155 psi dry and
170 psi wet after a squirt with 50w oil
Were the wide open throttle/ fourth stroke/
average readings at
140,000 miles on my
1995 Magnum 5.9 V8
Was your bad cylinder #7 ?
Even on a low rpm compression test my cylinder 7 read about 10 psi higher than the average.
The Mopar V8 firing order has number 5 cylinder firing just before number 7.
The "Kegger" Magnum intake manifold has the inlets for cylinders 5 and 7 side by side at the rear passenger side corner.
The air flow inside the Magnum manifold's large plenum gets moving toward the inlet for cylinder 5 and then only has to change direction slightly to begin filling cylinder 7 next. This "air monentum" boosting effect on cylinder 7 causes it to get a somewhat better percentage air fill, which for the same fuel injector squirt causes number 7 cylinder's air fuel ratio to run leaner.
The combination of "higher volumetric efficiency" due to air momentum,
and leaner air to fuel ratio
causes cylinder 7 of a Magnum V8
to be more prone to ping or detonate first.
The cylinder will also run hotter.
If by bad luck your driver's side block deck surface was milled at the factory to slope to the rear, the situation is even worse.
If tiny particles get by your fuel filter,
or come off the fuel line's inner liner,
notice that the fuel line feeds the fuel injector rails from the rear first,
which means particles will settle into cylinders 7 and 8's fuel injector "catch screens" first. If an accumulation of such crud restricts the rearmost fuel injectors output,
that will make an even leaner and more dangerous air to fuel ratio.
I have heard from other Magnum V8 owners that they have discovered one rocker arm so worn that the pushrod breaks through, causing one cylinder to have poor compression. The root cause may be a bad hydraulic lifter that is no longer pumping oil up to lube the rocker.
Magnum V8 combustion chambers are prone to carbon deposit build up. Official Chrysler Combustion Chamber Cleaner/Conditioner does a good job of cleaning the carbon out and generates an impressive black smoke cloud.
Intake valve upper surface deposits could cause low compression test readings.
High amounts of carbon deposits inside the chamber and deposits filling up the soapdish shaped cavity in the Magnum piston crown could cause 200 psi readings.
