BOV placement; hotside or coldside...?
 

Gentlemen,

I am trying to plan out my future intercooler build, which I'm hoping will take place sooner, rather than later. In my current setup, the BOV has been welded to the end tank of the intercooler, on the "hotside" or turbo side of the setup. I have no adverse affects from this and have been pleased with it.

There is a debate however that with the BOV positioned so close to the turbo, that you actually lose some responsiveness when changing gears. I have not noticed or felt any such hesitation in my car.

From my perspective, it would make sense to vent the pressure before entering the intercooler, for efficiency purposes. If you are allowing the charge air to exit the system after the intercooler, you're allowing air that's already been passed through the intercooler and cooled to then leave the system. It would seem to me, it would make sense to allow air that has not passed through the intercooler and therefore would be hotter, to exit the system before the intercooler.

It seems like most people agree to put it on the coldside however, and the "benefit" doesn't seem to be tangible. Does anyone have a definitive answer on this? I've seen plenty of opinions or "in theory" type posts, but I'm curious to find some data which can put the issue to bed, 100%.

Your thoughts, guys?

Nick

To be honest I've used them in either position without issue.

The EFR turbos have the BOV mounted on the compressor cover to allow recirculation, which improves turbo response. My SRT4 has the BOV mounted on the compressor cover as well and I have never had an issue. It doesn't seem to matter too much where it's placed, as long as it's capable of venting enough pressure to prevent compressor flutter.

Thanks for the responses guys. I've also seen them in both positions with no obvious issues to someone's setup or build. For some strange reason, my tech's setup is the same as mine, BOV on the hot side. Most seem to go cold...?

Maybe as you pointed out ACR RX-7, what matters most is the volume the BOV can release, to eliminate the possibility of that compressor surge. I would love to have someone step in that had some degree or experience in a race team or something that could just tell us without any shadow of a doubt which is best.

Both will obviously work, but which is better? That's the question I'd love to answer.

Nick

IMO, the better way of doing it for response is to recirculate the BOV. Yes, you will lose some of that sweet "phssss" noise, but I can still hear my SRT very easily and I prefer to keep my entire intake system drawing from filtered air. Recirculation from the hot side is closer, so maybe it would work better. ¯\_(ツ)_/¯

The main reason to recirculate, aside from packaging and emissions, is better response. I believe that is the reason BW did it to the EFR turbos. BW state that they did it for packaging and to avoid compressor surge on their site.

Compressor Bypass Valve (CBV). That is the actual term for a "BOV" and implies that it is recirculated . BOV is a racer colloquialism and marketing term which means non-recirculated CBV.

If you run an AFM that is not pressurized then you technically NEED to run a CBV. If you don't then you'll be venting metered air and have a rich spike (barring a way to account for the vented air). Certain cars were incredibly sensitive to rich spikes (DSM) but it seems most aren't.

As to hot size or cold side CBV placement, for best theoretical performance you want to place it on the hot size. This is because the closer you are to where the air enters the intake system the minimal work that has been done to the air. This is refered to as how "expensive" the air is. The more work being done to it, the more expensive it is considered.

Additionally, a CBV is what helps the system avoid a surge condition (particularly when the CBV is close to the compressor), not whether it is recirculated or not.

Now, to the real question, do any of this matter to the enthusiast? Not really. Whether to CBV or BOV is an important consideration but can be dictated by setup or preference, as to the rest... do what works.

Thank you for the detailed explanation. When recirculating, is it important where you recirculate to? Can you just plumb back into the intercooler piping or do you have to, or is it advantageous to recirculate before the compressor, say in the "intake pipe" after filter, before compressor...?

Nick

You're welcome, and yes, it matters where you plumb the air but more as guidelines than hard and fast rules (unless you're doing some system optimization and even then it can be fuzy).

As with water flowing, air flows from high to low pressure. You can technically vent any high pressure source into any low pressure source, however, the lower the difference in pressure (pressure delta) the lower amount of air you'll be able to vent.

This being said, in the case of cars, no, you cannot really vent air post-compressor to anywhere also after the compressor. There are some few exceptions (compount turbo) but there are other considerations to account for in such cases.

So... you need to vent your compressed air back into the intake tract AFTER the AFM. However, you don't want to recirculate it too close to the AFM OR the compressor. Doing so can cause some strange interactions in the air flow. If you have to pick a place though, it's better to be closer to the compressor than the AFM. How close is too close? IIRC it's ~12". Don't qoute me on that number though as I haven't had to play with turbo systems in quite a while. Either way, you don't want it too close to either as the vented air disturbs the flow and can (potentially) momentarily cause flow reversal through the AFM or cause part of the compressor to stall or lose a lot of efficiency.

Again, this is assuming you have an AFM and the system requires it. If you don't, it's easier to just vent it to the atmosphere barring other considerations (i.e. sleeper).

I'm not into Technicalities But I do have some Reasoning behind my answer.

I would put the BOV after the Intercooler (outlet side towards Throttle body).
My reasoning is that when that BOV is closed you will get "cooled down air" readily available from it already being IN the intercooler.
When you open the BOV it will get rid of the air BUT the intake will still be drawing air from the intercooler to send it to the Throttle body.
This makes cooler air always available instead of waiting for the intercooler to fill up.

I understand your reasoning, but when the BOV opens, isn't it venting the air behind it more than in front of it? I would argue that it's more likely to vent the air from the source rather than the destination. Meaning, the intercooler in your scenario would be somewhat evacuated anyway...?

Nick

I just look at what the factories do with their cars. Subaru mounts their valve pre-ic, my Dodge is in the compressor cover, I think the EVO has theirs on pre-ic as well.

Ive heard that the closer the BOV is to the turbo, the better it is for the turbo, but harder on the BOV.

Fuhnortoner, that is correct.

Brilliant7-LFC, That is correct as the air flow has some velocity. Since the throttle body is more or less "closed" any excess air will be vented from up stream (closer to the compressor) and the down stream air will equilize relative to the total pressure such that some of it will flow backwards to escape through the CBV.

misterstyx69, you bring up an interesting point where both application and situation can make a difference, however, both of those will likely be outliers and not the norm.

Yes, you can potentially heat up a portion of the air with several full boost to vent cases in quick sucession. However, the timing will have to be such that you manage to capture the same portion of air each time. In your scenario you do in fact cool the air twice but you have lower temperatures moving through the IC each time so the total energy still remaining in the air will be greater (i.e. higher avg. temp.). Additionally, with a portion of air moving through the IC at a higher temperature it will be more effectively cooled by the IC and the total energy still remaining in the air will not be as great. Keep in mind that going through turns, throttle body, and IC will cause turbulence which increased pressure drop, lowers system efficiency, and mixes the air. Only the last is favorable and really only matters if your setup is such that temperature differentials between portions of air as expected to differ noticably.

Do keep in mind that a CBV has two primary goals. To mitigate compressor surge and control pressure into the motor. Closing the throttle body largely decreases pressure doing into the motor and largely increase the possibility of compressor surge. Barring a situation where mitigating factors require it, put the CBV close to the compressor.

Thank you for that informative post valley! I'm convinced that placing it on the hot side of the system, closest to the compressor is the way to go.

For someone who may read this five years from now while searching for the answer to this same question, as I was...the final verdict is HOT SIDE and recirculating the pressure probably isn't necessary.

Thanks to everyone who contributed!

Nick