993,951,944

I made a discovery with the Varioram intake manifold on my '97 Carrera 4S. While probing
various electronic systems with an oscilloscope, I measured high voltage spikes as high as 237 volts and digitally captured spikes of 186 volts
between the aluminum intake manifold and engine
block/chassis ground. See attached Oscilloscope
photo link displaying 186 Volts:


<a href="http://boards.rennlist.com/upload/highvoltageonmanifold.JPG" target="_blank">http://boards.rennlist.com/upload/highvoltageonmanifold.JPG</a>


I demonstrated this to a few Porsche mechanics.
They were surprised and concerned about the
measured high voltage spikes. Although, one did
not believe the Porsche engineers would consider
it to be anything consequential. The others
believe the engineers never actually measured it
with an oscilloscope. They confirmed the
manifold is electronically isolated, having all circuit grounds returned through individual
ground connections.


The high voltage spikes are caused by normal
coupling of the spark plug wires and spark coils,
being transmitted to the electronically isolated
aluminum intake manifold. This matter may be
minor and insignificant, perhaps another red herring of concern, although high voltage spikes,
conductive carbon and close proximity wiring in
combination could add up to something to be a
little bit concerned about over time.


The high voltage spikes are being discharged
inside the intake boots, due to normal internal
conductive carbon, as evidenced on the attached
oscilloscope display. This discharge forms a
carbon arc conduction trail slowly attracting
and fusing carbon and electronically etching the
intake boot inner wall. Theoretically, if the
right combination of non-linear carbon and
dielectric conditions occur in low humidity,
elevated high voltages could build up reaching
several thousand volts. This elevated high
voltage could discharge resulting in misfires if
combustible fuel is in proximity, as well as a
discharge to a close proximity sensor or wire.
Furthermore, the high voltage spikes, theoretically, could cause interference with
operation of close proximity sensors and wiring
and even possibly trigger a false "Check Engine"
light. If close proximity wiring contacts the
manifold, the high voltage spikes would likely
damage the DME or other systems. There is a
very simple solution to this potential problem;
connect the manifold to the engine block using
one of two methods:


1. Connect the manifold to engine block or chassis by means of a simple direct wire.


2. Connect the manifold to engine block or chassis by means of a D.C blocking capacitor
and discharge resistor. See attached diagram:


<a href="http://boards.rennlist.com/upload/capacitorresistordischarge.JPG" target="_blank">http://boards.rennlist.com/upload/capacitorresistordischarge.JPG</a>


The purpose of using the capacitor is to filter
the H.V spikes and to block any D.C battery potential that would occur if a sensor or other
wiring insulation is worn or burned through and
comes in contact with the manifold. The resistor limits current and effectively discharges any
stored charge. I used a 0.22uF,200Volt capacitor
and a 1K ohm,1/2 watt resistor. This resulted in
reducing the voltage spikes down to very small
and safe 0.1 volt range. The capacitor and resistor values are not critical, as a 0.047uF
though 2uF /200volt capacitor, and 470 through
2K Ohm 1/2 W resistor will the job as well.
These parts are cheap, 10 cents to $2, and are
common at Radio Shack or other component distributor. You can connect the manifold
directly to the engine block using just a wire,
without the two components. Although, I recommend
using the components for the D.C protection
in the rare event of wiring harness insulation failure.


A common handyman's handheld VOM meter will not
measure this high voltage due to the short
intervals of the H.V spikes -as these meters are
designed to measure Ohms, D.C and simple
continuous sine-wave A.C. You can measure
the resistance/continuity of the manifold to
chassis. If you measure approximately 1 ohm or
less, ( good test leads required) your manifold
is already connected to the block by design.
An isolated manifold will measure as open or
near infinite Ohms. Some slight high resistance may be measured due to normal conductive carbon
inside the intake boots. I have measured from 59K
Ohms (cold) to 85K Ohms(hot) on my C4S from
manifold to chassis ground for openers.
I measured similar results testing another C4S
manifold, slightly different Ohm values 51K(cold)
and 78K(hot) and high voltage spikes of 167
volts. (Do not measure an isolated manifold Ohm
values with the engine running as this could
destroy your hand held meter due to the high
voltage spikes.) Ohm values will vary due to the
model of meter used, type of gas used, driving
habits, type of carbon, and temperature.
The Ohm values will measure somewhat unstable
and constantly changing regardless of engine
temperature -this is typical of electronically
unstable carbon.

<a href="http://boards.rennlist.com/upload/highvoltageonmanifold.JPG" target="_blank">http://boards.rennlist.com/upload/highvoltageonmanifold.JPG</a>

Ruf-Racer

David,

Very interesting! Are you able to capture & post pix showing a sequence consisting of several events? Is the engine block/frame ground the reference!

R

993,951,944

"Very interesting! Are you able to capture & post pix showing a sequence consisting of several events? Is the engine block/frame ground the reference!"


Yes. The engine block/frame is the ground reference. The aluminum intake manifold is
electronically isolated from the engine/block
frame. They used the rubber intake boots to
reduce heat from coupling to the manifold,
which improves performance and stability.
However, this leaves the manifold suspended
and receptive to the high voltage spikes
generated from the spark plug wires and
spark coils. A high voltage spike will occur
with every spark plug ignition. Which means
the H.V spikes occurs 6 X RPM. At idle this
equals 4,800 H.V spikes per minute and at 6,500
RPM, it equals 39,000 H.V spikes per minute.
(assuming the twin plugs fire in exact sequence)
If you look at the digital oscilloscope display
you can see a break down point resulting from
unstable inner boot carbon. The High voltage spikes fluctuate in voltage ranging about 130
volts to 237 volts. This is due to the spark
plug wire lay out proximity and the unstable
carbon.

993,951,944

I received a few emails inquiring about where
to attach the intake manifold connection to engine
block - to eliminate the high voltage spikes.
The attached photo should help.

<a href="http://boards.rennlist.com/upload/manifoldground.JPG" target="_blank">http://boards.rennlist.com/upload/manifoldground.JPG</a>


Note the capacitor and resistor are small enough
to fit within a two inch length of 1/2 inch heat
shrink tubing and are hardly noticeable.
Other engine block or chassis ground points will work as well.


All the best,