Just noodling something over.

When the bypass valve opens, where does the air go? Is it dumped into the atmosphere, or routed back into the intake tract? If it's routed to the intake tract, does it enter upstream of the throttle body or downstream - between the tb and the supercharger?

 

I think it returns to the intake...

 

it was explainedwell some time ago by the doc or somebody . but what i got from it was the bypass is open at idle (full vacuum) then as the revs go up and vacuum then decreases the valve closes to allow full boost. but don't think that at idle it's bypassing the s/c . it's just recirculating through the bypass . it's just a big loop circuit through the s/c until it shuts . at least this is what i got from the discussion. you do have to wonder what the definition of by-pass is though . it would make more sence that the valve bypasses the s/c until shutting then it's boost all the way . this would allow for the fuel economy . not fully but the system would balance itself as to the strength of the spring .some is bound to still go through the s/c . until fully shut then the s/c gets it all . at least i see it that way .

 

Yeah, I got all that, but I'm wondering exactly where in the intake tract the air is dumped back in.

Do you have any idea where to find the thread(s) that you're talking about?

 

use the search program at the top. just type in bypass valve

 

It's on the other side of the TB.

 

I've tried that looking for other related subjects. That's a LOT of threads to wade through. :impatient Just thought I'd ask if you remembered off the top of your head a particular thread title. Maybe I'll do some poking around later.

 

it's a loop . just think throttle body to s/c to i/c to intake the bypass when open just just opens the loop at the throttle body to intake . the air can then just recirculate . so no boost mentionable will build . it does not dump to atmosphere like a turbo .

 

What I'm wondering, though, is WHERE the air gets dumped back into the system. Into the airbox? Into the tube between the airbox and throttle body? Somewhere AFTER the throttle body but BEFORE the supercharger? It seems that different locations could conceivably have different effects on the performance of the engine, as well as the performance of the bpv itself.

 

The BPV is plumbed right after the TB in the SC duct, and connects to the IC outlet right before the intake manifold, number 22 in this pic...

 

Thank you!! That's exactly what I was looking for. So, when the valve is open, it dumps the already-charged, already-intercooled air right into the path of the *fresh* air coming in through the throttle body. I'm wondering if this has any negative effect on intake temperatures for cars with "cold-air" intakes, or if it affects the operation/response of the bypass valve's vacuum diaphram, which feeds from the same area.

 

Wow! Tons of information I just learned. A wonderful feeling!

 

I'm sure to some extent, but I seem to have done something to help...
https://www.northamericanmotoring.co...d.php?t=101043
Do a search for "VGS" it's about sourcing vacuum for the BPV in another location. My car doesn't like the VGS, but a lot of people have been running it with no problems.

 

Fascinating!! I'll have to read that epic... er, thread now. I hadn't previously.

I'll look up the VGS thing again. I was curious about that, too, for informational purposes. It seems that the best place to draw vacuum for the bpv is right after the throttle, but I'm not so sure about that being the best place to dump the air from the bpv.

 

If you look at the setup on your car, you will see that there really is no other place to plumb the BPV without some custom work. Then it comes down to if it will be any better plumbed a different way.
My IATs are 5F to 10F above ambient while cruising now, so i'll leave the setup as is and work on a different valve and/or a different way to open and close it.

 

Yeah, I figured that plumbing to a different location might be a problem. My original thought regarding this wasn't so much with IATs, though, but with getting a "clean" post-tb flow for the bpv to "read". I was thinking about the "Throttle Response and Boost Gauge" thread, and how to optimize the throttle/bpv response to pedal input. Thinking about it now, what I thought might be detrimental might actually be a positive in that regard - extra air in the tube between the tb and the sc to "preload" the bpv vacuum pick-up.

That issue aside, I still think it would be better for the "bypassed" air to be routed to the airbox, but the "hassle to benefit" ratio probably wouldn't make it worthwhile. Plus, I'm not an engineer or mechanic, so I may just be talking out of my rear.

 

The BPV does not allow air to "bypass" the engine like a blow off valve, it allows air to bypass the supercharger directly into the engine. It is there for two reasons. 1. To reduce drag from the supercharger when the throttle is closed by equalizing pressure on both sides of the charger. 2. To increase MPG by reducing the amount of time the engine sees boost, specifically at mid throttle. If anything the bypass valve gets cooler air into the engine by mixing it with the air that passes through the hot supercharger. Remember, while cruising, the air is hot from heat exchange from the hot charger not becouse it has been compressesd by the charger.

Don't believe me, think about how the BPV is actuated. It uses vacuum, when at part throttle to open and bypass air. The engine is moving more air than can be passed through the throttle, creating the needed vacuum. If you can't even supply the engine with enough air during this time, how could you ever supply the supercharger enough to create boost. At full throttle, more air is available than the engine needs and manifold vacuum is low (about the same as atm.) there isn't enough pull on the BPV and it shuts. Now the pressure on each side of the supercharger are no longer equal and the and the extra air (made available by the open throttle) lets the boost increase with the supercharger rpm. Is is mainly throttle position that dictates the boost level, then supercharger speed. You can see that there is never an overboost situation, because the air needed to create boost is imediately cut off when you close the throttle. You can test this by zip tying the BPV shut, which people have done to increase boost response. If you have a boost gauge, you'll see that not only is there no overboost when closing the throttle, but that boost is directly controlled by throttle position.

