You just have a diverter system.
When you let off the throttle, or create an environment of enough vacuum, the diverter(s) (you have more than one )crack open. How far open, depends on the vacuum, this sends a large amount of the charge air back BEFORE the supercharger, to be recycled back though the intake system.
As CV said you just have a diverter system.
Rather than having a blowoff valve, any additional air flow is bled off by using the valve overlap from the camshaft timing and the physical limitations of how much air can be pushed through the supercharger by the pulley size and gear setup combination inside.
By the way, a blowoff valve can be thought of the same way as a waste gate for a turbocharger since it tries to maintain a certain pressure. Since we don’t have one though, the boost pressure can change by physically changing hardware. For instance on my B6 S4 I saw 6.5 psi of boost while stage 1 but when I installed the intercooler kit without changing the SC pulley size, I saw 5.5 psi of boost. The engine made a little bit more power from the bigger intake to the SC, less restriction to the throttle body after the SC from the bigger openings, and the cooler intake air temperatures from a combination of the intercooler (the KC cooling off the water further compounds this) and less pressure forcing the air into the engine. Then a more refined tune made it even better!
Does this make sense?
EDIT: Wanted to give you some applied theory as an example since it was asked about the V3 specs.
When the 3.0T guys change the pulley ratio for the 1320 supercharger by switching out to a smaller SC pulley and/or bigger crank pulley, they are trying to reach that desired bypass valve pressure quicker than normal. That is until the software is changed to allow the bypass valve to not open until a higher pressure. There is a point of demenishing returns though because the 3.0T guys have pullied (not sure how to spell that) up the supercharger so badly that it is so far out of the designed speeds that it was meant for that a ton more heat is generated. The additional heat starts to outweigh the additional air flow and take its toll on the components. Like how Loe keeps causing the intercooler cores to fail and leak, the weak stock catalytic converters melt and clog up, and eventually the bearings/rotors/internal components of the supercharger will fail but that is of course if the engine itself holds up. Good thing for the 3.0T guys that the long blocks are pretty tough!
As always jimmy and CV great posts.
really appreciate the technical information provided guys. really thankful for this. I am using an obd Bluetooth dongle which is connected to my mobile phone to which I am running the app “torque - lite”. it has a boost/vacuum dial and I do see every time 0.6 bar / 9 psi when I step on it. Cant recall the exact increase in pressure but I will log it and add it for your reference here.
I’m an anxious RS5 owner waiting patiently for the JHM supercharger. I often “cross shop” other models as there’s relevant information and experience that I can gather. I enjoy reading all of this stuff no matter what chassis we’re talking about. But admittedly, there’s a connection between the RS4 and RS5.
One clarification, it’s not cam overlap, it’s valve overlap. Depending on the intake and exhaust cam timing (duration for each, the degree of crank rotation the valves are open), there’s a certain amount of overlap where the intake AND exhaust valves are open at the same time. In terms of boost modulation, this is important as boost pressure can be bled out via overlap when the exhaust valves are still open. Not the most efficient in terms of energy loss but it is a useful tool for a belt-driven supercharger that doesn’t have a hyper-reactive wastegate and boost controller like a modern turbo system.
Also, the KC system…is there a link or better source of more detailed info? I recently ran into a new Infiniti Q60S that was running a tune and using the a/c system to cool the relatively small heat exchangers. The owner had been letting it idle for two hours straight and the exchangers were ice cold. Given the the summertime temps here in Texas where I live, that’d be a good, good thing to keep everything in order and the engine intact.
Guys quick question on the above. For stage 1+, do you know what is the spring rate / pressure that the DVs open?
The DV valve is not a pressure valve so much as it is a vac valve. The valve is held shut until there is a vac situation and that diverts the air back to the air filter.
I don’t know if that made sense or not.
It makes perfect sense! …thank you very much for the explanation