78SC vacuum advance..beneficial? (long..sorry)

MotoSook · 07-22-2003, 10:30 AM

OK, I'm back on that dang vacuum advance that gave me such headaches a year ago.

I have a 78 SC motor with the vacuum disconnected at the distributor. The timing is set at about 10-12 degrees advance at idle which gives me about 34-36 of full advance (haven't checked lately, and the brain drops old data). The car runs very smoothly and loves to live above 3500RPM.

So last night I was fooling around in the garage drinking a Guinness (dinner, 'cuse Heather is out of town), and I decided to connect the vacuum to see if it would cause me to stumble at light throttle still.

I went out for a drive in the country (only after one Guinness) to test it, and it was not too bad, the stumbling.

Well, I went out for lunch today, and the stumbling and lurching was just now bearable, so off can the vacuum line and on went the port plugs.

So the big question that bugged me while I was munching on my meatball sandwich is: Why vacuum advance?

The vacuum advance only puts about 3-5 degrees of advance on the dizzy. With my 10-12 degrees advance at idle, haven't I got what ever advantage the vacuum advance would give me?

I know the v-advance comes and goes when I'm on and off the throttle, but what am I missing out on if I leave the vacuum advance disabled?

tsuter · 07-22-2003, 10:43 AM

Souk man, come on now you know the answer: going up a steep hill slowly with the engine turning 3000 rpm requires different advance needs (actually less advance) compared to a car traveling the highway at 3000rpm. Without going into too much detail, rest assured that there was a reason that Bosch didn't stick with the all centrifugal advance distributor on the later distributors and introduced vacuum advance and also retard. Current EFI systems use RPM and MAP both to enter the ignition advance tables - same thing - as MAP goes up advance is less for the same RPM.

MotoSook · 07-22-2003, 11:06 AM

Hi Thad..damn! Now I have more work to do.

I agree that Bosch had good reason, but I guess I need to know what the difference gained/lossed is, so I can decide if I want to compromise with no advance.

I tested with a vacuum guage and my vacuum was fairly stable with no load on the engine. The car runs great as usual with the v-advance enabled if I am medium to heavy on the throttle. It's the light thottle (just as the butterfyl starts to open) at low RPMs that is a PIA.

It's either a dist. rebuild or I get you to come down and build me an EFI

What I'm I not seeing here?

Bill Verburg · Bill Verburg's Garage

It's been a longtime since I messed w/ that stuff but here goes. Are you sure it is a vacuum advance? Some of the cars from that era used a vacuum retard. It's an advance if disconnecting at idle causes the timing to go from say TDC to say 5&degATDC or 5&degBTDC to TDC

The spec for a '78 US SC is 5&degBTDC @925 &#177 25rpm(vacuum disconnected) going to 28&deg &#177 4&degBTDC @6000(vacuum disconnected) ~23 - 28&deg of total mechanical advance

My old C3 engine was 5&deg ATDC@900rpm(vacuum connected) going to 35&deg &#177 3&degBTDC @6000(vacuum disconnected) I believe that the vacuum was a retard on this engine. giving total of ~35&deg of mechanical advance

If your #s are correct you have a total of ~25&deg of mechanical advance which is right on the money for the model

The only reason for the difference in spark timing specs being emissions. I would try having the distributor recurved to give a total of ~38&deg of mechanical advance@600rpm, set idle timing to TDC and forget the vacuum unless you want to try to get better part throttle fuel economy.

The original purpose of vacuum advance was to increase fuel milage. The leaner mixtures at part throttle(low MAP/high vacuum) want more spark advance than richer full throttle mixtures(higher MAP/low vacuum).

The port where the vacuum comes from can also make a difference, some are subject to full manifold vacuum others have a regulated exposure.

MotoSook · 07-22-2003, 01:51 PM

Bill,

I've got the just vacuum advance (78-79 SC).

The ports that I have available for vacuum to the dist. are really the same vacuum point on the SC throttle body. I have tried two of them with no change in my part throttle behavior.

I am pretty sure the vacuum source is good and direct, so I suppose a distributor rebuild is in my future. Until then, I'll just run without it, and give up some mileage.
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The problem seems to be at very small butterfly opening, the vacuum is not very smooth and the advance is not consistant nor smooth. It seems to pulse, or the vacuum advance mechanism creates the pulse (sticky stator?) The result is that the ignition (spark) is not consistant. The vacuum pot is new, and if I suck on the vacuum line I can see the stator move, and snap back when vacuum is released. Unfortunately, the Souk vacuum test is not scientific enough to detect a sticky stator.

Is that sympthomatic of anything some of you more seasoned 911 owners have seen?

Could I have some bad valve seats?

Bill Verburg · Bill Verburg's Garage

If the vacuum port is close to the throttle butterfly you may be getting a weak/modified vacuum signal. Try picking up the vacuum from somewhere as deep in the intake manifold plenum as possible to get a cleaner vacuum signal. Avoid the runners for the signal.

