Noob here with some questions. (Wastegate, Blow off)

Nixlimited

Not sure how much you know (and wikipedia is a good general reference), but a wastegate is used to divert exhaust flow around the turbo so the turbine speed is limited thereby limiting the boost pressure. Some wastegates are internal (i.e. built into the outlet side of the turbo) and some are external (e.g. Tial wastegates). Wastegates have sizes, usually in mm (e.g. 38mm) that refer to the size of the exhaust path through the wastegate. The bigger the wastegate, the more exhaust it can flow. Wastegates also have springs that limit the wastegate valve from opening before a certain pressure. The springs are typically rated in PSI of pressure. E.g 6 PSI spring. In concert with the spring, a boost controller can be used, which siphons pressure off the positive manifold pressure to push against the wastegate valve and keep it closed (in some designs). This is how you achieve boost pressure above a wastegate spring pressure. Typically speaking, if you want fast response, you want to keep the wastegate closed as long as possible before hitting your boost target so you use both a spring and a boost controller of some sort. While you don't need to buy a huge wastegate for no reason, like anything else, the wastegate is an exhaust flow restriction and bigger can be better. Also, typically, you will have a wastegate for each turbo. You don't really tune a wastegate other than choosing the right spring. You tune the action of the wastegate with the boost controller.

The blowoff valve serves a different function. As you create boost with some sort of compressor (e.g. SC or turbo) it forces air into the combustion chamber. However, when you suddenly let off the throttle, the throttle plate (if your car has one) closes. Because of all the pressure in the intake charge rushing towards the throttle plate, you want to vent that pressure rather than damage the throttle plate. Accordingly, a blow off valve opens to vent the pressure to the atmosphere or back into the intake tract depending on your setup (OEM is usually back into the intake because the MAFs have already metered the air). When they are vented to atmosphere (VTA), then you get the typical "blow-off" noise, which may be a whistle, whooosh, pshhh, or otherwise, depending on design. A blow-off valve typically uses a spring too (though not all designs do) to force the valve shut (i.e. to not blow-off). The blow-off valve may be triggered by an imbalance of pressure between the compressor and the manifold.

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