Ignition timing question
 

I have a couple of questions regarding my distributor set-up. I'm using my '101 Projects' book to check the timing on my 76 2.7. I couldn't find the answers by doing a forum search, so here are my questions:

When I pop the distributor cap, in place of the points is a black box with '22T-V4' on it. Is this a PerTronix unit? I went on their website and couldn't find anything with that number.
http://forums.pelicanparts.com/uploa...1313948552.jpg


To adjust the timing, do I just need to turn the distributor (and not concern myself w/ the setting the dwell angle)?

The timing was set to 0 TDC - the sticker on the fan shroud says it should be set to 5 degrees before TDC. Does the timing position stay the same if the car is equipped w/ points or a PerTronix unit? When I adjust to 5 BTDC it does seem to idle a bit better.

That looks like a Pertronix in there to replace the original points. It is a good upgrade - no need to set the points or dwell anymore.

You just set the timing as normal by turning the distributor. Your specs for 1976 should be 5 deg ATDC (after TDC) with the vacuum line connected at idle (900 RPM). Then check for timing at 6,000 RPM - it should be 36 to 38 deg BTDC with the vacuum line disconnected and plugged.

The idle timing will set your "initial" timing, and the higher RPM shows your "total" timing which includes both the initial and the centrifugal advance (the weights inside your distributor). If both are OK, then your distributor is working correctly. If the high RPM doesn't advance as much as it should, your distributor may need cleaning and lube.

Yes, you are correct! My engine decal says ATDC.

So, should I time to the first notch in the pulley or the second w/ the timing light?

Here are the marks on the pulley. Now that I look at the photo, I've got it timed to Z1 -- the mark to the right.

Is the mark to the left the 5 deg ATDC mark? (the marks on my car are flipped from the photo in the 101 book -- the book has Z1 as the mark on the left, but the book also references the timing mark to be 5 deg BTDC)
http://forums.pelicanparts.com/uploa...1313954935.jpg

Yes, the one to the left in the picture is 5 deg ATDC.

http://forums.pelicanparts.com/uploa...1313956650.jpg

On your crank pulley, about 1mm is equal to 1 deg of timing. So if you measure the distance between 2 marks, you can approximate the degrees. The distance between Z1 and the mark to the left should be about 5mm or 5 deg.

I see what you mean. The picture from the 101 projects book is labeled 911SC with a 3.0 liter engine. Here is an example of the timing for a 3.0 SC which is different from yours:

http://forums.pelicanparts.com/uploa...1313957490.jpg

Nice illustration....but a question. Is the total advance on the 3.0 that much less (25 degrees BTDC versus 32/38 BTDC for the 2.7) because the 3.0 is checked at 4000 rpm and there is another 7+ degrees of centrifugal advance+vacuum advance between 4000 and the 6000 rpm measurement point for the 2.7? Or does the 3.0 need less total advance?

D.

Excellent! Thanks for the quick responses, and for the diagrams.

My timing was correct in the first place, but hey, I learned alot today :)

The 3.0 needs less total advance. I also believe that the timing is all in at 4,000 RPM and there really is no reason to go above 4,000 RPM. If you watch the timing change while reving the engine, you will see it advance until it stops at some RPM - probably around 4,000 RPM.

Maybe someone can comment on why the 3.0 has less total timing - I can't remember if it is because of higher compression or more emissions restrictions.

Great illustrations!
To raise a new question, if I am not using a standard timing light but one with a dial to check the vac. advance.
To I set the dial on the light to the desired timing, and then turn the distributor until the Z1 mark lines up?
How then do I differentiate between BTDC and ATDC with this method? The dial on the light is 0-60, is this only ATDC?

Not clear what this "dial" thing is, so pictures are a must.

The dial on the back of the timing light.

No dial on my timing light. What's it for?

It allows you to check ignition advance while aligning the tdc to the reference mark and not having additional markings.

As I understand it, wiith your type of light, set it on zero on the dial. Idle the engine and rotate the distributor until the 5 degrees BTDC mark is at the fixed mark on the engine. Then increase the revs to the full advanced setting and rotate the dial on the gun until you see the 5 degrees line back at the engine mark again. Read the number in the window on the gun. This is how many degrees the electronics had to "delay" the spark to cancel out the mechanical advance in the distributor.

Note that the reading is the degrees between the two settings, not the absolute settings. It is the amount of advance. In a case where idle was at 5 ATDC and full advance was at 30 BTDC, the gun should show 35.