![]() However as shaft speeds increase, compressor surge should be avoided. In fact many vent-to-atmosphere BOVs will still experience a negligible amount of surge, as the engine is not producing enough vacuum at very low revs to open the valve. The impact of compressor surge can be minimal at low boost levels as shaft speeds are low. The open valve ensures the air flow continues in the charge pipe, eliminating the pressure spike and (referring the graph above again) ensuring that the turbo can continue in the ‘zone’ without tripping over the surge line.Ī suitable blow-off valve will open quickly enough, and have the flow capacity to avoid compressor surge, while equally being able to close and seal rapidly when the throttle is open again to build boost and aid throttle response and acceleration. To do this, it opens as quickly as it can when there is a rapid change in engine load from boost to vacuum - such as when a throttle plate closes. The purpose of the blow-off valve is to eliminate compressor surge. The closer to the throttle body they are, the better, as proximity increases the response to mitigate surge. These are fitted between the turbocharger and the throttle body. This is where a blow-off valve comes into its own. Love it or hate it, it’s doing nothing good for your turbocharger performance or reliability. That fluttering noise is the sound of a turbo operating in compressor surge, as the compressor ‘chops’ through the air rather than pushing the air into the engine. This situation forces the ‘intersection’ of pressure and airflow across the surge line, as the pressure momentarily peaks and airflow virtually stops, forcing the turbocharger into compressor surge. Close the throttle, however - for example when changing gear, or decelerating quickly - and the pressure stays up due to the momentum of the turbine wheel spinning incredibly fast, but the airflow has stopped because the throttle is closed. These two things intersect on this graph, where it’s all working as the turbo was designed to do. ![]() When your turbocharged engine is under load, the engine is consuming air (airflow), and your turbo is creating pressure (boost). ![]() This is called the ‘surge’ line, and forcing the turbocharger to operate to the left of this line causes ‘compressor surge’. The turbocharger will not always operate in this zone, and performance will then suffer as a result.īack to compressor surge - you will see in the graph below, the red line down the left-hand side of the compressor map. The area in red in the graph above is the area of greatest efficiency: when pressure and airflow intersect at a part of the map that has the turbocharger working ‘in the zone’, it is at its most efficient. Operating inside this maximum efficiency island is the ideal scenario, although it is sensitive to the set-up and characteristics of the engine and driving conditions. It is a misconception that both airflow and pressure are the same thing - they’re very different.Įvery turbocharger has an area of maximum efficiency, and whether it is a small area or a large area on the map comes down to what it was built for. Below is a graph which shows the airflow through a particular turbo on the horizontal axis, and the pressure ratio on the vertical axis. Bear with us, we’ll try to keep things simple. To explain it properly, we need to delve into the physics of turbocharging. Keeping things working as they were intended is more important than ever.Ĭompressor surge is a real thing, not something Turbosmart made up to sell blow-off valves. The inertia of these high-speed spinning wheels means that when a rapid change of speed is demanded from the system, enormous load is placed on bearings and rotating components that operate under tight tolerances. This whole process requires the shaft at the center of the turbocharger to spin extremely fast, and it’s only getting quicker with newer designs and increased efficiency, although the speed reached depends on the size of the turbo wheels. The more air we put into our engine, the more power it can produce. ![]() It does this by using the energy in wasted exhaust gas (on the turbine side) to spin a turbine wheel, which then (on the compressor side) pulls cold, fresh air in and pressurizes it for our engine to consume. Turbo basics: exhaust energy used to create positive airflow (boost)Ī turbocharger is an air pump, that is, it is a mechanical component that pushes air.
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