Electronic Ignition - Carb ported vacuum or manifold vacuum?
 

I have a 1964 MK2 3.8 MOD 8:1 compression. I am installing a 123 ignition JAG 6-R-V unit. I have read as much as I can find about the subject of maximum advance. I plan to start my engine in the next few days. The engine was run-in briefly (not on a rolling road) with the 123 unit with no vacuum connected following its rebuild. It is currently on the number 2 clickable setting. Now the engine is back in the car. New coil, new plugs, new ignition wiring etc.

My question revolves around the subject of the source for vacuum advance. My engine rebuilder has come to the conclusion that it is manifold vacuum that should be used rather than carb vacuum, although the 123 installation instructions specifically indicate the use of carb porting.

I have done some follow-up research and I found one article that advocates for the use of manifold vacuum to be pretty convincing. I have attached the article for your review.

I am interested in thoughts and comments from the group.

Lin

http://chevellestuff.net/tech/articl...r_manifold.htm

There's a vacuum port on the front carb just before it bolts onto the manifold, that's where the vacuum advance goes for the distributor.
Nothing changes because you're running an electronic ignition as far as vacuum advancement is concerned.


It's in that spot for a reason, so the butterfly valve on the front carb affects the vacuum advancement or retard.


Connecting the vacuum advance diaphragm directly to the manifold no where near the butterfly valve would have very little effect on the ignition timing.


It would almost be advanced all the time with the possible exception under very hard acceleration.

Hi Lin,

I have read the article, and I think you may have the wrong end of this confusing stick.

Jeff is correct it is in that position for a reason.

From the article the "ported" type is above the butterfly, what is not clear, but is my interpretation if you bear with me ... if you view the butterfly from the side with the manifold to it's right, the butterfly rotates clockwise to open and is angled from 1 o'clock to 7 o'clock when shut. If the port is above the butterfly, it is actually behind it also and therefore sees no manifold pressure at idle when the butterfly is closed. (common setup on many carbs)

The other option is that it is below the butterfly and therefore in front and sees full manifold pressure. The position close to the butterfly means that it will change much quicker than if it were simply in the manifold, and the swings are also larger.

This can also change of course, on the SU H type carb the butterfly is as I described above, but on the HD type the butterfly sits at 11 o'clock to 5 o'clock and rotates anticlockwise, therefore the Vacuum port is above the butterfly, but works in the same manner as described above, and also the preferred manifold option as per the article.

I hope this has not confused matters more !

Jon, I though you were going to comment.
I did read the article and I thought it would have been very odd for Jaguar to use the vacuum advance unit to control emissions.
It was 1964 after-all who cared about emissions back then.

Oh dear am I that predictable !!

I don't know when that idea re emissions came in, can't believe it would have been until after the oil crisis at least .

Maybe a little predictable... :icon_wink:

My 77 TR7 had a vacuum retard unit on it to reduce emissions.
I disconnected it and messed around with the distributor, but it was a bit of slug anyway.
It was still running Stromberg's, so one couldn't expect much anyway.

Oh dear I had better keep my head down a little then Jeff

Don't do that, you're like me, always willing to help when you can.

Cheers Jeff
:icon_toast: