[Kingtal0n]

Originally Posted by ADIDASilvias

I get what you are saying. Do you think the longer run of vacuum tube from intake to BOV will result in any BOV lag?

Longer vacuum lines and longer intercooler plumbing will create more lag in setups, slower response times, larger pressure differentials per unit time. Large diameter, short vacuum lines work better (more unit area) than long, thin vacuum lines.

It wasn't necessarily the idea that you need to protect the TB or equipment, but moreso that the pressure wave would be caught early and dealt with prior to the shift event ending and the TB opening again.

I believe you are overlooking the pressure in the hot side as a significant factor. When you lift to shift (or from WOT), the pressure is already high in the hot side (whether you have a spike or not in the cold side) and it needs to be 0psi or less, in the next instant. If the pressure at your "relief valve" drops 1psi, it must then drop the pressure in the nearby plumbing 1psi as well, releasing air molecules until that happens, and then the intercooler, and then the hot pipe, and then finally the compressor outlet will see the 1psi drop after all those air molecules are let go from the other side. The quantity of air molecules being vented in that situation is too great to expect a hole of that size (typical bypass) to vent the entire intercooler plumbing and both intercooler tubes in the time it takes to let go of the throttle. Some stock engines use a cold-side bypass (S15 silvia iirc) but they have a relatively tiny volume of plumbing to deal with (side mount intercooler). A cold side bypass becomes less useful as the plumbing volume prior to it becomes larger. Think of a situation where you install a 10000L intercooler and fill it with boost pressure. Now try venting all of that air through a cold side bypass- It would never drop the pressure on the hot side in time to shift, the compressor would be surging the entire time.

I am unsure of the speed of the pressure wave and whether it reaches back to the BOV next to the turbo quick enough to open the valve, vent the spike, and close before the exhaust is spooling the compressor again.

Any spike in pressure is brief and negligible, as the engine turns and continues to turn, even at idle, air molecules will quickly be removed from the cold side of the plumbing first. It isn't the spike you should worry about, it is the pressure already built in the plumbing from the boosting engine and the sudden stop to flow that accompanies the shutting throttle body. The pressure doesn't need to spike; it's already very high (boost pressure), and needs to vent from the compressor wheel outlet. The reason we vent is to keep the compressor wheel from feeling the high pressure at the outlet while also having a no or low flow situation, which runs the compressor off it's map and into surge. If the compressor cannot freely turn and send air molecules into the outlet, whether there is pressure or not, even in a vacuum, there may be consequences. It will not be beneficial to any setup.

Could this result in overspinning the compressor wheel? By that I mean, if the BOV is open and the turbo is being spooled could the compressor rpm spike without the pressure building in the IC plumbing?

To spin the wheel you need exhaust, and if the user shuts the throttle body the exhaust will suddenly also diminish accordingly. So regardless of whether the compressor is even hooked up to anything at all, it cannot spin without the accompanying exhaust flow. The biggest contributors to overspinning compressors is users who push their turbo past its limitations, and users with boost leaks that don't know it.