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Originally Posted by Slick
Please exuse me if this has been discussed before, I couldn't find anything with a few searches. I've been going around in circles with a friend discussing the design of a TT system, neither of us are to sure of the pressure regulation methods (I guess that's a good way to word it). I see systems with wastegates on the hot side and blowoffs on the pressure side....some with just wastegates and some with just blowoffs. I guess my main question is what are the theories behind when to use which? I've built enough cars/stuff to know it's all about balancing tradeoffs so I'm more interested in the thoeries/ reasoning to using each type than having someone tell me "just use this"...if it doesn't make sense to me I can't get the check book out if you know what I mean 
For some background...my friend is building a TT Camaro and is all about drag racing speed so his reasoning comes from that background, he's using wastegates and pressure side blowoff (alright it's a Banks system which varys a bit but basically pressure side blowoff). My take is that he's trying to get every last bit of power out so he wants to keep both pressure sides as even as possible which makes sense to me. My build is honestly more show than go (not that is won't be driven but I won't be trying to shave a .1 off my 1/4 mile time). My theory is forget the wastegates (yes I'm trying to be cheap ) and just use blowoffs, might not be perfect for drag racing but is there an issue?
Enough rambling, lets here the real reasons from people that know
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The wastegates control the exhaust pressure to regulate the boost made. They will open at a set pressure and vent excess exhaust flow (pressure) out the exhaust, bypassing the turbos. No wastegates, no boost control. A blowoff on the pressure (intake) side is for when being in boost and then snapping the throttle shut, it opens and lets the boost out to prevent damage to the turbo impellers and throttle blades. It is not used to control boost.
They each have a separate purpose and both should be run. A blowoff on the intake will not regulate boost so if you run that and no wastegates you have no way to control boost. It opens under vacuum, not boost. I guess there's something that could be plumbed into the intake to vent at a preset boost, but it better be pretty big if you're not regulating boost with a wastegate.
Do it right, run both.