Plugged timing advance hose...

jstein · 10-21-2004, 12:56 PM

It looks as if the PO cut the vacuum hose to the advance on the distributor, and 'plugged' it with a grease fitting. Was this done during/because the CIS was removed for Webers?

More questions:
1) Is this an accepted practice?
2) I hate having a zerks fitting someplace it doesn't belong, I shudder to think what would happen if someone decided to 'grease' the distributor - is there a better/neater way to handle this?
3) I assume this means I'm running mechanical advance? Any way to check that?

RoninLB · 10-21-2004, 07:39 PM

I don't know much about an SC CIS.. I disconnected the vac when I had CIS to eliminate the distributor retard..

jstein · 10-22-2004, 07:54 AM

bumpage

911pcars · 10-22-2004, 10:49 AM

Ron is correct. SC distributors use a vacuum diaphragm to provide timing retard at idle (for smog purposes). Once the engine is off-idle, the vacuum decreases and the mechanical advance takes over from that point. Plugging the vacuum hose has the same effect as disconnecting it from the engine.

Sherwood

Jeff Higgins · 10-22-2004, 11:23 AM

1 & 2: Yes, this is accepted practice. Just not with the zerk fitting. Most auto parts stores have a variety of rubber caps designed to fit over unused vacuum ports on carburators and the like. It should be easy to find one to fit.
3. Just hook up a timing light and rev the motor. As a matter of fact, I believe you need to set total timing at 6,000 rpm and let the idle advance/retard land where it may. That's the case on my '72, anyway.

I've been running my car with the vacuum retard plugged for some time. With the timing set at about 37 degrees total at 6,000 rpm, it idles at about zero advance rather than 5 degrees retarded. Not a big difference, but it does seem to idle better.

911pcars · 10-22-2004, 11:37 AM

It might idle even better with slightly more advance and have more low-end response with more advance, but then you'd have to limit the total at the top.

Should try to mimic the advance curve of a 2.7 RS or early S distributor or even better, run on a chassis dyno and have them perform a spark test to determine the best advance curve for YOUR particular engine.

Sherwood

Jeff Higgins · 10-22-2004, 12:56 PM

I wonder how you would limit the total at the top. Some kind of mechanical stop internal to the distributor? As a side note, I guess I have never seen an advance curve plotted for these. How fast does the advance come on with these? In my old drag racing days (with big block MOPARS), we always reworked our distributors to hit full advance at about 1500 rpm. Of course they were never asked to pull much below about 5,000 rpm, so we never worried about low rpm detonation. I really never ask my 911 to do much below about 4,000 rpm, so is it at full advance by then? Is there any advantage (or danger) in playing with advance curves on 911's?

RoninLB · 10-22-2004, 05:24 PM

Quote:

Originally posted by Jeff Higgins

Is there any advantage (or danger) in playing with advance curves on 911's?

fwiw, on my old CIS the elimination of the retard 'caused the lower rpm to be cooler and less spastic, ie: smoother. I pushed the initial to a 5deg advance on 8.5 compression for home operation.. but in 110deg desert heat along with slightly warmer oil caused some lower rpm pinging under quick idle to 3k rpm. Over 3k was no prob. The SC has a different retard curve.

911pcars · 10-23-2004, 01:33 AM

"I wonder how you would limit the total at the top. Some kind of mechanical stop internal to the distributor? As a side note, I guess I have never seen an advance curve plotted for these. How fast does the advance come on with these? In my old drag racing days (with big block MOPARS), we always reworked our distributors to hit full advance at about 1500 rpm. Of course they were never asked to pull much below about 5,000 rpm, so we never worried about low rpm detonation. I really never ask my 911 to do much below about 4,000 rpm, so is it at full advance by then? Is there any advantage (or danger) in playing with advance curves on 911's?"

There is a mechanical stop in the distributor that limits how far the weights travel and/or how far the rotor shaft rotates relative to the contact points or optical breaker.

Here's a thread from the Early 911S Registry:
http://d240157.u39.zeonhost.com/forum/showthread.php?t=4497&highlight=advance+curve

The factory repair manuals also provide advance curve charts. Generally speaking, most advance in by about 2000 rpm (engine) followed by a slow ramp to max. advance by 5000 or so (or thereabouts).

The advantage in tailoring the advance curve is to maximize the power/torque for the variables of your engine and operating conditions (compression ratio, combustion chamber shape, altitude, volumetric efficiency, A/F mixture, fuel octane, etc.). And as you mentioned, to avoid detonation caused by too much ignition advance at any engine speed.

Hope this helps,
Sherwood

MotoSook · 10-23-2004, 04:26 AM

Josh, I don't run with vacuum advance on my 78 3.0..just leave it plugged and check the timing.

jstein · 10-23-2004, 05:18 AM

Thanks for the answers - seams that I certainly don't need the vacuum advance. I'll look for a little nicer way of capping off the end.

Now I just need to learn how to adjust the timing

MotoSook · 10-23-2004, 05:24 AM

Josh, it's easy! One 13mm nut and a timing light...rotate the distributor CCW to advance, and CW to retard.

jstein · 10-23-2004, 06:50 AM

Souk-

The technique sounds easy enough - it's the theory that is beyond me. How far to advance, retard, etc. What is the benefit to either? I did a search and I find this one of the topics that I should probably already know

Believe it or not, I don't own a timing light.

RoninLB · 10-23-2004, 10:29 AM

Quote:

Originally posted by jstein

How far to advance, retard, etc. .

set the initial timing at idle to Z1 notch. That should give you a max total timing of 25deg.