Chevrolet Distributor

[40 Deluxe]

Originally Posted by Gene1949

Our flatties respond very well to the use of a vacuum advance can. The auto manufacturers put a vacuum can on their ignitions for a reason. None of them ran a “centrifugal only” system.
Something that needs to be said here, depending on driver input/throttle position.
Ported (venturi vacuum) ADDS advance.
Manifold vacuum SUBTRACTS advance.

I chose to run manifold vacuum on the ’49 and the ’69 BBC Camaro because of slow speed/cruising cooling. ALSO, drivability, fuel economy and idle quality especially on performance motors all played a part in that decision.
Next time you are futzing with your tune-up jack up the initial timing a bit. Immediately your idle speed increases. Reason being more of the idle mixture is being used. Since you can close the throttle plates AND lean out the idle mixture to maintain the same idle stands to reason you are using less fuel and creating less heat at an idle.

Ported vacuum and venturi vacuum are two different things! Venturi vacuum is sourced from the small diameter of the venturi and is commonly used to open the secondary throttle valves on a vacuum secondary Holly four barrel. It was also blended with manifold vacuum to provide the advance on '49-'56 Ford non-centrifugal ignitions.
Ported vacuum is sourced from an opening that is situated just above where the throttle plates are situated at idle, so there is no vacuum advance until the throttle plates are opened slightly.
When you "jack up the initial timing", you are also adding to the total timing so you may be over advanced at high RPM which can lead to detonation which can lead to damage even if you can't hear it! Use a timing light.











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[flatjack9]

All is not clear to me, you will have to explain your position which I deem wrong. I stand completely behind my post above. (This is in response to post #20). 40 DeLuxe has it right.

[Ol' Ron]

In cruise you have max Vacuum advance. When you goWOT you loose the advance. Kinda nice, don't ya think???

[Gene1949]

Originally Posted by flatjack9

All is not clear to me, you will have to explain your position which I deem wrong. I stand completely behind my post above. (This is in response to post #20). 40 DeLuxe has it right.

In a closed throttle position the manifold vacuum is high resulting in the maximum advance of the can. When you open the throttle plates that vacuum is lessened essentially SUBTRACTING advance.

[Gene1949]

Originally Posted by 40 Deluxe

Ported vacuum and venturi vacuum are two different things! Venturi vacuum is sourced from the small diameter of the venturi and is commonly used to open the secondary throttle valves on a vacuum secondary Holly four barrel. It was also blended with manifold vacuum to provide the advance on '49-'56 Ford non-centrifugal ignitions.
Ported vacuum is sourced from an opening that is situated just above where the throttle plates are situated at idle, so there is no vacuum advance until the throttle plates are opened slightly.
When you "jack up the initial timing", you are also adding to the total timing so you may be over advanced at high RPM which can lead to detonation which can lead to damage even if you can't hear it!
!

Arguing semantics vs concepts is a losing proposition. How about the "timed" vacuum port. There, feel better?
My "jacked up" comment was to prove a point, our flatties like a lot more advance at an idle. I posted the timing I use at the beginning of the thread.

Originally Posted by 40 Deluxe

Use a timing light. !

Without knowing true TDC AND verifying the timing curve in your distributor a timing light is a feel-good device demonstrating a “ball park” setting.

[Krylon32]

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I have used non vacuum advance Chevy distributors from Bubba and Charlie in 6 engines so far. I have installed them set the timing and went down the road. I dam sure ain't no mechanical genius but I do know when an engine sounds and runs right and is happy. In my simple mind that's good for me.

[tubman]

I had a converted SBC distributor with vacuum from the best source on my "Hot Rod" engine. It ran fine, but I decided that I wanted a "flattop" Mallory, mainly to provide a complete "old-timey" look. To my surprise, it ran as good, if not better with the Mallory than the conversion unit.

[40 Deluxe]

Originally Posted by Gene1949

Arguing semantics vs concepts is a losing proposition. How about the "timed" vacuum port. There, feel better?
My "jacked up" comment was to prove a point, our flatties like a lot more advance at an idle. I posted the timing I use at the beginning of the thread.


Without knowing true TDC AND verifying the timing curve in your distributor a timing light is a feel-good device demonstrating a “ball park” setting.

Who is "arguing semantics vs concepts"? I am merely stating the facts of the matter. Without understanding how advance mechanisms work and what they accomplish, we cannot tune an engine to maximum efficiency. And, is not "timed" vacuum just another name for "ported" vacuum?
Of course we need to be "knowing true TDC and be verifying the timing curve in your distributor"! That goes without saying. It gives us a reference, a starting point, in making adjustments. Setting the timing by ear or by using a vacuum gauge is truly the "feel-good" method for getting in the "ball park" (maybe, but more likely missing the ball park!).