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November 09, 2002
Hey, Guys!!
Bree was kidding in her response to my
supercharger posts!! Really!!
But...if I were to do this again, I would
spring extra bucks for the Shorrocks rather than the less expensive
Judson, most especially on the larger bore
engines in view of the fact we know the
Shorrocks has a "yellow" zone beginning
at approx. 7,200 and a "redline" of 7,500.
All you'd probably need then for a 6psi
boost curve on the larger capacity engine
would be a smaller blower pulley. I'd
not be surprised if Jon Peck of BCSC
hasn't had a few of these made up for
his 1275 customers.
For bigger bucks MiniMania offers
it's kit which seems very complete. Also
it's not a vane type blower so you haven't
got this particular "compression heating"
problem to deal with and considering
today's pump octanes, that's gotta be
a priority.
Incidentally, I ran my old stock '67's
1275 with both 4 and 6 psi curves "cause
with the standard pulley (which gives
6psi to the 948, 5 to the 1098) 4psi is
all you get with the Judson.
4 gives you nothing at the bottom and
very little in the middle. It all comes in
at the top and it doesn't do much for you
up there, either. So for the street, all this
boost curve does is make for heating of
the combustion intake charge. This beomes very noticeable on light pedal
acceleration at the torque point of the
cam profile. Guess this is why they want
you to run with the vacuum advance capped off when running a blower!
6psi is a different story alltogether. It goes
like stink, really. This is THE curve you
want. Helluva difference in low and mid-
range torque. And here is where you
begin getting into belt slip as well with
the Judson's single belt set-up. This
is where the Shorrock's more positive
double-belt setup would be more desireable for sure, if you're
overdriving
the set-ups for the larger capacity engines.
But the engine couldn't sustain the use
of this curve for any length of time without
overheating. We were using the stock
radiator set-up which was known to be
in good operating condition and a thermo.
blanking sleeve as well and NGK BPR8ES
plugs. We had sufficient fuel/jetting to the
extent we were exhausting some black
smoke on hard acceleration. Not a lot,
mind you, but you could definitely make
it out were you driving behind.
With the timing backed off a few degrees
from stock and the curve shortened to
24 degrees total advance, the engine
could sustain it a bit longer. Figure most
of the "heat" was from massive compression heating of overdriving the
vane type blower combined with increased friction of loads with respect
to vane against bore caused by higher
centrifugal forces (7,300 blower rpm+-)
to get the 6 psi out of the Judson to the
larger capacity engine. We never exceeded 5,500 engine rpm getting this
performance by the way. Not necessary.
The whole situation was a matter of low
and mid-range torque for the street so
no reason to go beyond 5,500 considering
where the next highest gear would put
you "torque-wise" with the Datsun 5 speed
we were using. Thus the Judson would have been turning over 7,000rpm at
this
engine speed, probably closer to 7,300.
The immediately foregoing took place with
a mix of 93PON unleaded pump "hi-test"
and enough CAM2 racing fuel to give us
more than 100RON in the tank. So the
engine's overheating had to be due to
increased blower "compression heating"
of the intake charge and vane/bore friction
as we drove the Judson beyond it's "normal" rotational design speed.
To more or less prove this, we went
over to 100% CAM2 (purple)112RON and
the situation got a helluvalot better. When
we went to 100% CAM2 (light blue?) 119RON(?), the engine didn't overheat
at all...just went like hell!! For $8.00+ per
gallon it damn well should've, eh? And,
we were able to restore the stock ignition
advance as well. Now it really went at the
low end!! Would the stock half-shafts have
stood up to this over time? I'm pretty
smooth and easy on the clutch and drive-line in my operation but, I
dunno!
So, assuming you don't want to spring for
racing fuel, a water/meth delivery (not
injection) system is pretty much a "must
have" with the Judson and an injection
system for the Shorrocks (I would think) perhaps predicated on boost,
assuming
the octanes of readily available unleaded
pump hi-test is what you want to use while maintaining the stock
ignition advance rate, "capped" at a reasonable
24-26 degree "total". Jon at BCSC already
has distributors made up this way or he
can sell you the proper size bushings to
limit the total advance in your own unit.
Foodtime! I'm starved! :)
Cap'n. Bob (Ret'd)
'60 Frog
PilotRob@... 16264
Posted on November 9, 2002 09:13 AM
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