Posted for "Ed Anderson" <eanderson@carolina.rr.com>:

Tommy, the Mazda manuel indicates (depending on model year) between 32 and
40 psi.  I run my at a higher pressure mainly because that is what I needed
on the orignal injectors (not Mazda).  I have an after market, Bosch
adjustable, manifold referenced ,fuel pressure regulator.  I could set the
pressure lower, but prefer not too.  The regulator is suppose to adjust
itself to manifold pressure/vacuum such that the pressure differential of
the fuel in the fuel line at the injectors and the manifold remains
constant.

Apparently 33 psi is OK with the Mazda injectors, but consider that we are
not driving along at 2000-3000 engine rpm, we are hauling _ss at a constant
5500-6500 rpm for hour after hour, therefore more fuel is demanded of the
injectors, so my rationale is the additional pressure pushes a bit more fuel
through the injectors (although not anything significant).
I just like a bit more margin between my normal operating pressure and the
point that the engine starts to run erratic.

One pump will give me around 38 psi on the ground, while both on will give
me 43 psi.  Interestingly enough, once airborne the single pump will give 40
psi.  I think perhaps the line voltage is stable and a bit higher at cruise
than during run ups on the ground.
Besides, you need to check whether your fuel pressure gauge is accurate.  If
using an electronic pressure sensor try a mechanical pressure gauge.  Fuel
Injection Test kits come with one that if you know a friendly mechanic might
let you borrow to check.

Ed Anderson