Knock detectors & sensitivity

[Barry Tisdale]

Recently, after assembling my Sy engine, after adding 1996 heads & Isky
cam, I was having some knock detector problems.  The ECU was showing
*extreme* knock @ exactly 1975 RPMs, right up to the set max of 19°.  Cut
the spark right down to zero advance, wether accelerating, cruising or
coasting.  A bit hard to drive.  The kicker was, that it would do exactly
the same thing w/ the KS disconnected and a 3.5k resistor in its place
(that's about the internal resistance of the Sy KS).  The true absence of
knock was verified by using the acoustic stethoscope hookup (described
somewhere on the net).

Re-loomed ignition wires, poked and prodded everything wire-wise,
repositioned stuff as far from the dizzy & coil as possible, checked all
grounds (& added some), etc.  Ok, knock w/ the resistor in place was now
gone, still there w/ the KS connected.

I also have a MSD knock detector w/ the KS mounted on an adapter stud
(supplied by MSD - 3/8x16 to 1/4 NPT) in the intake manifold near the
thermostat housing.  The MSD saw none of this bogus knock.  The MSD KS
measured around 80k resistance (as does a standard 1991 GM 4.3L KS), so for
test purposes (and until I got the proper replacement KS), I put a 3.9k
resistor in parallel w/ it, hooked it to the ECU.  1975 RPM knock now gone,
"real" knock heard accurately (as far as I could tell) w/ the stethoscope.

Got the proper replacement KS, installed in place of the kludged MSD, back
in business.  Now hears only the 'real' knock.  I can stop driving around
w/ the tubes in my ears now.

The only change I made at the rear of the engine, near the stock KS
mounting position,  was to modify the distributor mounting.  Had to remove
the mounting hold down boss & install one about 3/8" higher on the dizzy to
allow the dist to engage the oil pump properly; new head mounting raises
the intake manifold by that amount, so had to set the distributor deeper to
compensate.  Must be generating a new noise interpreted as knock at that
certain speed (?).

Just a tale of personal experience, YMMV, naturally.

Barry - Sy#26
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