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November 07, 2002


Yes and no, Alan!
Don't you love answers like that...clear
as mud!
Both Shorrocks and Judson are vane type
blowers but, the Judson's vanes run an
interference fit within the blower's bore
as opposed to the Shorrocks who's vanes
run just clear of the bore.
While the Judson's bearings are self-
contained and do not require oiling, it's
vanes do considering their interference
fit with the blower's bore. Thus, a "total loss" oil system is employed
with it's own
discrete oil supply to provide lubrication
in this respect, which results in oil being
ingested into the combustion process
diluting octane, accordingly.
This "dilution" of octanes was not
too important in consideration of yesterday's readily available 100+RON
leaded octanes. It could be tolerated.
Not so with today's relatively lower octane
unleaded premiums. Thus,
the Judson is bad news in this respect.
The Shorrocks vanes do not ride against
the bore. They are matched to ride just
shy of this. So, no friction (heat produced) in this respect, and no oil
required for lubrication in this respect either. So, less
oil in the combustion process so less
dilution of octane. Score one for the Shorrocks, accordingly.
However, the Shorrocks bearings which
carry the rotor do require oil. Unlike the
Judson they are not self-contained.
This lubrication is supplied by the engine's
normal oil supply. You will have to check
with Chris what effect this has in terms
of oil used in the combustion process. I
believe it is way less than the Judson's
quart every 700 miles or so from the
Judson's own, discrete oil supply tank.
If this is so, than dilution of octane is affected to this extent.
The Shorrock's has a twin belt drive,
which is far more positive than the Judson's single groove belt drive,
which
is important insofar as the larger Series
"A" motors require a smaller blower pulley than was initially provided
in consideration of the fact the Judson application was intended for the
original 948cc motor, in
terms of provding the 6psi boost curve
which results in a 40% increase in torque,
especially in the low and mid rpm range
which is the target range of low pressure
supercharging for the street...which is the
whole point of the application to begin with.
That means to keep the 6psi boost curve
which "produces" the expected result in
line with the financial investment in the
blower, you must "overdrive" the blower,
accordingly. The Judson with it's single
belt set-up tends to "slip" in the smaller
blower pulley. Not so, as I understand
it with the Shorrocks twin-belt drive.
Chalk up yet another one for the Shorrocks design.
Further, the Shorrocks is more user
friendly when overdriven due to the fact
the blades don't ride against the internal
bore of the blower case. The increase
in load and friction (HEAT) of overdriving
the Shorrocks for the larger bore engines
in retaining the "punch" of the wanted
6psi boost curve thus, does not exist.
We know the Shorrocks can be safely
overdriven (up to 7,500rpm+-) for short
periods of time. We know no such thing
about the Judson. Will the Judson's
cloth/phenolic blades take the loads?
Probably would be better off with a
high heat resistant Delrin self-lubricating
vanes. But, those tried
thus far have a melt point of 200F or
so. Not far enough away from the Judson's documented off-boost normal
running temp of 160F+.
So, here again, chalk another one up to
the Shorrocks design, thus far.
Now we get into "heat issues" caused
by insufficient octane in consideration
of today's readily available octanes. Don't
forget, these blowers were intended for
use when leaded 100+RON fuels were
readily available at the corner pump at
reasonable prices. These fuels are still
available....as racing fuels at US $4.50
a gallon and up!! 119RON leaded CAM2
sells up here for $8.20 per gallon. A 60%
"mix" with 93MON unleaded will yield
a little more than 100RON. This is what
you would need to run the stock ignition
advance curve with a static compression
ratio of no more than 8.3-1, assuming a
set of colder spark plugs than stock are
used.
If you can't get the octanes, you must
retard the timing to prevent excessively
hot combustion temperatures. Retarding the timing (most especially in
terms of
rate of advance) is counterproductive
in terms of performance...."torque" in
this case.
And what is low-pressure supercharging
designed to do in street appications?
Increase torque in accordance with the
cam profile of the (normally) stock engine.
So it can be obviously seen that
if you can't get the octanes, rate of
ignition advance might have to be retarded (to prevent pre-ignition
and/or
detonation) to the extent you'd see little
if any increase in torque from stock, even
with the blower producing a 6psi boost
curve. This is especially the case with the
Judson when ambient air temps get beyond about 70F.
Now vane type blowers heat air by
compression. In the case of the Judson
we get further heat of the friction of
vanes making contact with the bore.
Further in the case of the Judson, we
introduce into the combustion process
the oil that is required to lubricate the
vanes as they run against the blower's
bore, even further diluting octanes....
really counterproductive!!
The only thing which will allow the Judson
to be used effectively today on a daily
driver would be water/meth. injection.
Without it, a diet of a racing fuel mixed
with readily available pump octanes is the
only way to go, assuming you wish to keep the stock ignition advance
curve,
shortened perhaps to a reasonable 24-26
degrees.
So you can see that in many respects,
the Shorrocks has advantages over the
Judson in many ways.
Now, let's talk carburettors used with
each application.
The Shorrocks uses the variable venturi
SU which is an excellent choice considering it's well earned reputation
for simplicity and flexibility. It is very
user friendly.
On the Judson (for the Series "A") comes
a Holley single barrel fixed venturi type
with an accelerator pump system which
is a gigantic PITA to get "right". Once it
is right it is fine but getting it there can
be a long, drawn out, frustrating process
involving the correct setting of float bowl
level which is difficult to get at and adjust,
as the floatbowl cover is not of the transparent variety in any case.
Can take
literally hours and hours of adjustments
and "suck it and see" minidrives.
With the SU you simply use different grades of oil in the damper and a
K&N
fuel ratio meter or something like that
and your done. Meanwhile the engine
is at least running without "snatch" and
a "hole" between the idle and main system which can be very hard on
clutch,
gearbox and driveline in general as you
attempt to get the Judson's acc'l. pump
circuit correct.
The Holley is also quite susceptible to
carburetter icing under those types of
atmospheric condtions wherein temps.
fall into the mid-30's with high humidity
to match.... to the extent the car
can become unsafe in traffic as it quits
unexpectedly and refuses to start until
engine heat radiates into the carb. throat.
Then it'll start but immediately begins
to ice up again and the process repeats
itself on a continuous basis. Meanwhile,
it is running richer than hell and the plugs
start fouling accordingly. And gasoline
is washing the bores and diluting your
oil supply while all this is going on.
Truly a bad scene in all respects.
That's why I bought a 1.75 SU to take
the Holley's place. When a water/meth
delivery system is perfected that will
allow the Judson to be the bolt-on system
it was designed to be using today's
93MON unleaded premium, I'll buy
another one and try again.
Until that time, I am going normally
aspirated using the HIF44 in place of
the stock 1.25 set-up.
Cap'n. Bob (Ret'd)
'60 Frog

PilotRob@... 16218

Posted on November 7, 2002 11:02 PM
Posted to category(s) Carburetor | Cooling System | Cylinder Head | Electrical System | Engine (external) | Engine (internal) | Forced Induction | Fuel System | Gauges | Ignition System | Intake System | Interior | Lubrication System | Miscellaneous | Racing | Restoration | Suspension & Steering | Technical Info | Transmission | Tuning

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