Turbo lag is the delay between pressing the accelerator and the turbocharger delivering boost. A turbo is driven by exhaust gas, so when you accelerate from low engine speed, there is not yet enough exhaust energy to spin the turbine up to speed. Until it gets there, the engine behaves like a smaller naturally aspirated engine; then boost arrives and power surges. Larger turbos generally have more lag; smaller turbos spool sooner.
Turbo lag vs boost threshold
Boost threshold is the lowest engine speed at which a turbo can build meaningful boost under load. Turbo lag is the time delay to build boost when you press the throttle above that speed. Both make a turbo engine feel slow to respond at low rpm.
Why turbo lag happens
- The turbine and compressor have mass that must be accelerated.
- Exhaust flow is low at low engine speed and light load.
- A larger turbo, built for high power, needs more exhaust to spin up.
How manufacturers reduce lag
| Method | How it helps |
|---|---|
| Smaller turbo | Spools faster, with less top-end power |
| Twin-scroll turbo | Separates exhaust pulses for faster spool |
| Variable geometry turbo | Vanes act like a small turbo at low speed and a large one at high speed; see variable geometry turbo |
| Twin or sequential turbos | A small turbo for low speed, a larger one for high speed |
| Electric assistance | Some modern systems use electric motors to spin the compressor |
Turbo lag in diesel engines
Diesels make strong low-speed torque, but older fixed-geometry turbo diesels could feel flat until boost built. Modern diesel pickups mostly use variable geometry turbos for quick response; see 6.7 Cummins turbo.
Driving around lag
Keeping engine speed in the turbo’s efficient range, by downshifting before you need power, keeps the turbo spinning and reduces the delay. Modern turbo cars with quick-shifting automatics and small, fast-spooling turbos make lag far less noticeable than older turbo cars did.
When lag means a problem
If a turbo engine suddenly feels lazier than usual, look for a boost leak, a sticking wastegate, a failing turbo or a dirty air filter; see turbo failure symptoms, what does a wastegate do? and car loses power when accelerating.
Does turbo lag hurt the engine?
No. Lag is a characteristic of how turbos work, not a fault. What matters is that boost builds smoothly and reaches its normal level. A sudden increase in lag, new whistling or smoke are the signs to look into.
Turbo vs supercharger
A supercharger is belt-driven, so it delivers boost immediately with no lag, at the cost of some efficiency; see supercharger vs turbo and what does a blow-off valve do?
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