littleball_s4

Hello,

I'm learning a bit about SC setups at the moment, and one thing is a bit misterious to me:

What happens at part throttle with the typical kits we have available? I've seen some setups with blow-off valves, which open when they see less pressure after the throttle valve than before. Which is fine in theory: when at WOT it's shut and keeping pressure inside, and when you lift to shift gears or brake, it vents.

But I can't imagine how it works at part throttle, specially high rpm. Say you are using 10% throttle at 4000rpm in 2nd. Then you slowly open the throttle. Close to say 90% throttle, the pressure diff is small enough for the BOV to close. Then what, the BOV closes and the car suddently becomes boosted?

I wonder if anybody tried two throttle valves, one the actual throttle, the other the blow off (normally open), sequencially activated. That's easy to do, almost inmediate, and the transition from boost to no boost could be smooth. Also, it would be a clue for the driver, as a slight hard spot could be sensed on the throttle pedal, when the boost is allowed.

Anyone uses that in their kits?

Z

The Murf928 supercharger kits use a blowoff valve that is not just a full on or full off valve, with instant switching between those two positions. The valve moves in relation to a balance of spring pressure, pressure before the throttle, and pressure after the throttle. There are no sudden unexpected surprises in boost or power when you hit some particular point of throttle opening. They're smooth and progressive with how much you step on the gas pedal.

littleball_s4

I see. I should have guessed. Being proportional is ok.

Still curious as what reaction times the thing has, probably not the best but now it makes sense perfectly. Going mechanical would need little work, but certainly more than zero, in the throttle link, and would make very little advantages, save reliability

Many thanks

Cosmo Kramer

The BOV is only open when manifold vacuum is present. In your example of 10% throttle and 4000 rpm, the manifold would be under vacuum and the BOV would be open. The main reason for having it is to release pressure when the engine doesn't need it, or when the throttle plate is restricting airflow to the point there is manifold vacuum. Once this differential is overcome the BOV closes. There is still air pressure in the intake tract up to the throttle plate when it is open as it isn't all bled off, and it is helping to feed the vacuum of the engine at part throttle.

My car transitions from vacuum to boost at about 1/3 throttle, this varies a bit with rpm.

Imo000

Is a blink of an eye fast enough?