Direct 'Dear Dave' tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.
Q. Ask A Simple Question
I have a question on blow-off valves. I already know what they're used for and why, but I don't know how they work. How does the blow-off valve open when the throttle closes? Where does the vacuum line that is attached to the blow-off valve go? And what is it used for? I've noticed that some BOVs, like the GReddy Type-RS, are adjustable from soft to hard. What is that adjustment used for and what do the different settings do? Is there an advantage in setting it to 'hard' or 'soft' or is it better to try and stay in between?
Dexter Alleyne
Redmond, WA
A. When you're on boost with your foot pinned to the floor, there's a happy turbo pumping a huge volume of air through the intercooler, past the throttle body and into the engine. When you close the throttle in the midst of all this happiness, it's like when the cops come to shut down a party. The guy who answers the door gets the cold chills right away, but it takes a while for those in the backyard to get the message.
When the throttle closes, the air right in front of the turbo stops immediately, but the air behind it just keeps coming. Air piles into the throttle body until it builds up enough pressure to turn around and push back the other way. Eventually (in a party like this, eventually is a fraction of a second) this big wave of panicked air runs right back into the compressor outlet and tries to push its way out the back door.
Since the turbo is still spinning, this doesn't usually work. All the backpressure slows the turbo down quite a bit, but it's still pushing air into the system and the panicked mob of air turns around again and runs back for the throttle, where, predictably, it runs into a wall again and bounces back once more.
On a car with a big intercooler and long plumbing, this buzzkill makes a clearly audible choo...chooo...choooooo sound. With a smaller system with more compact plumbing, it will be a faster chi-chi-chi-chi. Either way, the sound is bad. At best, it tries to blow intercooler hoses off and slows the turbo, causing more lag when you finally get your foot off the clutch and get on with driving. At worst, it overloads the turbo's thrust bearings, leading to early failure (ball bearing turbos have strong enough thrust bearings to shrug off all this BS).
The blow-off valve is basically a side door. When the throttle slams shut, the side door opens and all the boost runs out there instead. This prevents the choo-choo train, and lets the turbo coast freely, so it's ready to rock as soon as the throttle opens again.
It's worth pointing out that if the engine uses a mass airflow meter (MAF), the blow-off valve should be plumbed so the air running out that side door runs back into the inlet of the turbo somewhere after the airflow meter. This prevents the MAF from telling the ECU that all the air running out the side door is going into the engine. Most people say that it's OK not to recirculate the blow-off valve, but most people have low standards for driveability, gas mileage, emission, and sooty rear bumpers. You just paid $12.95 a gallon for that gas, do you really want your ECU to go dumping it into your engine whenever the throttle is closed?
OK, so you claim you knew all that, so here's the part where I stop wasting time and answer the question:
There are three forces acting in a blow-off valve. First there's the air pressure acting on the bottom of the poppet valve itself. If there is 18psi in the intercooler pipe and the bottom of the valve is one square inch, that's 18 pounds trying to blow the valve open.