Lei Siew Long

am i correct to assume that it opens when you close the throttle to shift? i can't distinguish between the sound of my BOV and the wastegate.

thnx in adv

Lei Siew Long

????

TurboMiata

the wastegate opens when the preset boost pressure is reached. A wastegate operates much like a more complex ball spring valve. Say you have a ball bearing, and a small spring (shitty diagram below)

<--bled off pressure heading out wastegate /////////O <--boost pressure pushes this way

As pressure builds from the turbo spinning, a vaccum line feeds a signal to this ball spring type (its not really a ball spring, but this is simplified to avoid me typing a report ) vavle. The spring is a predetermined pressure. It will hold this ball bearing air tight to the end. When the pressure exceeds the springs ability, the ball bearing will push backwards. Now the entire area past the ball bearing is exposed to boost. This tells the wastegate that it needs to reroute boost past the exhuast housing to slow the turbine down so that it doesnt over boost. This happens pretty quickly as the BB moves back and forth to keep boost level where it is supposed to be.

Your BOV opens when the pressure on the exit side (where it vents) becomes higher than the pressure inside your intake manifold (or whereever you happen to have the vaccuum line from the BOV running to) A BOV works like this. There are two areas that are exposed to pressure/vaccum. One side is the side that is the exit for the air that rushes out and makes the PSHHHFFF sound. The other is the side that connects to a vaccum line. There is a cylinder inside the BOV that moves back and forth, with each face of the cylinder exposed to one of the two pressure areas. When you go into boost, pressure begins to build. You have your throttle plate open so the pressure inside of the throttle body is the same as the pressure where your BOV is located. (hopefully right before the TB) This means that pressure on the two faces of the piston are the same (remember, the vaccum line coming off the non-exit side of the piston goes to the throttle body, and pressure equalizes itself...) When you let off of the throttle, your throttle body slams shut. The engine consumes the boost that was left inside of the throttle body, and once again the throttle body reverts to vaccuum. At this moment, the pressure on the non-exit side of the BOV becomes vaccuum. This sucks the piston towards this side. At this exact same time, the pressure on the exit side has become greater since the momentum of the pressurized air was so high it continued to head towards the throttle body. Pressure becomes momentarily higher than the rest of the system (also keep in mind your turbo is still attempting to spin due to its momentum, so it is still making boost) it trys to push the BOV cylinder outward, towards the vaccum side. The two work together, and push the cylinder all the way to the vaccum side. WHen the cylinder gets so far there is a hole that becomes opened up to the exit side, and the air rushes out this hole. The BOV doesnt open while under boost and the throttle body open because pressure is equal on both sides of the cylinder.

CLIFF NOTE: Nice summary of how wastegates, and BOV's work.

Lei Siew Long

good detailed description of how each one works.

so, if i understand this correctly, say my wastegate has a 6.5 lb spring in there. so, if i exceed that amount of boost the wastegate will open? whether it is WOT or if the throttle is closed, once i reach full boost it'll open?

so, if i boost only through the wastegate w/o a boost controller, the wastegate will not open and dump exhaust gases through the dumptube? and if i set the boost controller to an amount of psi that will exceed the spring's capacity, the wastegate will open in this situation?

thanks again. i appreciate the well detailed explanations.

HXMan

A boost controller will modifiy the boost signal going to the wastegate spring. Fooling it into thinking it is seeing less boost then what is really being produced.

The WG will open without a boost controller, but often it begins to open BEFORE the boost reaches the spring setting. Say the spring is 6.5 psi...the wg will begin to open at 5psi, waisting exhuast, and slowing your transition to the full 6.5psi.

TurboMiata

Yep. What he said Boost controller modifies the signal going to the wastegate. Thats why if you remove the signal line, and the wastegate see's atmospheric pressure all the time it will never open (assuming pressure doesnt force it open, which can happen) Try it sometime if you want proof. HOWEVER, be advised that boost will build REALLY FAST. Make sure you let off ASAP. Tried it once back when I was first getting into turbos. Was planning to let off at 6psi (what was back then the limit of my injectors) Got to 10-12psi before I could let off of it. Luckily miata engines own and are really strong