GS guy wrote:I appreciate your thoughts David.
However, the valves by design are metal to metal seal. Any slight leakage could be compensated for by the Megasquirt ECU - in fact, I think some of these valve designs are intended to not 100% seal as they just don't need to. It may be the wrong valve design for this application? Maybe the alternative is to instead fit the simple on-off IAC valve instead of the sophisticated stepper motor controlled valve, for a potentially better "sealed when closed" design?
Jeff, first realize many, many boosted cars are running around with stepper IACs without issue. You are anticipating an issue that simply is not one. Metal-to-metal seals are generally fine - just look at your intake and exhaust valves that don't leak a squeak if in good condition. Also, you hit a big one with "Any slight leakage could be compensated for by the Megasquirt ECU" and for most of what you say, it's true. Yes, an on/off air control will sidestep your concerns (and what you want is a primary consideration), but if the GM IAC is not an issue, then why?
GS guy wrote:In the "downstream inlet" installation, the BOV could effectively cancel out any uncontrolled boost situation, except it is likely the manifold won't see vacuum since it is supplied boost, bypassing the throttle valve at least for some period after the throttle is closed, and can't do it's job and vent the intake system. The TPS is closed, but the MAP still registers manifold pressure and turbo still providing residual boost - what will keep the injectors from supplying fuel to match the MAP? If the TPS shuts off fuel flow, then I see a potentially destructive lean condition..... (edit - I'm talking about when the valve is open during warm-up, not when it is closed after the engine reaches operating temperature.)
As you said, MS compensates as part of normal running. Besides, you have another huge leak from around your throttle blades with boost behind it, and that's not an issue either. The manifold will see sub-atmo (vacuum) condition with your engine decelerating with a closed throttle and activate your BOV normally. I think you are anticipating a hurricane of flow through that little IAC gap. BTW - the TPS does not shut off fuel flow except for cranking in flood clear mode (or some non-standard settings), and that doesn't apply here. MS doesn't care if air is coming through the TB, IAC, or a hole drilled in the manifold, it sees air and adds fuel if tuned to do that as is normal, whether in warmup or otherwise. When tuned 'normally' the addition of a small (or actually any) amount of air will be met with fuel so there is no lean condition. But this is a relatively small amount of air, and as I said, it would only make a barely noticeable difference in decel even if held wide open. Much of what you are thinking about lean-outs could be true with a carburetor - not EFI.
GS guy wrote:As much as I'd like to have the convenience of easy start and warm-up with the IAC valve, it's now seeming too much of a potential liability for a supercharged/turbocharged application! I may have to shelve the valve and plumbing and "experiment" with it at some period well after the car is tuned and running......
And there's your answer. For you. Even if I and others tell you it's not a problem and we drive with them every day on boost, you need to do what makes you comfy. No one will tell you you're an confused person for not doing it or vice-versa. Just do what you feel good with and play with it later if that's your choice. But, just to leave you with one more thought - if the stepper IAC is a concern during warmup, then why isn't an open on/off valve or even your foot holding the throttle open a bit of any concern?
David
Jeff[/quote]
