Tuning the C20LET - Breathing/Dump Valve

Also known as a blow off valve. For some people the sound of the Dump Valve is the only reason to own a turbo. Personally I think it sounds like a lorry and it can only attract unwanted attention, but in fact there are even questions in forums about fitting DVs to n/a cars, even diesels!

Hey, it's a turbo, innit? I WANT a Dump Valve!

But you haven't got a throttle, what do you need it for?

..Silence...

OK, how can I fit one anyway?

Maybe they think it's a babe-magnet, who knows. The aural effects of the Fast&Furious scarred their memories forever...Below is a sad contraption sold on eBay as "Dump valve for diesels":

There is an even more ridiculous version: it will fit any car, bicycle or skateboard:

Sunami turbo noise simulator! (local copy here)

Turbowhistler (local copy here)

These are the same people who might use stickers like this one

So what does it do and why don't manufacturers fit them as standard? Don't they want us to be kewl?

Current OEM designs do fit them, but they're usually recirculation valves, so that noise is minimised. They don't want us to procreate, apparently...

This is the LET recirc valve:

The first dump valve was fitted to the Porche 924turbo, if I'm correct. No self-respecting manufacturer would miss it now, same as the water jacket around the turbo bearing (Porche again was first)

Here's how it goes: Happy Larry tries to impress his friends, and gives it some stick. Full throttle creates a rush of air in the cylinders, lots of exhaust gases are produced, the turbine spins like hell, the compressor is also forced to spin at the same speed (they share a common shaft!) so lots of boost (compressed air) is being shifted down the pipes, through the wide-open throttle. Larry's happy.

All good things come to end unfortunately, either the long straight ahead is over, or blue lights are flashing in the rear-view mirror.

Time to get off the throttle - suddenly the throttle plate is SHUT, but all that boost has nowhere to go. It's trapped between the throttle plate and the compressor, that still spins for a fraction of a second, as inertia and a few remaining exhaust gases still try to spin the turbine. That is a wall of pressurised air that won't let the compressor turn as fast as it needs to - but the turbine doesn't know that yet (it will, in a while, because no combustion creates no more exhaust gases, but it takes some time to respond).

So the turbine tries to spin the shaft, but the trapped boost forces the compressor to stop spinning. That can't be good for long-term reliability. Run a bit too high a boost and after a few gearchanges something will give.

This condition is compressor surge, and it means that the compressor creates boost that the engine can't swallow quickly enough. A very distressing sound usually accompanies surge. It can easily turn ugly, with shattered wheels or shaft bits all over the place. The technically-savvy might know it as as the no-go area at the top left on a turbocharger pressure map:

The dump valve prevents this condition, because as soon as the throttle is shut, it vents the excess pressure from the pipes, preventing this surge. Women love it, too.

Somewhere here I have to mention that there is always the odd moron who tries to create surge on purpose, because the sound of it turns him on - maybe it attracts a different sort of his female species, who knows...

 

Placement

DVs are best placed after the intercooler and before the throttle plate(s). They vent the excess air into the engine bay, creating the wholesome muscular 'whoooshing' sound. Some even have a megaphones and trumpets attached to amplify the sound even further (I kid you not!)

Some people think that the DV should be right next to the turbo, because that's where the manufacturer placed theirs - they forget the manufacturer was trying to cut costs and incorporated the whole thing into the turbo (along with the wastegate!) These people also believe that it's more efficient because nice intercooled air is not 'wasted' though the DV. I'm not sure how long the diaphragm will live in there under those air temps though...

Others go to the other extreme and fit it right next to the throttle plate. This one is part of the 'top hat'. Quite an efficient design, minimising disruption to pipes and hoses.

There is a general snag though: the dump valves are after the AFM, so the air that is dumped has already been metered. Since it has been accounted for, it's share of fuel has been set aside by the ECU. But that air mass has been vented into oblivion, creating a rich condition for the remaining air that has to live with the extra fuel.

With some dump valves this is a serious problem and the engine won't even run properly. Double-piston DVs (like the above Bailey model) respond much quicker and are much better in that respect.

Dump valves are not fit-and-forget items, they DO go wrong sometimes and the engine runs funny. If a DV is stuck cracked open, the boost leak will force the turbo to spin harder trying to reach the preset boost pressure at the manifold. If it's stuck closed, then the surges will damage the compressor.

There are (non-cowboy) turbo setups with a couple of DVs before the intercooler. These are meant to act as pressure regulators instead of a wastegate - different kettle of fish.

 

What to do with the recirculation valve then?

When fitting a dump valve you can't just leave the stock recirc valve lying there without the vacuum hose attached. This will lead to boost leaks. The recirc valve has a weak spring keeping it closed, it can be pushed open by just a few psi worth of pressure. This spring is meant to work in conjunction with the vacuum (and boost) pipe: Whenever there is boost on the inside of the valve, there is meant to be equal boost on the other side of the valve too (via the pipe). These pressures will cancel each other out, and the weak spring will be enough to keep the valve closed.

A good idea is to install a DV and also keep the recirc valve in place as well. Both can be hooked to the vacuum pipe with T-junctions. There's enough vacuum to keep everybody happy, including the Fuel Pressure Regulator that's also using the same vacuum source.

The advantage of this setup is that there is a lot of recirculation, which is a GOOD thing, as less unmetered air escapes. Also the dump valve doesn't work as hard, so it will last longer, and it will be quieter, making the car more 'stealthy'. Responsiveness is also increased marginally, as the recirc valve is as close to the turbo compressor as it gets. The overall dump volume capacity is increased as well, with both working together.

If it's decided to totally bypass the recirc valve (because it's faulty, or we want to make the DV louder - work for it's money!) then a spacer a few millimetres thick should be placed inside, to stop the diaphragm from opening under boost. A slice of garden hose would do nicely, but a silicon hose (or aluminium spacer) would be more heat-resistant.

 

 

On to the Intake Manifold...