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The performance of various BPVs (and BOVs) have been discussed many times in the past. And there are some good reviews of several different valves on the net - flaws and all.
The whole point is to dump the volume of pressurized air between the turbo compressor and the throttle plate as quickly as possible. I will not go into why we want to do that...
The valve has a certain flow rate, more accurately described as a resistance or lack thereof. The flow resistance will be characterized by the 90 degree Tee in the piping that feeds the valve, the valves own resistance, and the resistance of the valves discharge hose or pipe back to upstream of the turbo compressor.
'Maximun boost' suggests that one should avoid oversize pipes as this delays boost build up. Another implication is that larger volumes of the pressurized system also will effect BPV performance. So larger IC pipes and hoses, larger IC, larger IC end tanks and larger TB feed pipes increase the amount of air that the BPV has to dump. So the amount of compressed air volume in the system is an analog of a capacitor and the BPV is a discharge resistor. Larger amounts of air increase the discharge time constant. So any discussion of the merits of one valve been faster than another should also be in the context of right sizing the 'capacitor'.
Likewise, getting on boost pressure you have a current source, the compressor, the capacitance of the system air volume and a low resistance current loss through the TB. The current source, the compressor, is non-linear, and its current capacity increases non-linearliy with the current drain though the TB. All of that also has a time constant for the charge and that depends on the volume of the system. For a manual transmission where the whole thing is very road speed dependant, this may not be too critical. But when power launches, wheel spin or autobox torque converter slip is involved, the rate of boost gain, 'slew rate', is then strongly de-coupled from the road speed. In that case, the capacity of the system will have an effect.
So one objective of a turbo induction system is low flow resistance and low volumes at the same time. As long as one does not get carried away with oversizing pipes, the result should be decent. Pipes should not be larger than IC fittings or TB bores. Any increase-decrease in diameters will create dynamic flow losses and can only reduce responsiveness.
I have a large IC to install for my 9-5 and events and weather have been working against my getting this driveway install done. It has a huge end tanks and more core volume than the stock unit. The plastic feed pipe for the TB is also undersized, but its molded shape transistions are very smooth, so its dynamic losses (from flow speed changes) should be quite low. The metal TB feed pipes maintain flow cross sections. I am sure that both of these items do increase performance. The increase in the capacitance of the system will likely be more than compensated by the lower resistances.
Capacitance effects will effect responsiveness but do not have any effect once the system is up to boost pressure. So changes to capacitance only would not change power or dyno levels.
The only practical implications of all of this is to not oversize piping diameters, lengths or IC end tanks. One can then see how an air to water IC tucked under the hood, could effect a huge reduction in the system capacitance as the length of the sustem is then massively reduced. So one might then expect an air to water IC to show some increases in responsiveness that exceeds any expected effects of the IC heat transfer efficiency alone. (yes the air to water IC has huge heat sink effect too and it will not heat soak in the same way, but fast response transitions are different and distinct from WOT runs.) The ultimate goal would be to have the turbo compressor discharge directly into a TB feed pipe that was also an effective air to water IC. Now that would be simplicity!
Subtle changes in responsiveness can easily be hidden by boost controller induced boost lag. For a manual trany, a SMBC or MBC+A makes rate of boost dependant on the things discussed above. Many improvements can be 'lost' within the controller induced boost delay. For an automatic transmission, at least for the 9-5 Aero 2002+ with 5 spd auto tranny that I am familiar with, the boost can increase very fast and an SMBC is not really needed as the torque converter slip decoupled the TB flow rates from the road speed. Would a SMBC help on that vehicle?... I don't know yet, I will install one day and see what the wife thinks of that.
And back to BOVs and BPVs. The whole point is to drop the pressure that the tubro compressor sees to protect it from stall (speed loss) and shock waves from the 'water hammer' as a throttle snaps shut. So why then is the valve located downstream of the IC? Why not locate it between the compressor and the IC. Certainly the compressor discharge temperatures might trash a plastic valve in a hurry. But many of the good metallic valves would take that abuse. Has anyone see such a setup? There certainly would be more turbine noise this way, as the IC is an effective 'muffler' for that.
Another way to reduce the amount of air to be discharged by a BOV/BPV would be to move the throttle plate from the TB out towards a FMIC IC. Why not a remote throttle? Implicatations to ponder.
posted by 68.95....
, Thu, 4 Nov 2004 12:43:16
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