86-951

I thought it took close to 20 In Hg to open the factory CBV. If this is the case & this other valve stays open from full vacuum all the way to atmospheric pressure, it's not the same as the factory arrangement.

J Chen

I thought so too. Though in principle it
may be the same, I think theres more
to it that meets the eyes.

TonyG

The valve opens with far less vacuum that 20"

In fact, it opens closer to 3-4"

My previous red 951 only had 8" of vaccum at idle... and the valve was clearly open.


And with aftermarket valves, some of which are much larger (example: Turbonetics Godzilla), you can use different springs (spring rates) and/or set the valve preload.


TonyG

J Chen

Hi Tony,
Slightly OT question. I'm gonna install
a HKS BOV that's gonna dump air back
into the turbo. Since the spring is
adjustable, how do I set up the spring
tension for proper operation.

Thanks in advance

TonyG

JChen

I always set them up so they are as soft as possible, yet not allowing boost to leak out of the intercooler pipe.

How do you verify this? Good question!

You have to use a manual boost controller.

You simply set to boost to x psi boost, then do some test drives setting it looser and looser until you see your boost drop. Then go back a "notch".

The reason you can't use an electronic boost controller is that the electronic boost controller will keep shutting the wastegate more and more inorder to maintain the preset boost level. A manual boost controller will not.


In theory the actual spring rate necessary for proper operation should only need to be 1lbs because you have equal amounts of pressure applied to each side of the diaphram that operates the valve.

So if you're running 15psi of boost, your intercooler pipe has 15psi of boost pushing against the valve, and the vacuum line also has 15psi of boost pushing against the valve the opposite way, thus leaving the 1lbs spring to keep the valve shut.

That's why you want a very light spring. And because you want the valve to be open as much as possible during low load/under vacuum driving conditions so as to bypass the turbo compressor, the intercooler plumbing, and the intercooler....


TonyG

J Chen

Thanks again Tony for the
sound advice. Sorry Tony,
all the time I thought that
the vacuum is used to open
the BOV when throttle is shut.
Is this a wrong assumption ?

Thanks

TonyG

J Chen

Vacuum is used to open the valve when the throttle is shut. You're correct.

That same vacuum line is a boost line under boost.. which is used to hold the valve shut as well under boost.

The factory valve is designed in a way that it shouldn't need boost to hold the valve shut.. but not all BOV are designed that way.


When using a BOV valve in the true sense of a blow-off scenario, you are discharging to atmoshpere. In that case... the spring is set much tighter, so that it only opens when you lift.. and momentarily at that. You do not want to be sucking in unmetered, unfiltered air... under vacuum conditions while driving.

When using a BOV as a CBV, as the factory does... then you set it much lighter because you want it to open a lot more... so that you're actually bypassing the all the intake restrictions (compressor/intercooler, etc..).

And since in a true CBV setup (like the factory and the one you're doing), you're sucking metered & filtered air in.. so there's no harm to the engine or the a/f metering setup.

TonyG

J Chen

Thank you again Tony.

DanG

A blow off / compressor bypass / diverter valve won't help with the situation in question.

Quick diagram of a BOV from another site...



The situation we're talking about is low rpm, closed throttle transition to full throttle. Basically, you're coasting and then stomp on the throttle.

In this situation, ports A and C are at equal pressure (or better, vaccum), B is at atmospheric (approximations). So the point that the valve STARTS to open is the difference in area between the diaphram and valve minus the spring force. This obviously depends greatly on the spring force. Some BOVs have a very weak spring, others are pretty strong. The DSM BOV that most of the turbo dodge guys use will only stay open as long as the throttle is completely closed, as soon as its cracked, the BOV slams shut due to the strong spring (will keep the valve closed to 23 psi positive pressure).


Regardless, the point of this entire thread is that as soon as you stomp on the gas, ALL BOVs will slam closed because the spring force will overcome any difference in pressure.

10 in Hg vaccum equals -5 psi

Lets look at 3 different situations and whats happening at the BOV-

Situation 1: You lift throttle after building boost...
A = -5 (vaccum)
B = 0 (atmospheric)
C = 10 (boost)

So the valve flies open and vents the 10 psi of boost

Situation 2: Coasting about 1200rpm, no throttle.
A = -5
B= 0
C = 0

So its spring force fighting against the 5psi differential on the diaphram. Maybe the BOV is open, maybe its not. Maybe its venting air out, maybe its sucking in. Basically, it might be open, but its not like there is a ton of flow.

Situation 3: Same as #2, except you just floored it (the situation I've been describing as the reason for the new device)
A = -8
B = 0
C = -8

Its possible the valve can be open (depending on spring force again) but it will close well before you switch from vaccum to boost.


With the check valve I'm talking about, A=B=C in situation 3, so you'll never even see vaccum when you floor it (ideally, restrictions in the plumbing will probably keep it below boost for just a few hundred rpm).

Check out this graph. The data is all just estimates, but helps illustrate what the valve can do...

Edman951

Hi J Chen,
Good to see you are comming along good with you gt25 turbo upgrade.
Can wait to see how she goes. What kind of problem are you getting with the welding?

Me the car runs great. I'm going on the dyno in about 10 days. I'll be on vacation so plenty of time to go to the dyno.

