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There are fundamental reasons why you don't want to do this.
When a wastegate opens, it spills exhaust flow which limits the mass flow through the turbine, and also limits the pressure drop across the turbine. This reduces back pressure on the pistons.
With the blow-off valve running like this, it is more of a pop-off valve. After the CFM point where wastegate would be working, the turbine works harder and harder, the backpressures climb and temperatures go up and power goes down. The amount of air going through the turbo is more than the engine uses. So the compressor operating point will be often be in a less efficient part of the compressor map. So there will be more air than needed, and it will be hotter too. So the intercooler will be working harder than it needs to, and the charge air will be hotter, again, less power for specific boost pressure.
So in an application where a wastegate would not be opening much at all, a blow-off valve could work ok for that minor fraction of flow. But for other typical sitations the setup will be very inefficient and hot.
Its a case of energy control VS throwing energy away. Remove the throttle plate from your TB and control the power to the wheels by slipping the clutch and applying the brakes!
I have a TurboXS type S blow-off valve, converted to a pneumo-adjustable pop-off valve. This was done to 'eat' MBC pressure overshoots. If I backoff the pop-off pressure less than the MBC set point, the pop-off valve is flowing at peak boost, and I can really hear that through the firewall. I think than the idea setting for this setup would be for some flow at the boost setting so that it opens readily in response to the pressure overshoots. I currently have the pop-off valve adjusted to the point where I don't hear any flow at 16-17 PSI. To setup my system, the pop-off valve pressure point is raised out of the way, then the MBC boost point is adjusted to the desired pressure. Then the pip-off is softened up until its flow limits obtainable boost, then its firmed up until you get the boost set point back and you hear that the pop-off valve is not spilling significantly.
I made up a test manifold for the pop-off valve, where I can supply 15 PSI of shop air from the compressor's adjustable ouput regulator. Then I can adjust the 'control' pressure regulator that applies pressure to the blow-off valve to augment the spring pressure. By adjusting the small control regulator you can watch the blow-off valve piston move in and out, and into various rates of flow spillage. So that was instructive and confirmed the design. This rig is much more sensitive than a spring only unit. With a weaker spring and a pneumatic load on the back of the piston, the spring rate is still only that of the weak spring. So the valve will flow more in response to overpressures. The control regulator is of the type that will dump any pressures greater than its set point. So it applied a constant pressure and the air pressure forces do not go up as the piston moves.
posted by 207.43.195...
, Wed, 23 Oct 2002 14:54:02
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