Basically, you need an open throttle to supply air to create boost, therefore no throttle, no boost, and definitely no over/excess boost when shutting the throttle. Thus no need to vent boost. The valve is there to equalize the intake tract pressure, reduce boost at mid throttle and probably (I haven't quantified it) increase throttle responce at part throttle.

If you wanted to lower IAT by bypassing the hot air theat passes through the supercharger, you would have to set up the valve to completely remove that air from the system and only let in the air between the throttle and charger.

There is at least one person that disagrees with this, but study the intact tract and think about how pressure (mainly vaccum) across the intake path, air volume control, BPV actuation and the path of least resistance work together work together. It will bencome clear.

 

VERY interesting points. Made me think of the workings of the system from a different angle. I *did* draw out a diagram of the intake tract, and it all... well, mostly, make sense now. At part throttle non-boost situations, the air is actually "flowing" through the bpv in the reverse direction of what I was thinking. That being the case, along with the other pressure/available-air realities that exist at part throttle, makes the question of intake air interference and bpv vacuum draw moot. It appears that the only time that pressurized (and heated) air would be flowing through the bpv into the path of intake air coming through the throttle would be the very momentary point between when the throttle valve cuts off air supply to the the sc and the cylinders pull in enough of the pressurized air in the manifold to eliminate that pressure. That would happen instantaneously, and at that moment, of course, closing the bpv quickly would not be the issue.

I still haven't quite wrapped my head completely around the idea of pressure equalization for the always-spinning sc. It seems that at some point (in the rpm range) the available incoming air would be drawn through the sc simply because the sc would be spinning fast enough to out-demand the cylinders. Maybe I'm wrong on that, though, and at part throttle the intake air ALL bypasses the sc and the rotors just spin in a vacuum esentially. Yeah, I guess that no matter how high the rpm, if the sc is not being supplied with a lot of air, the bpv is open and the shortest air path is through it and straight to the intake manifold. Hmmm.

So many variables, what with two air pumps (the sc and the engine itself) and two butterfly valves (throttle and bpv) controlling airflow. :impatient

Thatnks for your explanation.

 

"It appears that the only time that pressurized (and heated) air would be flowing through the bpv into the path of intake air coming through the throttle would be the very momentary point between when the throttle valve cuts off air supply to the the sc and the cylinders pull in enough of the pressurized air in the manifold to eliminate that pressure."

-This part is exactly right, including that it would happen instantaneously, but the second part of the next sentance makes me think you have the BVP positions backwards. BPV open=no boost, air is bypassing the charger.
BPV closed= boost, all air passes through the charger.


"That would happen instantaneously, and at that moment, of course, closing the bpv quickly would not be the issue."

"I still haven't quite wrapped my head completely around the idea of pressure equalization for the always-spinning sc. It seems that at some point (in the rpm range) the available incoming air would be drawn through the sc simply because the sc would be spinning fast enough to out-demand the cylinders. Maybe I'm wrong on that, though, and at part throttle the intake air ALL bypasses the sc and the rotors"

-The air is being pulled into the intake through the BPV and SC. Because the BVP equalizes the pressure between the intake manifold and the runner between the throttle + SC, (both sides of the SC) Then the pressure of the air passing through the charger and BPV should be about equal. If less air is flowing past the throttle than the engine wants, there will be vacuum and no way for the SC to make boost. The only other place to get air is to pull it back out of the engine via the open BPV. I am not physicist, but I doubt that would happen.

I will post a drawing later that will simplify the path. It is confusing the same way comparing an electrical schematic to a working model is.

 

No, I know the closed/boost, open/no boost operation. What I meant by my statement, "...closing the bpv quickly would not be the issue" was that at that instantaneous moment of pressure equalization after closing the throttle, the bpv would be wide open and you would be decellerating anyway. In that case, the speed at which the bpv closes based on vacuum reading is moot. I had originally (when I was thinking in terms of pressurized air flowing in the reverse direction through the bpv) been wondering about the effect of a "false" non-vacuum situation between the throttle body and the supercharger causing a hesitation in the bpv closing upon pressing the accelerator down. Now I see that at part throttle and with the bpv open, there is a vacuum condition in that area, so no problem.

I sketched out a simplified picture of the intake tract, and it looks basically like a lower case "e". Connect with the bpv in the appropriate spot, and imagine where the air would flow under vacuum (or boost) along the shortest path, and it becomes very clear how the system works.

Yeah, I think unless you do something REALLY wrong, nothing is going to pull air out of the engine when it's running. I just had to make myself remember that no matter what the throttle, bpv and sc are doing, the air is always flowing toward the cylinders.