mrfred

iTrader: (50)

I've been trying to dial in my TiAl QRJ DV responsiveness over the last few weeks by adding some spring preload. When I got to about 0.15" of spring preload, I found that idle had become totally unstable. It would drift high, suddenly decrease, almost die, recover, and do it all over again. I didn't realize it was the DV and was thinking it might be a weather-induced change in the tune. I eventually ruled that out, and realized it was the DV. Sure enough, removing the preload brought idle back to normal.

To try go get an idea of what was happening, I removed the DV from the IC piping (I plugged the IC piping as well) but left it plugged into the vacuum line, and then started the car. With no preload, the valve is fully open, and with 0.15" of preload its about half shut if the idle is steady. However, if the strength of the vacuum decreases a bit, the valve nearly shuts. Also the valve pulses very strongly with the intake manifold pressure pulses. It seems that either the pulsation of the valve, or partial closure (possibly putting more load on the turbo) is the cause.

Anyhow, something to keep in mind if you have idle stability issues.

alpinaturbo

iTrader: (5)

Cool find...I also noticed variation in idle with DV adjustments...

ExViTermini

iTrader: (25)

Are you still on maf?

mrfred

iTrader: (50)

Yep. As you suggest, it could be a non-issue for SD.

ace33joe

iTrader: (2)

Usually more preload = BOV shut at higher vacuum?

Anything special about TiAl BOV? I am satisfied with my GFB StealthFX BOV so far. No idle issues even with ~50% VTA configuration.

mrfred

iTrader: (50)

Correct, more preload means it shuts at stronger vacuum. Now that I understand the cause of the idle instability, its clear to me that my Cobb DV did the same thing, so its nothing unique to the TiAl QRJ.

ace33joe

iTrader: (2)

I think I didn't read your original posting carefully.

So slightly open BOV is needed for good idle with stock MAF?

I usually adjusted the BOV preload so that it is closed at idle but opens freely at part throttle / release condition to minimize compressor surging, but after reading this, seeing closed at VTA port does not necessarily means it is also closed at the bypass port (which is not seen and it does have some offset in position, which means it opens earlier than the VTA port).

Is this because compressor sees too much back pressure if BOV is completely shut?

Not much air is needed for idling so most of them should be recirculated after the MAF sensor / before compressor?

mrfred

iTrader: (50)

I'm unsure of the cause, but my experience thus far is that the DV needs to be at least halfway open at idle. Fully closed is causing idle stability issues.

ace33joe

iTrader: (2)

Interesting.

I know local JDM EVO VIII guy bought a GFB Response (StealthFX was discontinued) BOV after seeing mine, and he had stall / rpm dipping issue after he got cams (stock to GSC S1) even with softest preload setting. But this symptom was only happening at decel (low speed to stop), not during steady idling at full stop.

So we contacted GFB, and Brett sent us a softer spring, but this was way softer than the original one, so to prevent the BOV opening at idle (which will cause extra un-metered air to enter the engine), we had to go maximum preload + stuffed coins inside the piston to add more force, which solved the problem.

And coins were removed and reduced preload a bit after he got S2 cams (less vacuum).

I always believed there is a thin operating window of BOV - keep shut at idle vacuum, but opening easily after part throttle release, but after reading this, it is still a thin window, but slightly open at idle vacuum, but not enough to open at the VTA port (if VTA port is equipped).

This is why I thought Greddy's FV BOV might work better by providing wider operating window, but it may not?

http://www.greddy.com/featured/46

mrfred

iTrader: (50)

seems like there is something off about your observations of the effect of preload and spring strength. more of either will drive the valve to be shut at idle (the spring counters the vacuum force that pulls the valve open).

my dv is still setup to fully recirculate which kinda makes it counterintuitive that idle stability could be affected by a partly open valve.

vta+maf definitely needs to be shut at idle and all times except when blowoff is needed. i think that greddy bov would be perfect for vta+maf except i think it would have horrible part throttle stutter tendency since it appears to be designed to open very easily under any amount of throttle lift.

the best option for recirc or vta would be a fast acting actuator-based valve that can be controlled with the ecu. no such thing exists, at least not yet.

LetsGetThisDone

iTrader: (1)

Audi/VAG uses an ECU contolled DV valve on the K03 and K04 turbos. Seems to work fine in that application. Obviously its not a lot of airflow, and the valve is small, but it does seem to work..