Your tech columns have been invaluable
to me in my build. The route I’ve taken
with my 1968 Plymouth Sport Satellite, built
451˝, from day one is improved handling
while running 15˝ wheels/tires. I currently
run MagnumForce 2˝ drop spindles with
AR Engineering caliper adapters, 11.75˝ x
1˝ rotors, Wilwood forged Dynalite calipers
and Wilwood BP10 pads on the front end,
with the stocker 10˝ drums in the rear,
and a Wilwood adjustable proportioning
valve with an 8˝ dual diaphragm vacuum
booster and 15/16˝ bore Raybestos blue
box (midgrade) disc/drum master cylinder.
Pedal feel is pretty good, but I feel like
stopping power could be better. In the MA
June, 2013 issue tech topics Q&A “Drum
Beat” you explain the details of a rear drum
upgrade to finned 11˝ with reduced diameter
wheel cylinders and other improved
internal components. If I were to do nothing
else other than reduce my rear wheel cylinder
size while keeping the 10˝ rear drum
setup, would I still realize some noticeable
improvement in stopping or only if I upsized
to 11˝ with all of the other details outlined in
the article? I’m somewhat resistant to a rear disc upgrade due to bang for buck.
The rationale behind juggling wheel
cylinder bore diameters, vis-à-vis caliper
piston bore, rotor diameter, and drum
diameter is to get what I have taken to
calling “natural proportioning˝ as close
to ideal as possible. Prop valves all have
a cut-in pressure, below this point, typically
300-500 PSI, they have no effect. This
is by design, because, as you increase
pedal (and, therefore, line) pressure, the
car sees more weight transfer (a “harder”
stop), so needs less rear line pressure (as
a percentage of the front). What I’m trying
to dial in (via wheel cylinder swaps) are
those low-pressure stops think rain, etc.
where the car will swap ends if the rears
lock first.
Of course, this perfect F/R proportioning
is a constantly-moving target, this is
why height-sensing prop valves and RWAL
were invented. Still, on a basic build,
there’s no reason whatsoever to not do
the best with what you have. Whether this
would give you the braking improvement
you are seeking is open for conjecture, I’d
need more detailed info to even hazard a
guess. What problems, exactly, are your
experiencing, and under what conditions?
Frankly–and it took me many decades
to come to this realization: the Bendix duoservo
drum setups on all our muscle-era
Mopars are so hard to modulate on even
a moderate stop that RWAL is the only
real answer, other than a rear disc swap.
And, yes, that’s why rear discs are such an
improvement, fade resistance is not really
a factor.