As for the check valve. When i did my test i did not use a check valve.
I just checked if their was vacum or pressure in my pipe at idle.
And my car make pressure.
And when i say pressure i"m not saying boost. It just that the 3/4'' open tube shoots out pressure insteed of pulling vacum.
So no need to by pass the turbo for me.

J Chen

Hi Edman,
Nice to hear from you. To start off, my GT25 has a mega size TO4S housing.
Due to it's size, I had to cut the engine
mount down much further than your set
up. I also needed to change to a smaller
alternator. As the flanges are totally
different, I had to cut up part of the crossover
& down pipe. Heres the catch, I can't find
any mandrel bends here with suitable
dimension. So I had to scout for exhaust
shops to build the part for me. But I have
been quoted ridiculous prices just for
making & welding 2 short pipes. In anycase
by hooks or crooks, my engine should be
back in the car by next weekend.
As for the check valve posted by Dan, I'm
convince that it will work, how well I'm
notsure. Will have to try it out to see.
Will keep you posted Edman.

Thanks again

gregeast

Did anyone ever have a chance to give this a try?

DSROCKS690

Well, you see, most CBVs and BOVs do the same thing but in the opposite direction. They essentially either vent pressure upon throttle closure to atmosphere, or in the case of a CBV, redirect that pressure back to the compressor inlet to even out the pressure on both sides while not confusing the MAF MAP and O2 sensors. Basically when pre throttle body pressures exceed a certain limit, they get directed somewhere else instead of putting wear on the throttle and compressor impellers from chopping air.

This valve essentially does the same thing, but in the opposite direction. Instead of letting pressure out of the system when overshooting target boost or setting throttle position to 0, you're letting air into the system when there is a lack of pressure before the throttle body, allowing the turbo to spool up and the engine to put more power down in the low end. Essentially this increases low end torque and time to boost, with the caviat of more parts under the hood. The nice thing is that this mod doesnt require a tune (although I definitely recommend one), it works on any turbo car, and its very cheap. Its also very effective on large turbo cars as you get more exhaust gas because you have more air flowing in which makes the turbo hit target boost much quicker while also allowing for the engine to put more power down earlier.

Keep in mind this is the case for most factory turbo cars. I'm not a Porsche buff so this may not apply to your car, however I daily a few turbo Volvos and they are known for having some of the most clever turbo designs in factory cars, and they were one of the early adopters of the turbo. Heck they even offer engines with both turbochargers and superchargers in line with each other.

Anyways, there are a few caviats to this ALS, as there are for any other solution. This form of ALS keeps the turbine spinning just enough for the compressor to be at or just below atmospheric pressure just about all the time, which is terrible for cold start conditions because the turbine will be spinning considerably faster than it normally does before the oil in the engine is near operating temperature. Regardless of if you go for a metal flap or a rubber diaphragm (which is what I would go for to mitigate the risk of mechanical wear while also allowing ALS maintainance to be easier), be sure to consider some sort of filter post bypass before the throttle body to prevent the ALS valve from throwing foreign objects into the engine in the case of a catastrophic failure. This will affect the effectiveness of the system slightly, but it will make it safer. It doesn't have to be something extreme, just something like a doorframe wire mesh, or anything similar enough to be able to capture any debris that may get thrown into the engine in the event of a failure.

If you really want to implement this system in a road car, I would do this:

1) Wire a switch into the center console with a relay and inline that controls a butterfly valve. This valve will be placed between turbo inlet side of the bypass valve assembly so that you can block air coming into the system from bypassing the turbo. This is essentially an ALS switch so that you can prevent premature turbo failure.

2) Take a pvc or aluminium pipe, somewhere around 2/3 the diameter of your inlet pipe, and put your diaphragm in the center. Draw an arrow on this pipe to mark the air flow direction, and test the makeshift check valve to see if it works.

3) Take wire mesh over the outlet and wrap it around the sides of the pipe, secure it with a hose clamp or by some other method.

4) Attach your butterfly valve to the inlet of the valve and test if the switch works. If it does, you're ready to T into your turbo inlet and intercooler charge pipe.

*If you can, try to T this valve in after the intercooler as close to the throttle body as you can, itll make the antilag work much better. If you can't, just make sure that it is connected after the turbo to one of the intercooler pipes, you'll still see some improvement. Once you do this, you'll have a fairly robust, toggleable and universal ALS.*

*TLDR: This mod and factory CBVs do not do the same thing. In fact, they actually do the opposite. It is still potentially harmful, mind you, especially on cold starts. You see, the turbocharger will be outputting somewhere near atmospheric pressure whenever you're idling or cruising, the turbo will be spinning faster on cold starts causing more wear before the engine heats up, which is why you never see this tech on factory road cars, especially now that hybridized turbos exist.*

Mike Goebel

This probably wont make much of a difference on the 944 engine. What it does for the most part is bypass the flow losses of the intercooler and the compressor for the split second before the compressor provides boost.

Thanks
Mike G.

RoyaleWithCheese

I could see this helping on a big turbo drag car or something with a fair bit of lag. I agree that on most average street driven turbo engines the difference would be negligible.

That said, they do make reed valves for turbocharged 2 strokes. I would think that would be a safer valve option than a plumbing part from the hardware store if someone was determined to give it a go.