I just ordered the best popoff (relief) valve that I have found.  I looked
at this before, but could never figure out the pressure rating.  According
to the manufacturers page ( http://www.greddy.com/products/blowoffvalves.htm
)  it's adjustable from .08 kg/mm2 to 2.0 kg/mm2.  Converting that to psi,
gives a minimum setting
of about 114 psi.  Some safety device :-)    I noticed that some of their
other devices are listed with ratings in the kg/cm2 range.  Assuming that's
what they meant, the relief valve should have an adjustment range of 1.1 to
28 psi.  
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--- As a follow-up, I just got the relief valve, and the minimum setting is
probably around 12 psi.  The valve itself is nice, as long as I can get the
pressure rating down.  It's about 4" long total, and about 1.5" in diameter,
and has threads on the end that goes to the intake.  I got the adapter plate
too, so installation should be a snap.  The valve screws apart, and has a
pretty darned heavy spring inside.  I'm sure I can find a suitable
replacement spring to lower the pressure to our desired level.  I'll look
tomorrow, as well as trying to put together enough fittings to hook this
thing up to an air source and gauge for pressure setting.  

Rusty