Could my ABV (air bypass valve) be leaking under pressure?

douglas vincent · Post by **douglas vincent** » Tue Dec 21, 2004 6:42 am

I should be (theoretically) pushing more than 6 psi of boost. Could the ABV I am using (an old Supra one) be leaking under high boost. There is no fluttering (ie, no spiking) but the pressure level does slowly move up with acceleration (where as it should stay pretty constant).

vrg3 · Post by **vrg3** » Tue Dec 21, 2004 6:45 am

Where is the blowoff valve plumbed in?

If it's before the intercooler that could be causing it to open some when it shouldn't.

douglas vincent · Post by **douglas vincent** » Tue Dec 21, 2004 6:57 am

It runs from before to after.

I'm sure you knew this and you might say "Doh!' but...

An air bypass valve allows air to flow both directions. So when not under boost, air is "bypassed" past the supercharger so the air doesnt have to be drawn through a restrictive area. Then when under boost the boost causes the valve to clam up and hold boost. Then when you let off boost, the valve opens up, lets the pressurized air to back track and even pressure out, then the air (because the motor is now under "vacumn" pressure, flows back through the valve into the intake.

vrg3 · Post by **vrg3** » Tue Dec 21, 2004 7:12 am

Right, I understand how the valve works... Realize that it isn't symmetrical; the valve's position is determined by the differential pressure between vacuum nipple and the valve inlet. So my point was that if you have the inlet of the valve attached to the pipe going from the supercharger to the intercooler, then the pressure drop through the intercooler could potentially cause enough pressure differential to open the valve even when the manifold is at positive pressure.

I just realized my question was kind of dumb. I know you're using the AWIC, so there's no good place for you to attach the valve except in the place I just mentioned.

So I guess my answer to your original question is: maybe it is "leaking" under high boost.

You don't need the valve to protect the supercharger during shifts, right? If that's the case, maybe you could try reversing the valve's inlet and outlet. That way it'll act according to gauge pressure in the manifold only.

-K- · Post by **-K-** » Tue Dec 21, 2004 9:13 am

I thought the SC had a idle bypass built in? If so just run without the valve to see what happens.

douglas vincent · Post by **douglas vincent** » Tue Dec 21, 2004 9:19 am

-K- No, those kind of bypass valves are built into the intake of the supercharger usually. The Mercedes SLK kind didtnt have this.

vrg3 If you switch it around it dont run so good. Yeah I suspect it is leaking under the higher boost levels. I was just asking before I test it tommorrow by just sealing it completly off. If it was leaking, I will see the higher boost BANGO!

-K- · Post by **-K-** » Tue Dec 21, 2004 9:56 am

Ahh, then I say Forge.

vrg3 · Post by **vrg3** » Tue Dec 21, 2004 3:17 pm

Oh wait. I don't know what I was thinking so late at night. If you turned it around it'd still open on boost, and potentially earlier.

If you just want a blowoff valve that'll require more differential pressure to open, you should be able to use a 1st-generation DSM one or a new-age WRX one; both have metal constructions so you could crush the cap in a little and increase the preload on the spring.

Do you think there's any way you could fit the blowoff valve plumbing at the intercooler outlet (or throttle inlet) somehow? That would be the "right" way to do it.

douglas vincent · Post by **douglas vincent** » Wed Dec 22, 2004 5:56 am

It was leaking. like .5 psi. not much but it was. However, even though I get the extra "power", it is hard on my SC I suspect, having to back flow all that air.

vrg3 · Post by **vrg3** » Wed Dec 22, 2004 6:12 am

Wait. I take back my retraction of the reverse-the-valve thing... It shouldn't open that way unless there's a good amount of manifold vacuum.

Or is that the way you have it? And when you put it the "right" way you get lots of leaking which makes it not run so good?

Where is the valve getting its vacuum signal?

douglas vincent · Post by **douglas vincent** » Wed Dec 22, 2004 6:24 am

The ABV valve gets it boost signal from a vacumn line on the manifold.

I have reversed the ABV and it flowed both ways without restriction, ie open at vacumn, but also open at boost.

vrg3 · Post by **vrg3** » Wed Dec 22, 2004 6:42 am

Right, but when you reverse it you change the side that it's using as the baseline to compare manifold pressure to.

Which direction made it not run well?

douglas vincent · Post by **douglas vincent** » Wed Dec 22, 2004 7:00 am

Off the top of my memory, it ran worse when it could flow both ways, which of course would be the worst at boost, because some of the pressurised air would be forced back pre-SC which of course would cause poor psi results.

vrg3 · Post by **vrg3** » Wed Dec 22, 2004 7:01 am

I'm confused now.

It flowed both directions all the time when you had it in one particular orientation? Do you remember which orientation it was? Most BOVs have a little arrow on them that show the normal recirculation direction.

douglas vincent · Post by **douglas vincent** » Wed Dec 22, 2004 7:07 am

OK, before I say anymore, I will have to test it so I am not working off memory.

Talk to you on this tommorrow.

vrg3 · Post by **vrg3** » Wed Dec 22, 2004 7:14 am

'Kay.

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Could my ABV (air bypass valve) be leaking under pressure?

Could my ABV (air bypass valve) be leaking under pressure?