Yes, be sure that there is no mechanical stiction anywhere in either the vacuum or mechanical advance mechanisms. It is also possible for the vacuum diaphram to develop a leak, but I see that you have already addressed that possibility.

MotoSook · 03:05 PM

Thanks Bill. I'll try to find a better vacuum source further down the throttle body. If that doesn't work it'll be a distributor rebuild perhaps.

How about dielectric spray lube into the dist.? Do they make a dielectric lube spray? (Note: the distributor has seen it's share of WD-40 bathes during my past attempts to cure the problem) The thinking here to to make sure the advance isn't hanging up during advance.

Kemo · 07-22-2003, 02:12 PM

were there any year SC's or options that had the mechanical advance only? Since ive owned the car, I have not seen a Vacume advace apparratus. My distributor has a metal plate pressed in place where the vacume advance WOULD have been. I have an 82 engine according to the engine number.

MotoSook · 07-22-2003, 02:17 PM

Kemo, you should have a vacuum pot on that dizzy. Perhaps a PO got tired of the stumble and lurching I'm experiencing!

Kemo · 07-22-2003, 02:20 PM

i havent seen one! I wonder what I am missing? (no pun intended) My car runs like a demon from 3500 up to redline, and thats with the timing set at 7deg BTDC at 950 RPM.

MotoSook · 07-22-2003, 02:28 PM

I should have been less vague. The PO probably removed the vacuum pot, because he had problems like I do. Other than minor emmisions and feul mileage issues, I have not heard of any major drawbacks to running with no vacuum advance/retard.

Looks like your PO was not an idiot. If you run a search on SC timing, you'll see that a lot of guys are now setting their timing to 30+ degress of full advance for better performance.

tsuter · 07-22-2003, 02:36 PM

Mileage was part of it but retarding advance under load was also a big part of the design. Ie., less advance at higher MAP. Or really reverse and at low or no load you get vacuum advance plus mechanical advance. Higher load, less advance (mechanical only). Hard to hear detonation with the motor back there. I got a headset and mic you can borrow. Guys running over 30-35 degrees mechanical under load are probably detonating - they just don't know it. (at least not yet)
Incidently I use that vacuum line to suck water in periodically when I feel like flushing carbon build up. Its real easy, disconnect at the distributor and hang it in a water bottle.

Jdub · 07-22-2003, 02:45 PM

I saved this from a prior post either here or on Rennlist and thought it *might* be of some use, esp. concerning the vaccum sidepot.

With thanks to Lee Rice:

Technical & Safety: Distributor Troubles

The question came up about ignition problems. There seemed to be more than one Porsche owner that had to replace their ignition distributor because the advance was not working. I was asked is this normal and was there any way to fix the distributor. It seems to be a normal practice to replace ignition distributors when the internal parts do not work right. The advance mechanism is usually suspect as it is subject to a great deal of heat and high frequency vibration.

The requirements of a ignition distributor are very demanding and the least little wear and imperfection will cause odd behaviors. The installation of, a new distributor will usually return your Porsche to crisp performance, but they are not cheap. A new distributor for a 911 SC costing $800- 1,100 and the 1980 928 over 1,300! The re-built ones cost about a third of that. So what usually goes wrong? I have removed, disassembled, cleaned, inspected, lubricated, reassembled, and reinstalled and timed many ignition distributors.

My observation is that the grease in the rotor and stator plate becomes old, dries up, and the stator plate seizes. This will cause the vacuum retard-or vacuum advance (for cold start-up) to be ineffective as the distributor will be stuck in one position. I did a major service on a 930 Turbo with this problem. When the engine first started it wouldn’t idle at all. Finally the engine would burp and burbble up to something near running speed and as I accelerated the throttle a couple of times all of a sudden the engine accelerated to a high idle speed and stayed there with the throttle closed. Driving it was even more exciting as on one run it ran like a dog and the next run it accelerated very strong and "pinged" on boost. The culprit was a sticking rotor/stator plate.

The normal cleaning and relubrication restored the engine to it’s normal silky smooth running. This particular problem with the sticking rotor/stator plate seems to be common in the 1978 and later 911/930. Maybe the lubrication wasn’t up to the demand of the emission requirements and the turbo heat. Once in a great while one of, the two, mechanical advance fly weight return springs breaks. The small and delicate bushings that fit over the fly weight spring retainers will break-thus reducing the springs tension and the advance will be too much for a given rpm. These problems can be repaired if you have the parts.

The main reason to replace a distributor is for worn distributor shaft bushings. When the bushing wears out the rotor will fly out of it’s track and hit the distributor cap contractors. This can and eventually will break the distributor cap. The bushings can be replaced-if you can find them. If you are driving an old Porsche it would be a good idea to keep en eye open for a used distributor at the swap meets.

