BOV vs Recirculating

[TurboDave]

In racing scenario when you change gear at full boost, your foot is only off the throttle for a split second. At this point compressor surge is minimal, but the pressure of the intake will be increased - so when the throttle is open you have more compressed air mass ready to fill the combustion chambers.
More air and more fuel will give a better combustion, in turn this should give better response.

The expansion of a flexible intake hose means that the energy from the compressed air is being absorbed and stored. As soon as there is a condition where this energy can be forced away then it is. This would be when the throttle is open again.

A solid pipe will not do this, this will force air from back the way it came, back over the compressor turbine. The compressor will never stall, just slow down slightly. My old turbo done 87,000 miles with no re-circulation or atmospheric valve installed. I'm confident that at most boost pressures this will not damage a well prepared turbo.

A flexible "Boost" pipe will also result in more lag as the flexible pipe will expand when coming on boost as I am sure you already know. This is why they use hardpipe kits.

I agree in most applications a BOV is not needed but in some setups there is too much stall on the compressor wheel and a BOV is needed. Mine stalls too badly now without a BOV so I have to run one.

 

[GTti]

Like I said an atmospheric valve that works purely on vacuum are useless. I'm not saying all of them are useless.

If I increase the pressure of the intake from say 16psi to 20psi when the throttle is closed, this will not increase lag, but make it more responsive. I'm not wasting any pressure by creating a leak in the system. I'm not at all convinced with compressor stall rubbish either, a turbo spinning at 100,000RPM will not be significantly disturbed by a few PSI of air pressure!
When higher boost pressures are reached this is when spring based atmospheric valves come into play.

Their are many other downfalls of bling pipes, such as engine bay heat transfer to the air running through it. But this is off-topic.

It won't make a difference to my CB80, the whole intake is <40mm, the standard throttle body is 35mm. This gives good torque and good response, there are alot of calculations that come into play. It was designed to be a responsive engine to make up for it's lack of capacity and cylinder.