M30 vacuum advance/retard

[Dick Steinkamp]

My 72 Bavaria with the original M30 has the original points type distributor but with Weber DGAS 38/38 carbs.

The vacuum advance/retard as I bought the car was hooked up like this...

Manifold vacuum to the retard port on the dizzy. Single vacuum line off the rear carb manifold to the dizzy.

Ported vacuum to the advance port on the dizzy. A vacuum line from the ported vacuum connection from each carb, connected with a "Y" fitting then on to the advance port on the dizzy.



I've read here that the retard portion of the system does not enhance running or driveability in any way and to disconnect this line and plug it at the manifold. True this?

I can't see a good reason to have the ported vacuum ports on the two carbs connected. Is there a good reason for this or should I just run the advance off of one carb?


The consensus here is to run the vacuum advance off of ported vacuum. However, in the broader old car hobby there seem to be as many arguments to use manifold vacuum as ported vacuum for the advance. The vacuum signal is going to look the same to the dizzy after the throttle plates uncover the ported vacuum port so the difference is essentially that manifold vacuum picks up the signal at idle and ported vacuum doesn't. At any other high vacuum condition ported and manifold are seeing the same signal. The manifold signal at idle will likely increase the idle speed.

I will probably experiment with both manifold and ported vacuum to see what (if any) the differences are. Has anyone done this experiment and if so, what were the results?

 

[Stevehose]

Ditch the vacuum retard and cap the carb. This was used to heat up the exhaust gases at idle in order to pass emissions.

I suggest keeping the two teed ported vacuum lines to the advance. This smoothes out the signal. I also researched ported versus manifold vacuum and in reality they are the same but at idle you will be pulling advance when you don’t want to if using manifold vacuum and bmw didn’t design the advance curves for this.

 

[Dick Steinkamp]

Thanks, Steve.

One other thing. I don't really understand what's going on with the stock setup (retard at idle off of manifold vacuum...advance at all other conditions where there is vacuum). At light throttle cruise for example, there is a high amount (and the same amount) of vacuum at both the ported vacuum port and the manifold vacuum port. One is tugging the vacuum diaphram one way and the other is tugging it the other way...at the same time! It seems the result would be that the engine wouldn't get the advance that is needed under this vacuum condition. What am I missing?

 

[Stevehose]

Manifold vacuum is high at idle, ported is zero. At WOT they are both zero. At some point they criss-cross as the throttle opens then they drop together. Manifold vacuum drops out after idle, ported increases after idle and is strong at low throttle openings, so at cruise (low throttle opening) there is more ported (advanced) vacuum than manifold (retard) vacuum. Therefore at cruise the engine gets some additional advance to help with fuel economy. The ported advance drops out towards WOT so as to not advance the timing to detonation.

 

[Dick Steinkamp]

Manifold vacuum is high at idle, ported is zero. At WOT they are both zero.

I agree so far.

At some point they criss-cross as the throttle opens then they drop together. Manifold vacuum drops out after idle, ported increases after idle and is strong at low throttle openings, so at cruise (low throttle opening) there is more ported (advanced) vacuum than manifold (retard) vacuum. Therefore at cruise the engine gets some additional advance to help with fuel economy. The ported advance drops out towards WOT so as to not advance the timing to detonation.

My understanding is that once the ported vacuum port is uncovered by the throttle plates starting to open, the vacuum reading you would get at the ported vacuum port is the same as the vacuum reading you would get at the manifold vacuum port. They are effectively both measuring manifold vacuum at that point since the ported vacuum port is now "in" the manifold. Zero at WOT to 20+ inches of mercury at light throttle cruise. Either will advance the timing the same amount if connected to the advance side of the vacuum canister of the dizzy when the engine is producing vacuum. At light throttle cruise you are burning a lean mixture (as you are at idle) and you need more time for that air/fuel charge to reach max pressure before the piston gets to TDC. It does improve fuel economy because of this (also helps the engine run cooler and overall more efficiently).