You may find a perfectly good distributor but at least you will have some serviceable Darts. The choices for replacing or repairing are one of time and costs. I prefer the warranty re-built as long as the part numbers are the correct match. If the vacuum advance-retard units work correctly and the problem is old grease causing a sticky vacuum advance/retard, I wold have your distributor repaired, this shouldn’t cost much more than a hundred bucks. 930 Turbos: When making timing adjustments to the 930 Turbo you should be aware that the large vacuum unit on the ignition distributor do three things that all need to work together. All turbos have a vacuum retard feature. This retards timing at idle only with the throttle fully closed. As soon as the throttle is opened this retard is gone and the timing advances. The second vacuum feature is on the opposite side of the vacuum unit. This vacuum is used to advance the timing for cold start and warm-up. This vacuum supply is controlled be a electric switch for (‘77-82) and by a thermal switch on the breather cover(’83-95). The third source is boost pressure.

The 1978 California specification Turbo and all later ones require the high rpm timing check done at 4,000 rpm with the vacuum retard hose disconnected. This timing at 4,000 is taking into consideration that the vacuum unit is fully operational and at 0.5 bar boost it will retard the timing by 4-6 degrees. The workshop manuals do not give you a performance check of this, but I would recommend that this be checked. Especially if you encounter roughness and pinging when on boost. This will melt expensive things!

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MotoSook · 03:04 PM

The 78-79 SC's only have vacuum advance Thad, no retard. With vacuum advance (3-5 deg. emperical guess..is there such a thing) and a mechanical advance of 26+/-2 (per factory spec..or was it 28 deg), the factory deemed it safe to advance fully at 30+ degrees under certain conditions. With my low compression motor, I feel safe at 30-35 degrees running modern fuel, usually 89 Octane.

Should I back off the timing a few degrees?

It is not very scientific, but the last time I have my motor out, I peeked into the cylinder (best I could w/o a borescope), and did not notice any funny patterns on the piston or rather carbon coated piston. Edit: Can detonation be detected by irregular carbon coating?

I think it's time for a water treatment though...demineralized water right Thad?

Alan Cottrill · Alan Cottrill's Garage

does a 930 dizzy have a vacuum advance?

tsuter · 04:15 PM

Sorry Souk just bad engleesh. You get retarded advance under load. but not via vacuum retard. Via an absence of vaccuum advance. The mechanical advance is actually two separate advances (centrifugally) of about 14 degrees each (26-28 total as you say). Plus you get 4-6 of vacuum advance with low or no load. On mine to reduce detonation I have modified the two advance to a total of 20 dgerees or - 10 degrees each by bending down the ends of the little metal shafts.
I can't speak to the 930 but if there is a hose then it has vacuum advance. If there are two hoses then it has vaccum advance and vacuum retard both.
Souk: RE the water business I use distilled water but not sure it matters.

tsuter · 07-22-2003, 04:29 PM

Like I said anything over 30-35 under load. Best be careful and use good gas.

Bill Verburg · Bill Verburg's Garage

My '76 C3 w/ ~35&deg of total at 6000 ran fine on leaded regular, but had audible ping w/ unleaded regular. The solution unleaded premium.

The C3 was an unfettered version of a '78 SC. Just get the mixture a little richer than the factory called for and run timing similar to the C3 and 200hp@6000 is doable.

The situation w/ mechanical and vacuum advance is similar to that w/ DME engines. The max performance values are inherent in the mechanical curve while the vacuum advance provides the part throttle economy mode. W/ DME the pre loaded chip maps alone determine mixture and timing, at part throttle the O₂ sensor modifies both for better fuel economy and emissions.

P.S. I corrected a typo in my earlier post, the correct C3 max mechanical advance is 35&deg &#177 3&deg

MotoSook · 07-22-2003, 05:15 PM

Excellent posts gentlemen! I love it!

With my timing, I do run a bit rich..according to the girlfriend test..."ew! you smell like the car's exhuast, get back in the garage and take that shirt off!"

Seriously, I don't have an O2 meter, so I turn the mixture screw to hunting-rich, then back off an 1/8th or so.

The car runs so well without the vacuum advance, that I'm reluctant to mess with it unless I'm making a poor compromise. However! It seems there are two ways to get around this: 1) conservative setting like Thad with a good vacuum advance circuit 2) run purely mechanical advance upt o 35 degrees with good gas and a richer than stock mixture.

Sorry Thad, I'm going with number 2 until I get a turbo like you have

And Ron's fancy dandy mobile test bed gives me enough comfort! So far

Keep it coming gents..this is good stuff!

RoninLB · 07-22-2003, 05:35 PM

Quote:

Originally posted by Souk
And Ron's fancy dandy mobile test bed gives me enough comfort! So far

Keep it coming gents..this is good stuff!

hey Souk.
I just posted some info on that "anti-detonation " thread