Most turbo cars of the last decade have a factory blow-off valve installed as standard. Placed so that they recirculate air from the outlet of the compressor back to the inlet, they go about their business quietly and with no extrovert gasps and hisses. But what if you wanna let everyone know that you're driving something with a puffer under the bonnet? How d'ya make the factory valve behave like a mega-dollar aftermarket one?
The key to understanding how blow-off valves work is to have a look at the typical pressures that occur in the intake plenum. It's easy - you run 1 Bar boost? Okay - that's the maximum pressure that you'll see in the intake system after the turbo compressor wheel. At idle, a pressure of less than atmospheric will be present - say minus 0.5 Bar. And on the over-run - like when you lift your foot suddenly to change gear - a pressure of minus 0.8 Bar might be happening.
Blowin' In the Wind
So let's have a look at how factory-fitted blow-off valves work - without knowing this, not a lot of the following will make sense. In this diagram, the car is on boost - the throttle is fully open and the turbo compressor's spinning hard. There's a positive pressure being developed everywhere in the intake, including in the vacuum/boost hose that goes to the blow off valve. This boost signal keeps the blow-off valve shut, meaning that all of the air being pushed by the turbo compressor must go into the engine.
Now something's changed - the throttle is being closed and so a vacuum like minus 0.8 Bar is being created in the intake system after the throttle body. A strong vacuum signal passes down the vacuum/boost hose leading to the blow-off valve, and so the valve snaps open. The open valve connects the intake after the turbo to the intake before the turbo, relieving the pressure build-up that would otherwise occur in the plumbing between the turbo compressor and the closed throttle blade.
At idle, the blow-off valve in most factory systems stays open - the minus 0.5 Bar or so that's present is enough to trigger it. Why do manufacturers set up their blow-off valves like this, then? If the turbo is spinning at idle (it is in some cars but not in others), an open blow-off valve has the advantage that the turbo won't be pushing air into a brick wall (the closed throttle), meaning that it can them come up on boost faster when the throttle is actually opened.
Pssht!-ing Factory Valves
All aftermarket blow-off valves vent straight to the atmosphere, rather than returning the air to the intake system in front of the turbo. That's the reason that they make satisfying whooses - the air is dumped straight out. So how do you make the standard blow-off valve behave like that? In this diagram you can see that the connecting hose between the blow-off valve and the intake system before the turbo has been removed. With the original hole into the intake blocked off, the car will make that familiar Psssht! whenever the throttle is suddenly lifted.
But there's a snag. Remember how the blow-off valve on factory engines is usually open at idle? Well, when you disconnect one side of it a couple of things can happen when the engine's idling - and neither is something that the engine likes! Firstly, if the turbo is spinning at idle, it will be pushing out a bit of air. This air will find its way out of the blow-off valve, spilling into the engine bay. The airflow meter will be measuring this air (measured before it's gone into the turbo, of course) and will be expecting all of the metered air to make its way right into the engine cylinders. When it doesn't, the engine will run rich - it won't be getting as much air as it should have got to match the fuel being injected. That's one scenario - here's the other.
If the turbo isn't spinning, air will be drawn into the open blow-off valve. This is because when the throttle butterfly is shut at idle, air still needs to be made available to the engine if it's to run. This air is provided by a throttle body idle bypass, so there is still a route into the engine. The air that gets sucked through the blow-off valve (an easier path than through the filter and airflow meter in many cars) is then both unmetered and unfiltered, so it will cause the engine to run poorly (lean this time) and may also cause dirt to be drawn in.
So waddya do? It's not too hard, but it does require careful setting up. Go to the hardware store and buy two miniature plastic irrigation taps and a matching T-piece - about $2 for the lot. Insert the T-piece in the vacuum/boost hose leading to the blow-off valve, and then on the intake manifold side of the T-piece place one of the taps. This is the coarse adjustment tap. Add a short length of hose to the unattached arm of the T-piece and insert the second tap on this line. Cable-tie a scrap piece of foam rubber around the open end of the hose - this is a simple filter. With the factory blow-off valve disconnected from the intake so that it will Psssht! nicely, perform the following set-up procedure.
Open both taps and feel the open large diameter hose connection of the blow-off valve. With the engine running, air will be either passing into or out of the open hose connection. (If it isn't, you may not need this system. Simply drive the car, see that it goes Psssht! and that the engine idles and drives properly. Everything OK? - you're done!) However, more likely the car engine will be staggering and you'll be able to feel air passing through the valve. Shut the coarse adjustment tap and the engine idle should return to normal, open it and the idle should again be bad.
Okay. With the engine idling, slowly close the coarse adjustment tap, while at the same time feeling with a moistened hand the air flow into (or out of) the blow-off valve. When you get to a certain point, the airflow through the valve should suddenly stop and the idle should become good again. Open the coarse adjustment tap a tiny bit so that the blow-off valve just begins to flow again. Then slowly close the fine adjustment until the blow-off valve just closes - ie air stops flowing in or out. Idle should now be fine, and when you drive the car the valve should still go Psssht! satisfactorily. Slight further adjustments of the fine adjustment tap may be needed to get just the right compromise of idle quality and Psssht! noises.
Make sure that the plastic taps and T-piece are not mounted close to the hot exhaust manifold or turbo - they'll melt if you do this! Also make sure that the new small filter is not placed in a dirty location - you can always connect this hose to the filtered side of the aircleaner box if you like.
How it Works
This system alters the strength of the vacuum signal to the blow-off valve. The coarse tap acts as a variable restriction and the fine adjustments as a bleed. It's the same system as the AutoSpeed EXA boost control, except in reverse. With the coarse and fine taps just cracked open, the air being drawn in by the vacuum present in the manifold flows from two sources - the open (filtered) hose and the blow-off valve. This in effect 'dilutes' the level of vacuum, experienced by the valve. If the dilution of the signal is just enough that the valve closes at idle, the much bigger vacuum present on the over-run will still be enough to open it. The air being drawn in past the fine adjustment tap is not normally enough to have an effect on idle.