BOV pros and cons for fd3s rx7?
01-16-10, 11:41 AM
#27
BOV's aren't illegal in Canada, but apparently are in many parts of the USA, Europe, and Australia. That refers to blow-off valves that vent to atmosphere, while diverters/bypass valves that feed back into the intake, the norm for OEMs, are not. That FFSSSHT! sound does invite extra police attention, just the same, similar to a loud exhaust.
"Stalling" the turbo with the pressure wave building in the intake and rebounding off a closed throttle plate does not mean spinning the turbo backwards - as RPM noted, decelerating the turbo from 200,000 rpm to nothing, or even reversing it, all in a fraction of a second likely destroy it - instantly. That would be an amazing amount of acceleration, which if possible, would make turbo lag a non-issue (if it could be reversed that quickly by the intake over-pressure, the exhaust could easily accelerate it nearly instantly too). The intake over-pressure can force air back past the impeller, as RPM notes, but without stopping, let alone reversing the turbo.
The stalling refers to dramatically slowing the turbo, so it falls off boost and causes lag when getting back on the throttle, which the diverter/bov helps prevent - in the case of bypass/diverters, the pressure is fed back upstream of the turbo inlet, to aid in keeping the turbo spinning even as exhaust pressure drops from closing the throttle. In the case of bypass valves and MAF systems, it also serves a critical (for OEM) purpose of accurately metering fuel, since in a MAF system when the throttle is closed suddenly, the computer will meter too much fuel (for air already past the MAF that doesn't make it to the manifold, causing driveability and emissions problems
Intake pressure could never be sufficient to actually stop the turbo, let alone reverse it - it only lasts for a moment, while the turbo's speed imparts considerable momentum to it, and exhaust is still driving the turbine side, albeit weakly. For intake pressure to stop the turbo or reverse it, it would have to exceed the energy that was input from the turbine side - which would imply a perpetual motion machine. In other words, that violates the third law of thermodynamics. And that's real physics you can bank on. There cannot be more energy stored on the output side of the turbo than was input, so even if no energy was lost to heating the charge, internal friction, and so on, the intake pressurization can't be enough to stop the object which imparted the intake with stored energy in the form of pressurized air.
"Stalling" the turbo with the pressure wave building in the intake and rebounding off a closed throttle plate does not mean spinning the turbo backwards - as RPM noted, decelerating the turbo from 200,000 rpm to nothing, or even reversing it, all in a fraction of a second likely destroy it - instantly. That would be an amazing amount of acceleration, which if possible, would make turbo lag a non-issue (if it could be reversed that quickly by the intake over-pressure, the exhaust could easily accelerate it nearly instantly too). The intake over-pressure can force air back past the impeller, as RPM notes, but without stopping, let alone reversing the turbo.
The stalling refers to dramatically slowing the turbo, so it falls off boost and causes lag when getting back on the throttle, which the diverter/bov helps prevent - in the case of bypass/diverters, the pressure is fed back upstream of the turbo inlet, to aid in keeping the turbo spinning even as exhaust pressure drops from closing the throttle. In the case of bypass valves and MAF systems, it also serves a critical (for OEM) purpose of accurately metering fuel, since in a MAF system when the throttle is closed suddenly, the computer will meter too much fuel (for air already past the MAF that doesn't make it to the manifold, causing driveability and emissions problems
Intake pressure could never be sufficient to actually stop the turbo, let alone reverse it - it only lasts for a moment, while the turbo's speed imparts considerable momentum to it, and exhaust is still driving the turbine side, albeit weakly. For intake pressure to stop the turbo or reverse it, it would have to exceed the energy that was input from the turbine side - which would imply a perpetual motion machine. In other words, that violates the third law of thermodynamics. And that's real physics you can bank on. There cannot be more energy stored on the output side of the turbo than was input, so even if no energy was lost to heating the charge, internal friction, and so on, the intake pressurization can't be enough to stop the object which imparted the intake with stored energy in the form of pressurized air.
01-16-10, 03:04 PM
#28
Just one point of correction, the OP was talking about a PPSSSHT sound rather than a FFSSSHT.
I think the OP should test out the cops in his jurisdiction by opening his window when he is near a cop in traffic and orally emit the ppsssht sound of his choice.
I think the OP should test out the cops in his jurisdiction by opening his window when he is near a cop in traffic and orally emit the ppsssht sound of his choice.
