Posted on 31 March 2017
Turbo Speak - "Turbo Talk For Gearheads" By HotRod Magazine
Turbo geek-speak may sound like a foreign language, but here’s what the fancy terms mean in standard English:
Area/Radius (a/r) ratio: A term defining the properties of the compressor or turbine housing’s curved scroll. Compressors are relatively insensitive to A/R changes, but turbine performance is greatly affected by changing the A/R. Proper A/R is determined by application, engine size, and, ultimately, real-world tuning.
Blow-off or bypass valve (bov):
A valve on the compressor’s outlet side that quickly relieves pressure in a blow-through system when the throttle is suddenly backed off.
Blow-through system: A turbo system in which the throttle blades are located on the compressor’s outlet side. Most modern efi setups are blow-through configurations.
Boost: The amount of pressure above atmospheric pressure, expressed in pounds/square inch (psi) in the U.S. system or bars (b) in the metric system. 1 psi is about equal to 0.0689 B. A turbo EFI setup usually requires a 2B map (manifold air pressure) sensor.
Choke flow: The maximum flow for a compressor wheel trim and housing combo at a specific efficiency, usually 60 percent. This is the righthand boundary on a compressor map.
Compressor: The cold-air-into-engine side of the turbo. Intake air passes through this side of the turbo on its way to the engine.
Compressor surge: Reversion of air out of the front of the turbo–either when the throttle is suddenly slammed shut and air is caught between the compressor and throttle plate or under boost if there’s too much pressure in relation to the amount of airflow through the system. Cure the former with a bov and the latter by getting the right compressor for your combo.
Draw-through system: A system in which the throttle blades are on the inlet side of the compressor. Rarely seen today, it was mostly found on some old-school carbureted setups.
Intercooler or charge-air cooler: An air-to-air or air-to-liquid heat exchanger mounted between the turbo and engine that draws heat from the air exiting the compressor under boost before it reaches the engine.
Lag: Any delay between a change in throttle position and the production of usable boost.
Ported shroud: Antisurge relief channels used in certain high-tech compressors that allow some airflow to bypass the wheel.
Pressure ratio: The ratio of the total absolute pressure produced at the turbo outlet divided by atmospheric pressure. Turbo engineers usually prefer this standard to an outright expression of boost pressure.
Turbine: A turbo’s exhaust or “hot” side. Exhaust pressure exiting the engine passes through the blades on the turbine wheel, causing it to spin. Through a shaft, it spins the compressor wheel.
Wastegate: A boost pressure–actuated valve in the exhaust collector that allows only enough exhaust gas into the turbine to achieve desired boost, venting excess gas around the turbine.
Wheel trim: An area ratio defining the relationship between the inducer and exducer of both the turbine and the compressor wheels. A higher-trim-number wheel means more airflow; a smaller trim results in faster spool up. Changing the compressor wheel trim in particular can dramatically alter performance.
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