disconnected distributor vacuum line?

pmead · pmead's Garage

I just purchased a 76' carrera 3.0 Targa, 87K miles. Runs very smooth but seems to lack the power that the 3.0 should have. I noticed the vacuum line to the distributor has been blocked and disconnected with a cap placed over the distributor nipple. No advance? Why would this be done? Any ideas?

sc_rufctr · 03-13-2009, 04:05 AM

That sounds familiar. I own a 78 SC and mines exactly the same.
It's a ROW car and was original bought in France. (according to Porsche)

It has always run fine and it's not down on power. When I bought the car it had a red plastic cap cap over the vacuum port on the dizzy. It's still there.

Someone with more knowledge please chime in.

john walker's workshop · 03-13-2009, 04:16 AM

attach a length of hose and suck on it while the engine is running. if the idle slows, it's a retard and not needed. if it speeds up, it's an advance and should be functioning.

sc_rufctr · 03-13-2009, 04:45 AM

Quote:

Originally Posted by john walker's workshop

attach a length of hose and suck on it while the engine is running. if the idle slows, it's a retard and not needed. if it speeds up, it's an advance and should be functioning.

Thanks John... That's another thing on my to do list.

pmead · pmead's Garage

Thanks for the info. If not connected how does the advance work? I have been led to believe that this car was originally sold in the German market.

sc_rufctr · 03-13-2009, 05:12 AM

There is an advance built into the distributor. The Dizzy has centrifugal weights the spin out as the revs rise... Old school simple. Most older cars used the same setup... MSD Still has this setup in their aftermaket distributors

Jeff Higgins · 03-13-2009, 05:26 AM

These distributors have an internal, mechanical advance. The vacuum advance or retard is purely to aid driveability and/or emissions, introducing a crude "throttle position sensor" kind of function into the mix. They will all run just fine with no vacuum line hooked up, particularly if it is a vacuum retard mechanism. If it is a vacuum advance mechanism, it's probably more important (and noticable) whether it is hooked up or not.

Many distributors get "re-curved" to run purely mechanical advance. The advance mechanism is operated by a couple of flywheights retained by springs. Change the weight of either, and you can make full advance come in earlier or later. You can also eliminate the need for the vacuum assist. This may or may not have been done on yours. Unless you have specific reason to believe it has, I would suspect not. However, I do believe the "euro" distributors have a different advance curve, achieving full advance earlier than U.S. distributors. They weren't as concerned about emissions... If that is what it is, it may work just fine without vacuum.

It's pretty easy to check where full advance comes in. All you need is your timing light. Just run the car, slowly increasing rpm while watching the timing marks. At some point it will stop advancing. Some cars (like SC's) reach full advance very late - like around 5,000 rpm or more. Purely an emissions thing. The last two distributors I had recurved (by Barry Hershon) were set up to achieve full advance at 2,300-2,400 rpm, far below where these engines should ever be operated. Essentially, they only returned to "0" advance to help it idle. 935's and such, by the way, have fixed distributors - no advance mechanism at all.

Anyway, that's a kinda long-winded way to say it probably does not matter as far as performance. Set the advance to spec at 6,000 rpm, and let it settle where it may at idle. Having that hose connected or not will never affect power, just drivability at lower rev's.

sc_rufctr · 03-13-2009, 05:37 AM

not wanting to jack the thread but...

Thanks Jeff

I was considering locking out my mechanical distributor and using an MSD timing computer to do the same function. I got an MSD timing computer for cheap and I have the MDS 6AL in my car. MSD says it's more accurate and consistent than a mechanical advance.

My plan is to duplicate the stock mechanical advance but I like the idea of have it happen electronically instead of manual. My dizzy is working fine but I'm considering going to the Bitz CIS to EFI conversion. The plan is to get the car running as close to 100% before doing the conversion.

Have you personally seen this done on a 911? I have seen some cars here and the Bitz site with the timing computers.

sc_rufctr · 03-13-2009, 05:50 AM

This is a pic of my dizzy showing the red cap mentioned. You can see it on the vacuum port.

RoninLB · 03-13-2009, 05:56 AM

Most Bosch dizzys have vacuum retard. It's an emmissions thing. I believe only stock carbed engines had a vacuum advance without an additional retard.

A vacuum advance comes in at cruise. At steady state cruise the engine can tolerate some additional advance over the mechanical engine loaded timing, about 7deg. Not a performance thing, only for increased gas milage.

if I missed something correct me

Jeff Higgins · 03-13-2009, 06:46 AM

Quote:

Originally Posted by sc_rufctr

not wanting to jack the thread but...

