How do I measure ignition timing?

[bertdejut]

I've got a 911 2.4E (1972) and I tried to measure the timing with an Gunson Supastrobe.
The spec for this car says the timing should be 5 degrees ATDC at around 900 RPM. However, the manual of the timing light says that measuring ATDC is not possible but 'there are other ways around the problem', which I can't find in the manual.

Anyway, the measurements I get with the car are:

- Aligning Z1 with marking on the blower housing: 7.6 degrees on timing light.
- Aligning LE (is that 5 degrees ATDC?) with marking on the blower housing: 12.5 degrees on timing light.

Does this mean that the timing of my car is 7.6 BTDC, rather than 5 ATDC??

If I align LE (5 ATDC? - this LE mark is about 5 mm left of Z1) with the marking on the blower housing and the timing light is on 0.00 degrees, would the mean the ignition timing is set to 5 ATDC?

Thanks!!!

[Early_S_Man]

Bert,

You can check the timing with the vacuum disconnected and plugged -- it should be 5° BTDC at 850 to 950 rpm.

You can also check the entire advance curve from the following data:

[bertdejut]

Great Warren. Thanks a lot!!!

So this means that with the vacuum hose connected it should be 5 ATDC (as per the specs for a 911E 1972 onwards) and without the vacuum hose connected it should be around 5 BTDC.
In order to measure the 5 BTDC I suppose I need to align the Z1 with the mark on the blower house and the reading on the timing light should be 5. Is that right?