mickatbp

Firstly, thanks in advance.

I know original 2x4 C1s had dual point mechanical distributors with no vacuum advance. I've read the articles about where to source the vacuum advance on a WCFB but I am a little confused.

I though Vacuum advance was initiated when the butterflies were open to advance the timing for better higher rpm performance; for that to occur vacuum advance shouldn't be felt when the engine is idling .... or so I thought.

But most threads say to use the rear port in the base of the primary carb which is full vacuum at idle, doesn't this then give full vacuum advance at idle ....... is this what you want? My mechanic says partial vacuum is what is required for vacuum advance to work correctly ... so now I'm even more confused as I'd planned on using that tapping on the back of the base plate.

Lastly, the original wcfb clone 2x4s are on an original GM 2x4 manifold sitting on top of a mild 327. What initail timing, mechanical timing and vacuum timing are suggested.

I've read a heap of threads and apart from the woffle answers are very conflicting. I'd like some nice concise answers without the woffle.

AND some photos and how what you have done works for you.

Are distributors of today different than they were in the 60s in the way they used vac advance and that partial vac is used in modern distributors.

I also understand that wcfbs don't have a partial vacuum taping but what is the small tapping in the baseplate front side and offset to one side?

Frankie the Fink

First off vacuum advance is for LIGHT load cruising; it is useless at WOT which is why the high performance motors (e.g. 270 HP) don't even have it. There is an assumption they are going to be at hi revs most of the time.
Vac advance also has beneficial effects at idle.

Either your mechanic is f*ed up or you are misinterpreting him. "Partial" vacuum advance is different than "ported" advance. Ported vacuum (obtained ABOVE the throttle plates) is largely a contrivance to deal with emissions; our old car designers didn't care about such things 'back in the day' and ran FULL manifold vacuum (and so should you) from below the throttle plates. "Ported" vacuum provides NO vacuum at idle but acts the same as a full vacuum port above idle speeds - no difference.

Here is an excerpt from a article from our own JohnZ (the ENTIRE article is too big to attach; provide me an email address if you want it):

"At idle, the engine needs additional spark advance in order to fire the lean
(and exhaust-diluted) idle fuel/air mixture earlier in the cycle in order to
develop maximum cylinder pressure at the proper point after TDC for efficiency,
so the vacuum advance unit is activated by the high manifold vacuum, and adds
another 15 degrees of spark advance on top of the fixed initial timing setting. For
example, if your initial timing is set at 10 degrees, at idle it’s actually 25
degrees with the vacuum advance connected (a properly calibrated
centrifugal advance mechanism will not have started to move yet at idle rpm)."

There is no ported source for vacuum on a WCFB; ANYTHING you draw off the base will be FULL vacuum and THAT IS WHAT YOU WANT.

A good place to start with your timing is 10* initial (vac advance plugged) and then work back or forth a couple of degrees (my 270HP dual quad is real happy at 12*). If your centrifugal advance is 24* added on top of that then you are near the ideal 36* which small block Chevy's like. Ideally the distrib weights should bring this in at about 2500 RPM (in my experience) with the Duntov 097 cam (adjustable via distributor springs).

Add vac advance for cruising on TOP of that at about 12* and you are at 48* -- again nearly ideal for an original 283ci. Some cars can handle more but you have to watch for detonation - my particular car is perfectly happy at 52*.

I have attached a work sheet I keep on all my classic cars so I can track beneficial changes and other things and for future owners. There is some timing info there. You should get a digital 'dial-back' timing light so you can curve your distributor properly.

plaidside

I agree with Frankie and not your mechanic.
I have a 2 four 61 and I removed the dual point distributor and installed a single point with vacuum advance.
To make it look stock I installed the distributor 180 degrees to put the vacuum advance under the left side shielding.
I have been running the car with this set-up for about 12 years now.
Joe

SpartyGW

Hey! I also installed a vac adv distributor 180* out from "normal" to hide the vacuum can. Glad to see I'm not the only one trying to hide the canister.

When I made the conversion a few weeks ago I installed a distributor I totally rebuilt that I bought off ebay. In total I have about $70 into the distributor and the drivability improved immediately and I am just waiting for the next gas fill-up to see the mileage. I was averaging 10mpg before the swap. I am 100% positive it will be significantly better.

After I get more data (I record everything) for a few tanks I will report on the mileage increase.

John S 1961

Why is the VA connected to a ported source? Doesnt that give you 0 vac at idle?

Powershift

Answer is YES it does. But this thread is about 8 years old with no recent posts.........so it will be closed shortly.



Larry

SWCDuke

Optimum total idle advance to minimize fuel flow with lowest EGT is low twenties to low thirties, and the primary determining factor is valve overlap. If it's on low side like base engines the lower end of the range is optimum. The highest overlap cams like the 30-30 like the high end of the range.

Retarding total idle advance raises EGT, which helps oxidize HC and CO with injected air. SO PORTED VACUUM ADVANCE WHICH ELIMINATES VACUUM ADVANE AT IDLE IS AN EMISSION CONTROL STATEGY!!!

Duke