Thanks Jeff

I was considering locking out my mechanical distributor and using an MSD timing computer to do the same function. I got an MSD timing computer for cheap and I have the MDS 6AL in my car. MSD says it's more accurate and consistent than a mechanical advance.

My plan is to duplicate the stock mechanical advance but I like the idea of have it happen electronically instead of manual. My dizzy is working fine but I'm considering going to the Bitz CIS to EFI conversion. The plan is to get the car running as close to 100% before doing the conversion.

Have you personally seen this done on a 911? I have seen some cars here and the Bitz site with the timing computers.

MSD is way more focused on American V-8 performance than they are on our stuff. Much of what they tout as "performance enhancing" on big V-8's has no real apllicability with us.

A great example is their very company name - "Multiple Spark Discharge". The MSD box lights every plug some incredible number of times to make the car run better at lower rev's. Problem is, the "MSD" output stops somewhere between 2000-2500 rpm's and reverts to a single spark, so it does us no good whatsoever. Maybe it will help idle, but that's about it. No one should be rev'ing our motors that low, especially under any kind of load. So "MSD" is a moot point for us.

The same sort of thing happens with their electronic advance setup. If you are running a big V-8 at low rev's, and want power and drivability down low - like 1500-2500 rpm's, then that tunable advance curve is great. In our case, if you set the distributor up to hit full advance at 2500 rpm or less, the advance curve once again becomes moot. You will never drive it at such low rpm's. In that application, I can think of nothing more reliable than weights and springs.

Quote:

Originally Posted by RoninLB

Most Bosch dizzys have vacuum retard. It's an emmissions thing. I believe only stock carbed engines had a vacuum advance without an additional retard.

A vacuum advance comes in at cruise. At steady state cruise the engine can tolerate some additional advance over the mechanical engine loaded timing, about 7deg. Not a performance thing, only for increased gas milage.

if I missed something correct me

That's pretty much it. Note that vacuum advance is really only applicable on distributors with a very slow advance curve, like SC's. If the mechanical advance pegs it a full advance at some rpm below cruising, the vaccum advance becomes moot. SC's run great with recurved distributors, by the way. I had one recurved for my 3.0 liter MFI motor, then switched to Electromotive instead, so it wound up on my son's '79 SC. Full advance at 2300-2400 and no more vacuum advance.

sc_rufctr · 03-13-2009, 07:10 AM

Thanks again Jeff

My Bosche ignition failed so i replaced it with the MSD 6AL. I was quoted $1200 AUD exchange for a rebuilt Bosche ignition so the decision to go to the MSD was purely economical.

I have plotted my distributor and basically it's all in at about 3000 RPM so I'm guessing after reading your posts that it has been "tweaked" in the past. The pic attached is the Excel spread sheet I created at the time. As far as I can tell it goes from 5 to 25 degrees BTDC. Does this sound right?

I did this plot manually with a timing light and portable tacho. It's not 100% accurate but it wouldn't be far off.

BTW: It verifies everything you've been posting

pmead · pmead's Garage

Great info folks!!! I do come from the V8 camp....(factory 5 cobra with stack injection and stand alone ECU from SDS) I know how timing effects the performance with that engine and assumed that the porsche would be similar. I'll go ahead and set the timing as Jeff has recommended!! This is very good forum. Knowledgable people with the same sickness I have

Jeff Higgins · 03-13-2009, 07:43 AM

Don't get me wrong, I like MSD products and use them myself. I ran my 2.4 on an MSD 6AL for years after my Bosch gave up the ghost (it is now leading a new life on pwd72's old car, in Turkey, with Evren, after Warren rebuilt it...). When my current motor was still a single plug with a distributor, that's what I used. It's now on my younger son's 68 912/911.

Anyway, it looks like you are good to go with your advance curve. I'm not entirely sure, but I think most of these have about a 20 degree range of mechanical advance. That sounds about right. The ones I had Barry Hershon do have about that. I run 30 degrees total, and they idle at about 10 degrees.

Tadpole · 03-13-2009, 02:42 PM

Hey guys,
I have a '75 with a 2.7 and webers. The PO also disconnected the vacuum line to the dizzy. I thought maybe this was something he did because he switched to the carbs. But I tried putting a vacuum line on the dizzy liked John Walker suggested but it didn't change the idle either way. Do I have a potential problem somewhere?

Thanks
Mike

stlrj · 03-13-2009, 04:34 PM

Quote:

Originally Posted by sc_rufctr

As far as I can tell it goes from 5 to 25 degrees BTDC. Does this sound right?

That does sound correct according to my Spec Book and that's with the vacuum line disconnected just like you run it. But since you have the vacuum advance disconnected, you've eliminated 5 degrees advance and lowered performance as a result.

Cheers,

Joe
74 911 w/86 3.2 conversion