BLOW-OFF VALVES

A blow-off valve (often referred to as a dump valve or recirculating valve, such as those available from GCAP Performance) is simply a pressure relief valve allowing excess boost pressure to escape when the throttle is shut. On most conventional applications, when the throttle is wide open, the boosted air from the turbo is going straight into the engine, but what happens when the throttle closes during gear changes, lift-off or braking? In these situations, the boost air is stifled by a closed throttle body, encouraging air to travel back from whence it came. The air hits the spinning blades of the turbo, resulting in a distinctive ‘chatter’ sound (known as turbo stall, but often mislabelled as wastegate chatter). This condition is less than ideal, although on most moderately tuned road cars, isn’t massive cause for concern. For big boost race or drag cars, however, this can be hugely detrimental to the turbocharger’s health.

To prevent damage occurring, a blow-off valve is fitted between the turbo and the throttle, allowing stifled air to escape. The part is usually controlled by a vacuum source, which opens the valve and frees the air. Many modern turbocharged production cars feature a blow-off valve built into the turbo’s compressor housing. The valve fitted is usually a plastic unit featuring a small spring and diaphragm, which is fine for a standard road car, but a bad idea for a tuned engine enjoying enhanced levels of boost. Most modern aftermarket blow-off valves feature a robust body and a piston in place of the weaker diaphragm design, promoting longevity and reliability, as well as resistance to extremes of heat and pressure. Much like actuators, a blow-off valve can be fitted with a variety of different springs, enabling you to tailor the part to your Porsche’s needs.

The inner workings of a blow-off valve are simple, with differences in design being where the trapped air goes after it is released.

Vent-to-atmosphere

The simplest way to get rid of excess boosted air is to jettison it straight into the atmosphere. This is a fast and efficient way of relieving system pressure. It offers performance advantages too: when we vent air to atmosphere, the turbo keeps spinning, even with the throttle closed. In other words, the turbo is already spinning when we open the throttle again (after a gear change), enabling the immediate creation of boost pressure.

There are, however, drawbacks to this design. For example, when it comes to modern cars with mass airflow (MAF) sensors, unmetered air can be allowed into the inlet system, while already metered air has the potential to escape, momentarily encouraging the engine to run rich.

Recirculating valve

A recirculating-type blow-off valve operates in the same way as other types of blow-off valve, but rather than dumping trapped air into the atmosphere, it’s plumbed back into the inlet before it enters the turbocharger. As the air is channelled back into the system before the MAF, there’s no chance of unmetered air being introduced or escaping. This is obviously good news for the air/fuel ratio.

Dual port

Recirculating valves tend to be much quieter than vent-to-atmosphere dump valves, but whether you think this is an advantage or drawback is down to personal preference. What if you want a blow-off valve recirculating air (enabling you to avoid MAF sensor issues and emptying of your car’s inlet tract), but you still want to hear that joyous blow-off valve badoooosh?! Many aftermarket manufacturers produce a range of dual port products which do exactly that. These brilliant blow-off valves recirculate some of the air back into the inlet whilst dumping the rest of the trapped air direct to the atmosphere. This design enables quiet operation around town, with the benefit of quick expelling of trapped inlet air under heavy boost conditions.