Pop-off valve vs. wastegate

[Sivat]

What's the difference? It sounds to me like it does the same thing as a regular turbo wastegate.

[KE30MAN]

A pop-off valve sits on the inake side of the compressor and fastens to the plenum chamber. It controls the amount of boost at the intake. A wastegate gets it's feed from power from the exhaust maniold and bleeds of air at the turbine side. It's does however gets it's signal from the post compressor plumbing.

[StuiE]

So on a Champcar it's just working as an overboost valve?, why don't they use, like, and overboost valve for that...

[Snrub]

This from from the series website in the tech section:
Pop-off valve
Champ Car engines are currently turbocharged providing additional horsepower and torque by injecting air and fuel into the engine’s intake manifold under pressure. Champ Car limits the amount of pressure, or “boost”, which can be used in order to help keep horsepower within reasonable limits. To keep the playing field even (and the manifold pressures within the rules), Champ Car provides each team with a manifold pressure relief, or pop-off valve, to put on top of their intake manifold. It's called a pop-off valve because it makes a loud pop when it lets off excess pressure. The effect is a sudden drop in horsepower. Champ Car jealously guards these valves, and goes to great lengths to make sure they are both accurate and consistent. Each day of practice, qualifying and racing, Champ Car officials pass out the pop-off valves to the teams and collect them in the evening

Regarding wastegates, a turbocharger is a positive feedback device. Without the wastegate, the exhaust gases would cause pressure to build up very quickly and damage the engine. The wastegate is on the exhaust side and give exhaust gasses an alternative path around the turbine. In other words, it can stop slow the down the speed at which the turbo spins. The pop-off valve doesn't control the operation of the turbo the same, it can only vent excess intake pressure beyond a certain level.

I suppose you could look at the situation and say that if all pressure was vented from the intake, the engine would produce a lot less power, thus not have a lot of exhaust flow and not spin the turbine very fast, but it's not operated that way. I think that given that currently the turbo is operating at fairly low revs (and thus load), that even with a wide open intake valve the turbo would be spinning fast enough that it would produce boost if spinning at the same speed with the closed intake valve. (that is a bit speculative, so I could be wrong)