By Bill Wotschak
The turbocharger on your MR2 at spins at over 100,000 RPM, and creates nearly 400 CFM of airflow at 1 bar of boost. But what happens when you lift, and the throttle plate snaps shut. All of a sudden that energy that your compressor is pumping out hits a "closed valve". A huge pressure spike that puts a severe load on the compressor impeller, shaft, and bearings. And its like throwing the brakes on the impeller-dramatic loss of RPM and airflow.
But Toyota's designers have addressed the problem and installed a blow-off or bypass valve. I will get to the difference in those terms in a minute.
A blow-off valve is simply a kind of pressure-relief valve that opens to vent the airflow. This minimizes or eliminates the spike, and allows the compressor to "freewheel" under no-load condition so there is no significant loss of impeller speed. So it should be obvious that the important considerations are 1) how quickly will it open (to allow venting), and how quickly will it close (to restore boost pressure). And it should further be obvious that it must be capable of staying closed under any level of boost that you might run.
Most aftermarket valves operate on a spring vs. pressure principle. The valve body directly opposes turbo pressure on one side, and senses manifold pressure on the other side. By adding tension to the spring, you can set it hold higher boost. And when the combined spring tension and manifold pressure falls below the turbo pressure, it opens and vents. So it always operates at basically the same pressure. That means if often only cycles under full throttle, full boost runs. When making less boost, it may not open at all.
And it also means it closes at fixed pressure. So it may be slow to close and restore boost.
The Toyota valve uses a 2-chamber diaphragm to control the OEM valve. There is a hose from the manifold leading to both the top and bottom of the diaphragm. In the hose leading to the bottom there is a one-way restrictor (called a Vacuum Transfer Valve, or VTV). When pressure builds in the manifold, there is equal pressure on both sides of the diaphragm. And since force = pressure X area the larger area on top of the diaphragm develops greater force, and holds the valve closed. As boost rises, the more tightly it is held closed.
But, as soon as you lift, manifold pressure starts to drop, pressure on top of the diaphragm drops, but full pressure remains in the lower chamber, so the valve instantly pops open. Then as soon as you get back on throttle, equal pressure is restored on both sides, and the valve snaps shut.
And this works exactly the same, whether you are at 1-2 psi, or 20 psi. The "farting noise" that people report is just the sound of the valve closing as you transition from boost back to vacuum.
BLOW-OFF VALVE AND BYPASS VALVE
The OEM valve has an additional feature that NO aftermarket valve incorporates-a bypass mode when off-boost (probably 95% of your driving). Since the only path for airflow into the engine is across the compressor , the engine is actually PULLING air through the compressor when at normal cruise speeds, or even idling. This significantly adds to the pumping losses, making the engine work harder, and lower fuel economy.
Owners buy aftermarket BOVs because they like the noise. But to get the noise, it must be vented to atmosphere. The problem is, the ECU's engine management system uses airflow, measured at the AFM, as the primary determinant for fuel and timing outputs. To function properly, the induction system must be a closed system. If your BOV vents measured air out of the system, the ECU has no way of knowing that, and send fuel for that air that will never get to the engine. Result-a momentary over-rich condition every time the valve opens.
1) No aftermarket valve can open and close faster than the OEM valve. And that's what counts!
2) The OEM valve cannot leak under boost no matter how much pressure you run. Spring vs. pressure valves will open when boost exceed spring tension.
3) No aftermarket valve incorporates the bypass feature.
4) Any valve that is vented to atmosphere will allow
false data to go the ECU, resulting in some loss of
Questions, comments and criticisms can be directed to: Jeff