To recirculate or not - pressure vs. temperature
 

Hey guys -

One thing I've been wondering lately is should air from the blow-off valve be dumped to atmosphere or recirculated into the intake?

First off, this is taking out any discussion of noise or packaging.

This could apply to either twin or single turbo cars.

Stock, the BOV and CRV are dumped back into the airbox, most likely for noise and emissions reasons. Many people vent them to the air when the stock airbox is removed, mainly for ease of plumbing and packaging.

So, the BOV is releasing a certain amount of pressurized air. Seems like it would make sense to re-route that pressurized air into the turbo intake to help spool and give a "ram air" effect. (FYI, non-turbo ram air cars make less than 1psi in the airbox at 100mph!)

That said, the air has already been compressed, which means it has been heated up. So, you're adding to the intake air temps with hot air that needs to be compressed and heated yet again.

So, is pressurizing the intake with hot air worth it, or are you just unnecessarily adding more heat to the intake charge?

Dale

I think that the recirculating has more benefit adding more air to the intake because the intake temps are already hot and adding x amount of degree per psi isn't going to break the bank once both air temps are mixed. The engine bay is already hot, pipes are hot, turbos/engine are hot.

I think the engine / turbo is going to suck in however much air it is going to suck in, regardless of if the BOV dumps air into the intake or not.

Dump all the air you want into the intake, but the throttle body is the only thing that is going to allow any of that air to go anywhere or be of any use.

I can understand the potential benefit to MAF metered engines.
Just my opinion

I think you are better off getting rid of the long hose that the stock BOV sits at the end of. It seems like an awful lot of wasted space that is between the turbos and the engine needing to be filled with compressed air. The closer you can get the BOV to your piping the better.

I agree about having the bov hose short as possible to reduce lag. Also isn't a bov supposed to be as close to the throttle body as possible to prevent back pressure on turbos more efficiently. My feeling is that recirculating even the hot air would yield a performance gain than venting.
But I have no data to back that up. I'm sure someone has tested this.

What about the best of all scenarios you put a bov on greddy elbow and recirculate it back in. Then air has already been cooled by intercooler and isn't so hot.

Things happen pretty fast in this situation. I don't see how you could do a good test without fast sampling, sensitive lab grade equipment.

As Snook suggested, placing the bov on the elbow should yield cooler air into the turbo inlet. What if you run water injection and place it before the bov so that the water cooled air is recirculated into the intake? I would imagine it would yield better results.

I seem to remember asking Corky Bell (actually met him a number of years back) on BOV placement and he said that it really came down to packaging, there was no significant difference in performance if it was close to the turbo or the throttle body.

Thinking about it further, it's possible that the pressurized air in the intake from the BOV will not be pressurized for long. You have a nice big air filter open to atmosphere, that will kill the pressure off extremely quickly.

Dale

what is the volume of air that comes out of the BOV, as compared to the engine?

i think you'll find its a pretty small volume, so changing the plumbing won't do a lot.

you are right though, the OEM's have valves because the turbo is loud without them, and then they recirculate due to emissions.

Yes was going to mention that. I don't think there is any significant pressure increase since the intake filter is path of least resistance. If you were to recirculate at throttle body it's cooler and I could see a slight increase. another point... It must be a decent volume of air that is released. On a lot of cars like the supra when they vent the BOV the car will stall when it releases as it's measured air by the maf that it's expecting and not receiving. Seems significant if it's making a car stall. It also depends on how much boost you're running and on what sized turbo. But it stalls even the stock tt supras and other cars.

There is no performance advantage to recirculating a BOV. Air exiting the BOV will almost instantly expand to ambient air pressure, not remain pre-compressed when pulled back into the turbo. At least the expansion will cool it, but probably not enough to even get it back down to ambient temperature.


No, it's not a significant amount of air. Have you ever had a MAF equipped car with a tiny intake leak? It runs like crap. Metering is pretty precise and it doesn't take much to throw it all off.

Recently read an article on the web about this. The only reason to recirculate is noise control.


END OF DISCUSSION!

Someone never owned a DSM. CBV vs. BOV matters in the case of where/how you meter the air going into the engine. Like mentioned with MAFs, if you use a BOV after the MAF(s) then your engine will run like shit whereas a CBV after the MAF(s) (and recirculating after the MAF(s) too) will not.

I don't see water injection having a lot of effect. Methanol would probably be more effective, but would still need enough time in contact with the air.

When BOV is open, throttle is closed and manifold is in vacuum.
There's no performance to be gained in that scenario.
Recirculation vs. Venting is all about noise. Do you want it, or not?

Let's not get too sidetracked with MAF-equipped cars, the FD doesn't have one so it doesn't matter.

Chuck, any possibility you can find that article you are referencing?

Trying to think of a way to test this that doesn't require expensive equipment :).

Dale

I did not find the one I read but a simple search finds many such articles.
Only needed for MAF sensor car to prevent the AFRs from going rich but this sounds goofy from the way they describe it. Their description requires a slow computer that is dumb.

If the blow off valve were to increase pressure, it would be momentary, not consistent pressure. I dont think you could tune for that.

I don't personally think a BOV venting back into the system is going to increase air intake volume or pressure at all.

You need to look at the energy in the situation. The turbocharger is doing work. It is bringing in and compressing air. When the throttle plates shut, the BOV is venting that compressed air (simplistically).
Injecting the vented air back into the system with a plumbed back in BOV is just making the very small amount of air leave the system and then run through the BOV piping and come back into the system. In terms of "adding" anything, its not going to. Its self cancelling.

A very very dumbed down way of looking at it is deflating a tire, and taking a small amount of air coming out and plumbing it back into the tire. You gain nothing.

The best gains to be had from a BOV on an FD (if you can call them "gains") are

1. Keep the BOV hose short. Keep the volume of the intake piping that must be filled with boost small. You may gain some minute throttle response improvements or lowered lag by 0.5 or 1%. or more if you have a massive bling bling front mount intercooler with the volume of a swimming pool.

2. Increase the stiffness of the BOV piston spring to the point that the BOV only opens under wide open throttle/full boost gear changes. Even to the point of hearing compressor surge/flutter noises at part throttle/low/medium boost. This preserves boost in the system and GFB (who manufacture and design BOVs) have done testing showing that 1-2psi of boost can be retained in the system with a stiff BOV spring setting.
Conversely, having a very light/loose BOV spring setting means the BOV is venting and whooshing away your boost under all conditions including part throttle, which may sound cool but its not your friend.

Using the tight BOV spring method as described by the GFB engineers, I haven't heard my atmosphere venting BOV once in about 4 months of part throttle driving. But the boost is very slightly more eager.