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Any opinions on where to place the bypass valve (BPV) on a Subaru AWIC setup?

I know that the stock Subaru location for it is before the AWIC. But placing it before the AWIC would mean that for a fraction of a second hot air would be feed back to the turbo which in turn would heat the air even more. Would placing the BPV after the AWIC have a performance advantage?

I believe that is the general concencus.

Look at new WRX and STI BPVs. They are on the i/c after the core.

That's one of the reasons I didn't go with the saab i/c, since the BPV would be pre-i/c (and the shop that welded on the fitting to my pipe angled it the wrong way...).

I am currently trying to work out the same problem.
I would like to install it post core as well.
The only advantage I can see in going post core, is you are releasing the pressure where the majority of the volume is. And like you, I dont like the idea of the air going backwards.

I am thinking of attaching a nipple at the front passenger side mount. Where the alluminum is thicker, and since the core is angled, there is more room on the inside at this corner as well.

Silicone hose T-piece 3way 2.75 inch 70mm silicon

Do you guys think that something like this can fit between the AWIC and the Throttle body?

I suppose its possible. It might be tight.

napphappy wrote: I am thinking of attaching a nipple at the front passenger side mount. Where the alluminum is thicker, and since the core is angled, there is more room on the inside at this corner as well.

Are you planning to weld/braze the nipple in place?

I don't see what the volume really has to do with it. The compressor is moving so much air, and the valve is going to be the bottleneck to flow anyhow, no?

It seems to me that the heat argument could cut either way. With the valve at the intercooler inlet, the recirculated air does get compressed (and therefore heated) multiple times. But with the valve at the intercooler outlet, the air that goes through multiple times could heat soak the intercooler. I guess a good intercooling setup would favor the downstream location though.

Having the valve after the intercooler does helps keep the valve's behavior consistent. The valve acts on the pressure difference between its inlet and the intake manifold. Since the pressure drop across the intercooler is a function of flow, a valve positioned upstream of the intercooler sees an exaggerated differential as airflow increases. In principle the valve could start to open just because you're at high enough boost.

I have seen at least one example of a stock Subaru air/water intercooler with a bung added where napphappy is planning to add his (on the right side, which was the driver's side on his car). This person welded a 26mm bung on there so a Bosch valve fit perfectly. As a bonus, the Bosch valve's outlet was pointed towards the right side of the car so it was fairly easy to plumb into the compressor inlet.

I don't think that silicone T-piece would fit as is. With the intercooler pushed up against the firewall, there is nowhere near 25 millimeters between the intercooler's outlet and the throttle body's inlet. Maybe you could trim the latter two though, like some people have done to fit new age WRX intercoolers. You'll lose the beads that help keep the hose in place, but 70mm is the right size and there isn't a lot of room for the hose to pop off.

Well there is also the argument of air flow having to reverse direction with the valve placed before intercooler and once back on boost reverse back directions towards the throttle.
Also from what I recall the BPV is supposed to remain open during off boost/part throttle conditions if that’s the case then I venture to guess that there might be a slight improvement in throttle response if most of the air is bypassing the intercooler and turbo.

All of this is bench racing for me at the moment I can't test any of these thoughts or theories until I get my project car on the road. Just looking for opinions and experiances from others on what path to take.

btw I am planning to use a turboXs type H34, just happen to have it from a previous project.

vrg3 wrote:I don't think that silicone T-piece would fit as is. With the intercooler pushed up against the firewall, there is nowhere near 25 millimeters between the intercooler's outlet and the throttle body's inlet. Maybe you could trim the latter two though, like some people have done to fit new age WRX intercoolers. You'll lose the beads that help keep the hose in place, but 70mm is the right size and there isn't a lot of room for the hose to pop off.

After looking at some pictures I would have to agree from all the pictures I have seen the throttle boby and intercooler outlet are very close together.

I don't understand what you mean about reversing direction. What air is reversing its direction of flow? And why is it different depending on where in the stream the valve is?

original picture taken from to autospeed article: The Zero Cost Recirculating BOV

I modified the picture to help with the discussion (sorry for the quality)

vrg3 wrote:I don't understand what you mean about reversing direction. What air is reversing its direction of flow? And why is it different depending on where in the stream the valve is?

I am referring to BPV being placed before the intercooler. The intercooler in this case the AWIC has certain amount of pressurized air which is flowing toward the throttle plate. When the throttle plate closes the air for the system to equalize and not to stall out the compressor has to travel backwards opposite of the original direction of flow to the BVP and once again when you go back on boost the flow direction changes towards the throttle plate. I have no clue how great of a disturbance this is but never the less is a disturbance in flow. Probably has to do more with pressure waves than flow (but that would be something way beyond my weekend wrenching)

Ah, I kinda see what you're saying. I'm not well-versed in fluid dynamics, but it seems unlikely to me that it's a noticeable difference. The volume of air flowing through the entire system quickly drops anyway; that's what causes compressor surge in the first place. The blowoff valve just forces the pressure differential to drop too. It doesn't need to flow a lot of air to do that.

Where is your pressure source....the compressor or the intercooler/throttle body?

I'd want the valve closest to the outlet of the compressor. That would relieve pressure at the compressor the fastest.

However like Vikash said.....it really shouldn't be an issue though.

If the placement of the valve was so poor in the stock configuration the setup would have never been used from the factory, and the setups wouldn't be lasting the 15+ years and however many thousands of miles on thousands of different engines.