Here's one article that helps explain the relationship,,,

Everything You Wanted to Know About Vacuum Advance and Ignition Timing

Unplugging your vacuum advance is costing you mileage and efficiency. We help explain ignition timing and that little silver can.

www.superchevy.com

One more...

http://rockridgefarm.com/vettdoc/Timing_101.pdf

So, I still don't understand how (in a stock setup) one vacuum source can pull on the retard side of the vacuum canister and another identical vacuum source can pull on the advance side of the canister and result in any advance at all when the engine needs it. Somehow the canister must be blocking the signal from the manifold vacuum tied to the retard side of the canister when it starts seeing the signal from the ported vacuum tied to the advance side of the canister.

 

[Stevehose]

I see your point, maybe we should cut the cannister open!

So, I still don't understand how (in a stock setup) one vacuum source can pull on the retard side of the vacuum canister and another identical vacuum source can pull on the advance side of the canister and result in any advance at all when the engine needs it. Somehow the canister must be blocking the signal from the manifold vacuum tied to the retard side of the canister when it starts seeing the signal from the ported vacuum tied to the advance side of the canister.

 

[Dick Steinkamp]

I see your point, maybe we should cut the cannister open!

You first!

 

[bluecoupe30!]

You guys have to put together a PowerPoint presentation and share or at least a YouTube video after you have nailed all this down. Fascinating!

 

[Dick Steinkamp]

You guys have to put together a PowerPoint presentation and share or at least a YouTube video after you have nailed all this down. Fascinating!

I think at this point, we all (probably) agree that eliminating the retard side of the vacuum canister is a good thing. An open question (at least to me) is does the engine run better...better drivability...with the advance side of the vacuum canister hooked up to ported vacuum or manifold vacuum.? I'll try to run some (semi) scientific tests and report back.

 

[Stevehose]

It will advance your timing at idle without turning your distributor to do so which normally would give you too much all in advance. I run 25 degrees of advance at idle and the car loves it (triple webers, 123 distributor). So try it out, your idle rpm will go up, compensate with the idle screw back to 950 or so, you'll still get some cruise advance and it will drop out at WOT like the ported does. Engine may run cooler at idle due to the added efficiency. The drivability should be the same, the only issue may be off-idle as the manifold vac drops down but shouldbe ok. Cap the ported vacuum port.

 

[sfdon]

Canister has nothing to do with it.
Both ports are ported
Retard only exists at idle
Advance does not exist at idle
use your vacuum gauge and study it

 

[Mike Goble]

I always run full manifold vacuum to my VA can. Better idle, less run-on, cooler running. You need to have the VA can all in at the vacuum your engine produces at idle, otherwise it will hunt and eventually the engine will die.

 

[billpatterson]

I always run full manifold vacuum to my VA can. Better idle, less run-on, cooler running. You need to have the VA can all in at the vacuum your engine produces at idle, otherwise it will hunt and eventually the engine will die.

Hi
This is interesting stuff. I will need to reference this when I begin to reassemble my E3. A good amount of time will have past though once sorted I expect she will run fine with a rebuilt dizzy by Jeff at Advance Distributors and rebuilt Webers. After all, she ran great before disassembly Thank you for providing great advice!

What is the 'VA can'?

Regards,
Bill

 

[Stevehose]

Vacuum advance can - attached to the distributor

What is the 'VA can'?

Regards,
Bill

 

[Dick Steinkamp]

We had a sun break here today so I experimented with the advance.

First of all, during the test set up process I found out the retard side of my VA can was not holding vacuum. Since the manifold vacuum line went to it, I have been having a small vacuum leak at all times. I wasn't planning on using the retard side anyway, so no big deal. Because of this, when I plugged the manifold vacuum and left the ported vacuum in place (hooked to both carbs) on the advance side of the VA can the idle was too low. It was no longer getting the extra air through the vacuum leak. I set the idle up to 900 and went for a test drive. The car drove as it always had. The engine was smooth and responsive as usual. It always had just a hint of a surge or stumble with just the beginning of throttle tip in. Not enough to be annoying and most would not notice it (I'm sensitive )

Next up was to plug the ported vacuum and hook the advance side of the AV can to manifold vacuum. Idle speed increased by 250 RPM. I reset the idle to 900. The test drive was nearly identical to the drive with the AV can hooked to ported vacuum. The difference was that the tiny stumble or surge at throttle tip in went away.

I'll leave it hooked to manifold vacuum. Either way works well. Eliminating that tiny stumble/surge is reason enough but so is knowing the engine is happier on manifold vacuum as the idle RPM increase indicated.