What is Compressor Surge?

When people think of turbocharged cars, one of the first things that comes to mind is the sound of turbo flutter, that fluttering/chattering sound that happens when the driver backs off the throttle in a hurry, such as when changing gear. That noise may tell the world that you have a turbo, but the truth is, that noise can be doing more harm than good. Turbochargers are our friends, we shouldn’t let them suffer!

Turbo Basics – Exhaust Energy used to create positive airflow (boost)

compressor surge

A turbocharger is an air pump; that is, it is a mechanical component that pushes air. It does this by using the energy in wasted exhaust gas (on the turbine side) and spinning a turbine wheel, which then (on the compressor side) pulls cold, fresh air in and pressurizes it for our engines to consume. The more air we put into our engines, the more power they can produce.

This whole process requires the shaft at the center of the turbocharger to spin extremely fast, which is only getting faster with newer designs and increased efficiency, although this speed is dependent on the size of the turbo wheels. Due to the inertia of these high-speed spinning wheels, when a rapid change of speed is induced on the system, enormous load is placed on bearings and rotating components with tight tolerances. Keeping things working as they were intended is more important than ever.

Compressor surge is a real thing, not something we made up to sell blow-off valves. To explain it properly, we need to delve into the physics of turbocharging. Bear with us; we’ll try to keep things simple. Below is a graph which shows the airflow through a particular turbo on the horizontal axis, and the pressure ratio on the vertical axis. It is a misconception that both airflow and pressure are the same thing – they’re very different.

Every turbocharger has an area of maximum efficiency, whether it is a small area or a large area on the map, it comes down to what it was built for. Operating inside this maximum efficiency island is the ideal scenario although it is sensitive to the setup and characteristics of the engine and driving conditions.

Compressor Map – Showing the ‘Efficiency’

comp surge 1

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’ and, as such, it is most efficient. The turbocharger will not always operate in this zone, and performance will suffer as a result.

Back to compressor surge – you will see in the graph below, the red line down the left hand side of the compressor map. This is called the ‘Surge’ line and forcing the turbocharger to operate to the left of this line causes ‘Compressor Surge’.

Compressor Map – Showing the ‘Surge’ Line

comp surge 2

When your turbocharged engine is under load, the engine is consuming air (airflow) and your turbo is creating pressure (boost). These two things intersect on this graph and all is working as the turbo was designed for. Close the throttle however (as in when you would change gear or decelerate quickly), and the pressure maintains due to the momentum of the turbine wheel spinning incredibly fast, however the airflow has stopped because the throttle is closed. 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.

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. ‘Love it or hate it’, it’s doing nothing good for your turbocharger performance or reliability.

This is where a blow-off valve comes into its own. They are fitted between the turbocharger and the throttle body. The closer to the throttle body, the better, as it will increase the response to mitigate surge.

The purpose of the blow-off valve is to eliminate compressor surge. To do this, its job is to open as quickly as it can when there are rapid changes in engine load from boost to vacuum – such as a throttle plate closing. 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.

A 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 quickly when the throttle is open again to build boost and aid throttle response and acceleration.

The impact of compressor surge can be negligible at low boost levels as shaft speeds are low and the impact minimal. 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. However as shaft speeds increase, compressor surge should be avoided.

Many early factory turbo cars which tended to run low pressure turbos did not have blow-off valves at all, however as the technology and understanding of it has improved, these valves are common fitment from by the manufacture, simply venting the air back into the inlet system (recirculating) in front of the turbo to be used again.

The limitations of the factory fitted valves, however, are well known. They’re often made of plastic which gets brittle and fractures with heat cycling and age. They rely on rubber diaphragms which again age and fail and cannot handle increases in boost when performing mild upgrades and can certainly not handle the flow when performing major upgrades.

All Turbosmart BOVs are designed with maximum air flow in mind for performance and turbocharger reliability. They are designed with a unique ‘Boost Balance’ system which means they cannot be overcome by increased boost levels and use light pistons to ensure quick response to eliminate surge and then promote boost and throttle response equally, by closing fast, ensuring turbocharger momentum is maintained, reducing lag and improving acceleration.

Do they need to be noisy? No, but that is an option if you prefer.

An externally venting valve will make plenty of noise as they’re fitted to vehicle’s where the primary objective is to get that air out of the system as quickly and efficiently as possible. Refer our Supersonic and Race Port type valves as an example.

A Recirculating valve does the same job but is much quieter and requires addition plumbing in the engine bay and re-introduces the excess air in front of the turbocharger for re-use and also, minimizes the sound required to get the job done. Refer our Plumb Back, Dual Port and Kompact valves as an example.

What you don’t want is the sound of compressor surge which goes by a few other names, including, ‘flutter’, ‘dosing’ and ‘wastegate chatter’. These are all leading you toward a repair or replacement bill for your turbo sooner than you would like.

What you want is somewhere between a ‘Pssh’ sounds and absolute silence. Anything else is slowly but surely costing you money and performance.

Have any questions about blow-off valves or boost for the experts at Turbosmart? Fire away in the comments!