Flywheels: them big, round, heavy frisbe

[dlawson@ball.com (Dave Lawson)]

I spent an evening last week examining the flywheels from both an
1983 TQC and an 1987 5000 TQ to see if there are any differences.
>From a quick glance they are indentical. Both heavy, didn't weigh
them, with a stepped edge to the surface where the clutch disk contacts
the flywheel.

On the clutch side, there are 3 locating pins for the pressure
plate and then two longer pins close together on this raised step.
Both flywheels had these pins. Looking over my 1989-91 200
Bentley manual, these 2 pins are shown as being magnetic engine
test pins, or some such description. They aren't used for normal
operation, but for some diagnostic test, which I haven't found a
description for yet. They appear to be in the same location on
each flywheel.

Both flywheels had reference mark pins located on the engine side
of the flywheel at what appeared to be the same location. Each
flywheel has 135 teeth, yup I counted 'em. Each tooth therefore
represents 2.6667 degrees of crankshaft rotation. I found a -0- mark
location on the outside perimeter of each flywheel. Counting from
the 0 mark, the 5000 TQ flywheel had the  35.5 teeth to the ref mark
pin and the TQC flywheel had 34.5 teeth to the ref mark pin. So
there is a 1 tooth difference between the flywheels.

I don't think the 0 mark location is really the TDC mark on the flywheel
as this places the  ref mark pin roughly 92-94 degrees from this mark.
As RDH has figured out for a TQC, the ref mark pin signals 60 degrees
before TDC and the computer counts flywheel teeth from the ref mark
pin until the correct amount of ignition timing advance is achieved.

On my application where I am using the TQC flywheel and the
5000/200 ECU and ignition system, the 5000 ECU is expecting to see
the ref mark pin signal in a different location by one tooth. So this
would imply that my resulting timing would be retarded by 2.6 degrees
at every speed. I am thinking of just changing the timing maps in my
ECU by this amount to compensate for this bias. I know people have
said before that this is well within the range for the knock sensor timing
adjustment( ~7 degrees), but if I want to eek out that last bit of timing
advance I probably need to adjust the ECU map. Suggestions? Thoughts?

Another  observation is that the teeth on the flywheel ring gear weren't
lined up exactly the same on the 2 flywheels, so there could be a 1.3
degree error in tooth counting due to ring gear placement during
manufacturing. There might also be a small change in the location of the
ref mark sensor on the block, between the two cars, but not more than the
size of the ref mark sensor head. I need to check this closer.

More than anyone ever wanted to know about turbo flywheels.....
 -
Dave Lawson  dlawson@ball.com
Boulder, CO