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After reading everything I could find about the MBC+A concept, I decided to put something together. I already had 0-20psi ball/spring valve (the same one that Darren Dawes uses) lying around, so I thought I'd give it a try. Before putting anything into a car, I hooked up the system to an adjustable-pressure compressed air source to see what was really going on. I had calibrated digital pressure gauges on the inlet and outlet ends so that I could simulate actual turbo pressure conditions and see what the wastegate would see.
I was very disappointed in the Mahon/Dawes style valve to say the least. Mine (maybe there's something wrong with it?) leaks air all of the time, even when screwed all the way in. The way people talk about these valves, I expected to crank the inlet pressure up and see no more than a few psi at the outlet then a sharp pressure rise at the set point. This was not the case. Outlet pressure more or less follows the inlet pressure, with a few psi of drop.
I also happened to have a Norgren diaphragm-style releif valve lying around, and tried replacing the brass ball and spring valve with it. This valve gave me exactly what I expected: about 2psi outlet up to 80% of the set point, then full pressure comes on within a few more psi at the inlet. So what's the deal? Are all those Dawes MBC's not functioning as claimed? Has anyone else bench tested their valve, or is all information out there based on conjecture and in-car experience? Is it just my valve?
Just for the heck of it, I attached my 0-5 psi regulator in parallel with the Norgren valve and dropped it into my '95 Aero. Since I'd already set my pressures on the bench, tuning it in was very quick (just two adjustments). The Aero really screams with an MBC (I'd forgotten what it was like) and even in the 30 degree weather I experienced no overshoots or any significant droop at high RPMs.
The one thing that disturbs me is that my autometer vac/boost gauge (just it 'calibrated' using those digital pressure gauges) was reading sustained boost at 19 psi! Obviously, I didn't run it with the MBC very long as I still need to get a 3" downpipe in to reduce exhaust restrictions and don't want to blow anything up (especially at 19 psi).
posted by 68.226.9...
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