I need some back up on another forum and hoping that people here have the smarts. Seems this crap is on you tube aswell. People don't seem to understand physics.
I need to dispell 2 myths about recirc valves, using a 940 turbo as an example.
1. "A recirc valve recirculates the unwanted air so that the measured air stays the same and doesn't confuse the ECU"
This is nonsense. in fact the opposite is true. The standard valve (on a TD04 say) dumps the air out the air filter in order to decompress the inlet, partly because the air reading would be then invalid (although its probably done like that because it has to dump for other reasons), but also because the throttle body and idle valve is not designed to control the air at pressure for idle. Without dump (and this is easy to test by disconnecting the control pipe), the engine won't idle for 4 or 5 seconds after boost because the mixture is too weak, it just dies.
The ECU cannot deal with stored air, or air it measured some time ago. It can only measure air now, and assumes that is what is going into the engine.
2. "a dump valve dumps to the atmostphere, whereas a recirc valve recirculates"
A recirc valve just means it can stay open during idle, and has no remaining pressure.
Tip - because the air is reversing out it tends to pressurise the crank case via the big breather and makes redblock turbos a bit oily. In high boost it literally spurts out of every seal during dump. A one valve helps keep your engine clean (as used by most diesels).
I need to dispell 2 myths about recirc valves, using a 940 turbo as an example.
1. "A recirc valve recirculates the unwanted air so that the measured air stays the same and doesn't confuse the ECU"
This is nonsense. in fact the opposite is true. The standard valve (on a TD04 say) dumps the air out the air filter in order to decompress the inlet, partly because the air reading would be then invalid (although its probably done like that because it has to dump for other reasons), but also because the throttle body and idle valve is not designed to control the air at pressure for idle. Without dump (and this is easy to test by disconnecting the control pipe), the engine won't idle for 4 or 5 seconds after boost because the mixture is too weak, it just dies.
The ECU cannot deal with stored air, or air it measured some time ago. It can only measure air now, and assumes that is what is going into the engine.
2. "a dump valve dumps to the atmostphere, whereas a recirc valve recirculates"
A recirc valve just means it can stay open during idle, and has no remaining pressure.
Tip - because the air is reversing out it tends to pressurise the crank case via the big breather and makes redblock turbos a bit oily. In high boost it literally spurts out of every seal during dump. A one valve helps keep your engine clean (as used by most diesels).