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WHAT’S THE DIFFERENCE BETWEEN TURBO LAG AND BOOST THRESHOLD?

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The boost threshold of a turbocharger system is the lower bound of the region within which the compressor operates. Below a certain rate of flow, a compressor produces insignificant boost. This limits boost at a particular RPM, regardless of exhaust gas pressure.

Newer turbocharger and engine developments have steadily reduced boost thresholds. Electrical boosting (“E-boosting”) is a new technology under development. It uses an electric motor to bring the turbocharger up to operating speed quicker than possible using available exhaust gases. An alternative to e-boosting is to completely separate the turbine and compressor into a turbine-generator and electric-compressor as in the hybrid turbocharger. This makes compressor speed independent of turbine speed. In 1981, a similar system that used a hydraulic drive system and overspeed clutch arrangement accelerated the turbocharger of the MV Canadian Pioneer Turbochargers start producing boost only when a certain amount of kinetic energy is present in the exhaust gasses. Without adequate exhaust gas flow to spin the turbine blades, the turbocharger cannot produce the necessary force needed to compress the air going into the engine. The boost threshold is determined by the engine displacement, engine rpm, throttle opening, and the size of the turbocharger. The operating speed (rpm) at which there is enough exhaust gas momentum to compress the air going into the engine is called the “boost threshold rpm”. Reducing the “boost threshold rpm” can improve throttle response.

TURBO LAG VS BOOST THRESHOLD

Jason Fenske of Engineering Explained is here to break it all down for you. The thing you’ve been describing as turbo lag? It’s probably got to do with boost threshold, the RPM at which a turbocharged engine can begin to push enough exhaust to start spooling up the turbo. You can calculate an engine’s boost threshold using its displacement and volumetric efficiency; it’s a factor of engine design, one that you can’t change without completely altering the way the engine moves air. What is turbo lag? It’s the delay between when you open the throttle and when the turbo begins delivering boost—when the engine is above its boost threshold RPM. Some engines have nearly zero turbo lag, some have a ton. But there isn’t an engine out there that can deliver an immediate hit of boost below its boost threshold RPM. It’s simply not possible. Turbo lag is the additional time delay above boost threshold, when the engine’s throttle valve is open and the turbocharger needs to accelerate to deliver positive pressure to the engine. I think there’s a lot of confusion on what the term “lag” actually means. It is my understanding that “boost threshold” is the RPM at which the turbo will be capable of building full boost. For example, if you put your car in 3rd gear, at 2000 RPM, and you hold the brake to prevent acceleration, and floor it, the turbo will not spool up to full boost. If you do the same thing at 3000 RPM, the turbo will build full boost. “Lag” is the time it takes the turbocharger to spool to full boost, once past the boost threshold. Example, you’re in 2nd gear spinning at 5000 RPM with your foot off the throttle. You suddenly go full throttle, and there is a lag time before the turbo gets to full boost, despite being past the boost threshold. This is what I see “lag” as being. Therefore, when you gut your up pipe, etc, you aren’t affecting lag, but your boost threshold.