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 which happens when the driver backs off the throttle in a hurry, such as when changing gear. That noise may tell the world you have a turbo, but the truth is, the sound signals a problem. Turbochargers are our friends, we shouldn’t let them suffer!
Turbo basics: exhaust energy used to create positive airflow (boost)
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) to spin a turbine wheel, which then (on the compressor side) pulls cold, fresh air in and pressurizes it for our engine to consume. The more air we put into our engine, the more power it can produce.
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 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. Keeping things working as they were intended is more important than ever.
Compressor surge is a real thing, not something Turbosmart 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, and whether it is a small area or a large area on the map comes down to what it was built for. 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.
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. The turbocharger will not always operate in this zone, and performance will then 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’.
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, where it’s all working as the turbo was designed to do. 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. 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. These are fitted between the turbocharger and the throttle body. The closer to the throttle body they are, the better, as proximity increases the response to mitigate surge.
The purpose of the blow-off valve is to eliminate compressor surge. 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 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 rapidly when the throttle is open again to build boost and aid throttle response and acceleration.
The impact of compressor surge can be minimal at low boost levels as shaft speeds are low. 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 the manufacturer, and simply vent the air back into the inlet system (recirculating it) in front of the turbo to be used again.
The limitations of the factory-fitted valves are, however, 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.
Selecting the correct BOV
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, they 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.
Several factors need to be considered when choosing the correct blow-off valve, including the performance requirements, and the size of the air-intake system. Larger intake systems (big intercoolers, charge pipes etc.) hold a larger volume of pressurized air, so need a big Blow-Off Valve to vent the larger volume of air, whereas smaller systems may not need as large a Blow-Off Valve, as they hold a lesser volume of pressurized air.
Kompact: designed as a bullet-proof replacement for OEM parts — where space is at a premium.
Type 5: designed as a high flow/performance alternative to OEM parts, or custom turbo set-ups. Type 5 valves are well suited for both street vehicles and race machines.
Vee-Port PRO: maximum performance in a lightweight, compact package. Ideal for performance situations when space is limited, such as boosted motorcycle or powersports applications.
Race Port: our highest-flowing BOV. A pure high-performance motor sport Blow-Off Valve. Compact and very light weight.
Big Bubba: Size matters! Designed for arge-capacity intake systems for both turbo and supercharged V8s, a modified turbo-diesel engines.
Do they need to be noisy?
No, but that is an option if you prefer. If you like things loud, then an externally venting valve will make plenty of noise, as they’re fitted to vehicles where the primary objective is to get that air out of the system as quickly and efficiently as possible.
If you prefer to keep it stealthy, a recirculating valve does the same job but is much quieter, and requires additional plumbing in the engine bay as it reintroduces the excess air in front of the turbocharger for re-use, and also minimizes the sound required to get the job done.
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 all lead you toward a repair or replacement bill for your turbo sooner than you would like.
What you want is somewhere between a ‘pssh’ sound and absolute silence. Anything else is slowly but surely costing you money and performance.
Every application is different. However, ideally Turbosmart recommends mounting the Blow-Off Valve after the intercooler and before — and as close as possible to — the throttle bod, because the airflow through the intake system continues to flow towards the intake manifold when the throttle is shut and the blow-off valve is venting, but when the throttle is reopened, air is still travelling in the same direction, meaning less energy and time before the turbocharger returns to operating speeds.
This article was originally published in NZ Performance Car Issue No. 230. You can pick up a print copy or a digital copy of the magazine below: