake and exhaust paths reduce lag.
Variable geometry turbochargers have movable vanes that allow the exhaust gas flow to be optimized based on load conditions. This significantly reduces turbo lag across the engine’s operating range. It allows boost to come on sooner and taper more gradually.
Is boost available all the time?
How consistently the turbo boost is available depends on several factors:
- Engine load – Light throttle will not have the exhaust flow to spin up the turbo quickly.
- RPM range – Staying in the engine’s optimal boost RPM range provides continuous boost.
- Wastegate – Controls boost level but may bleed off at higher RPMs.
- Boost control – Aggressive boost control can minimize lag but may cause abrupt gains/losses.
- Engine mods – More radical camshafts can limit low RPM boost due to reduced exhaust velocity.
For the most consistent boost delivery, you want to choose a turbo setup matched well to the engine size and power goals. Staying in the peak boost RPM range also helps. ECU tuning can optimize boost thresholds and control strategies.
Signs that a turbo isn’t boosting properly
Some signs that your turbo system may not be performing optimally include:
- Higher than normal turbo lag
- Peak boost not reaching expected PSI levels
- No boost at RPM range where boost should activate
- Check engine light illuminated
- Black smoke from exhaust tip indicating rich fuel mixture
- Lack of power when accelerating, especially at higher RPMs
- Typical boost sound not heard when accelerating
- Rattle or metallic knocking sounds from turbo area
If you notice any of these symptoms, inspecting the turbocharger and boost control system for faults would be advised. Potential issues include blowing seals, sticking wastegate, clogged pipes, or failing actuators or sensors.
Does higher boost always mean more power?
More boost pressure from a turbocharger does result in more engine power, but only to a certain point. Here are some factors to consider:
- Efficiency – Each turbo has an ideal RPM and pressure range where it operates most efficiently. Too much boost can decrease efficiency.
- Detonation – Too much boost can cause pre-ignition and detonation which can severely damage the engine.
- Fueling – More boost demands more fuel. Exceeding the injectors’ capacity will lead to lean conditions.
- Mechanical limits – Very high boost levels increase cylinder pressures which can exceed the limits of stock engine components leading to failed bearings, rings, pistons, head gaskets, etc.
Running higher boost than a stock engine is designed for requires supporting modifications like lower compression, added fuel capacity, stronger internals, and tuned engine management.
Symptoms of too much boost
Signs that a turbocharged engine is being pushed with too much boost include:
- Knocking or pinging sounds from the engine bay
- Mis-shifts and transmission slippage
- Intermittent power surges or losses
- Check engine light with possible cylinder misfire codes
- Increased exhaust temperature
- Visible white exhaust smoke under heavy load
- Failed or blown head gaskets
- Melted pistons
- Bent connecting rods
If such failures occur, reduce boost levels immediately or severe engine damage can result. Always tune turbo boost conservatively and use high octane fuel to prevent detonation issues.
Adjusting boost threshold and maximum boost
There are a few ways to adjust when boost comes on as well as how much maximum boost is produced:
- Turbo size – A smaller turbo will spool up faster while a bigger turbo can produce higher peak boost.
- Wastegate – Adjustable wastegate actuators can be used to increase or decrease boost pressure.
- Electronic boost controller – Can modify desired boost curves and maximum boost levels.
- ECU tuning – Aftermarket ECU flashes allow tweaking turbo thresholds and control strategies.
- Manual boost controllers – Bleed valves to change boost pressure based on spring rates and adjustment settings.
Care should be taken not to over-boost the engine or exceed the capabilities of fuel injectors when increasing boost levels. Conservative and gradual boost changes are recommended.
Turbocharged vs. Supercharged
Superchargers are similar to turbos in that they force more air into the engine for added power. But whereas a turbo uses exhaust gases, a supercharger is belt-driven off the crankshaft. Some key differences:
| Turbocharger | Supercharger |
|---|---|
| Delay in power due to turbo lag | Power increase directly proportional to engine speed |
| Peak efficiency at higher RPMs | Consistent boost across engine’s RPM range |
| Wastegate regulates maximum boost | Positive displacement design produces fixed boost levels |
| Tends to be more fuel efficient | Roots style blowers have higher power draw from engine |
| Quieter operation | Roots type superchargers tend to be noisier |
In general, turbos are preferred for their efficiency while superchargers provide instant throttle response. Many high performance applications now use both a turbo and superchargers in combination.
Conclusion
While turbochargers may appear to be relatively simple devices, there are many factors that influence if and when they start to produce boost. The engine RPM range, exhaust gas flow, turbo size, and boost control strategies all play key roles. By matching the turbo well to the engine and monitoring for signs of improper boost, enjoyment of the power of turbocharging can be maximized.