Blow-off valves and diverter valves

A diverter valve or blow-off valve is a valve in the pipe between a turbo and the throttle that opens when you lift off. How a turbocharger works showed the problem it solves: at full boost, that pipe is full of squeezed air, and when the throttle snaps shut the air has nowhere to go but back through the compressor. This lesson looks at what that does to a turbo, how the valve knows when to open, and the different kinds.

The names get mixed up, so here’s how this lesson uses them:

• A diverter valve, also called a recirculating valve or bypass valve, lets the air back round to the turbo’s inlet. It’s quiet, and it’s what nearly every turbo petrol car leaves the factory with.
• A blow-off valve lets the air out into the open air, with the well-known “pssh”.

In the UK, both are often just called a dump valve.

The demo plays a gear change over and over: floored at full boost, lift off for two seconds, then back on. The compressor on the left feeds squeezed air along a pipe to the throttle, and on into the engine. The valve stands on that pipe, with a hose from the intake manifold, behind the throttle, to its top. Use the buttons to change the valve, and the stage buttons to jump to each part of the gear change.

It starts with a diverter valve:

1. Full boost. The throttle is wide open and the compressor squeezes the air to 1 bar (15 psi). The valve’s spring holds it shut.
2. Lift off. The throttle snaps shut and the manifold behind it drops to a vacuum. The hose carries that vacuum to the valve, which pops open, and the squeezed air flows back round to the compressor’s inlet. The pressure in the pipe falls to below 0.2 bar (2.9 psi) within a fraction of a second.
3. Back on. With no exhaust to drive it, the turbo has been slowing down, but it’s only been working gently, pumping air round in a loop, so it’s still spinning at about 93,000 rpm. Boost is back to 90% about 1.2 seconds after you’re back on the throttle.

Now press No valve. When the throttle shuts, the pressure in the pipe spikes to nearly 1.3 bar (19 psi), higher than at full boost. Then the trapped air starts bursting back out through the compressor in pulses. That’s surge, and every pulse brakes the turbo. By the time you’re back on, it’s down to about 52,000 rpm, and boost takes about 1.6 seconds to come back.

Blow-off valve works just like the diverter for the turbo. The only difference is where the air goes: out into the open air, which is where the sound comes from.

The model is simplified, but its numbers show the right kind of difference. In a real car, the surge pulses come faster still.

A compressor can only push air against so much pressure, and it needs a steady flow of air through it to do so. Lift off at full boost and both go wrong at once: the throttle stops the flow, and the pressure in the pipe climbs.

The air flowing over the compressor’s blades then breaks away from them, and the compressor can’t hold the air back. It rushes backwards out through the wheel, the pressure in the pipe drops, the compressor catches it and pushes again, and the cycle repeats many times a second. That’s the fluttering “chu-chu-chu” you hear from turbo cars with no valve.

Every reversal shoves the compressor wheel back towards the turbine and slows it hard. The thrust bearing, which stops the shaft sliding end to end, takes the shock, and the wheel’s blades flex. A turbo will survive a little surge, but a lot of it, all its life, wears the bearings sooner.

Surge isn’t only a lift-off problem. Engineers draw a compressor map for every turbo, showing how much pressure it can make at each flow. Its left edge, the surge line, marks where it would surge. A turbo that’s too big for an engine can cross that line at full throttle and low revs, which is one reason a bigger turbo isn’t always better.

A simple diverter or blow-off valve has a piston or rubber diaphragm, a spring holding it shut, and a small hose from its top to the intake manifold, after the throttle.

• Under boost, the manifold is full of boost too. The pressure pushes on the piston from above (through the hose) and from below (from the pipe) about equally, so the spring keeps it shut.
• When the throttle shuts, the engine is still turning and sucks the manifold down to a strong vacuum. That pulls on the top of the piston, while the trapped boost pushes up underneath. Together they beat the spring and the valve flies open.
• At idle, there’s a vacuum above the piston but no boost below, so many valves sit slightly open. That’s normal: there’s no boost to lose.

The spring has to be right. Too soft, and boost can creep past the piston at full throttle; too stiff, and the valve opens late or not fully, so the turbo surges a little on every gear change.

Many modern engines, including VW Group TSI engines, use an electric diverter valve instead, often bolted straight to the compressor housing. A solenoid opens it the instant the ECU sees the throttle closing, without waiting for the vacuum. The early ones on VW Group 2.0-litre TFSI engines had a rubber diaphragm that was known for splitting and leaking boost; VW replaced it with a revised piston-type valve.

Whether a car can use a blow-off valve depends on how its ECU measures the air coming in. That’s covered in Oxygen sensors and other engine sensors; there are two common ways.

• An airflow meter (MAF sensor) sits in the intake before the turbo and measures the air as it comes in. The ECU adds fuel to match. A blow-off valve lets some of that air out again after it’s been counted, so the engine gets the fuel for it but not the air, and runs rich for a moment. You feel it as a hesitation or stumble after a gear change, sometimes a puff of black smoke, and on some cars a stall as you come to a stop.
• A manifold pressure sensor (MAP sensor) works out the air from the pressure and temperature in the manifold, after everything else. Air let out before the throttle was never counted, so a blow-off valve does no harm. That’s why blow-off valves are common on tuned cars that run on a manifold pressure sensor.

A diverter valve sends the air back in after the airflow meter, so it’s counted once and reaches the engine once. That’s why makers fit diverter valves to cars with airflow meters. A dual-port valve is a compromise: it lets part of the air out for the sound and sends the rest back.

Diesels set their power with the amount of fuel, not with a throttle, so the air always has somewhere to go, and they don’t need either valve.

• A split diaphragm or worn seal. The valve leaks boost back to the intake, so the turbo works harder for less boost. You may hear a hiss under boost and feel less power, and the ECU may log P0299 (underboost).
• A weak spring. Like a leak, but only at high boost: power fades at the top of each gear. It’s common after a remap raises boost past what the original valve can hold.
• A valve stuck shut, or a split hose to its top. Without the vacuum signal, the valve doesn’t open when you lift off, so the turbo flutters on every gear change.
• A faulty electric valve. Wiring or solenoid faults on electric diverter valves log P0033 and similar codes, and the car may go into limp mode.

A quick check on a simple valve: pull the hose off its top with the engine idling and you should feel the vacuum; suck on the valve’s port (or use a hand vacuum pump) and it should open and stay open while the vacuum holds.

• An uprated diverter valve, with a metal piston instead of a rubber diaphragm, holds more boost without leaking and doesn’t split. On a remapped car it’s often the first thing to fit, because the original can start to leak at the higher boost.
• A blow-off valve is fitted mainly for the sound. On a car with a manifold pressure sensor it does no harm. On a car with an airflow meter it can cause the rich stumble, and on some cars a fault code. A dual-port valve, or one you can adjust to let only some air out, is the usual compromise.
• Removing the valve, or blocking it, for the flutter sound makes the turbo surge on every gear change. It hammers the thrust bearing and shortens the turbo’s life.