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post #1 of 49 (permalink) Old Apr 15th, 08, 05:48 PM Thread Starter
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Initial timing and Source Vacuum

Ok say you have just set up initial timing at 19 degrees on a BBC with carb adjusted to idle at 800 rpm using a vacuum advance distributer. The vacuum line to the distributer was disconnected and the ported vacuum source on the base of the carb is plugged in this particular example for the purpose of accurate Carb setting and establishing what the engine wants as initial advance (or advance at idle). When you reconnect the distributer vacuum advance the timing will advance and the rpm will increase somewhat. What do you do? Retard the distributer to 19 degrees or turn out the throttle adjusting screw with the anticipation that the vacuum will drop enough to retard the timing back to 19 degrees? I want the engine to Idle at 800 rpm with an Initial Timing (n the case of a Vacuum distributer - Idle Timing) of of 19 degrees.

I understand that a vacuum distributer say idling at say 19 degrees will start the engine at a lower timing to make easier starting and less work for the starter in which case this factor could be considered as the initial timing.

Also, is there any benefit, when running long duration cams, to tap a vacuum advance distributer from the manifold vacuum rather then the carb’s ported source? Another thing to consider some Performance Carbs do not have a Ported Source.

I’ve run Mechanical Distributors for so long that I’ve lost some of my neurons concerning Vacuum Distributors - LOL

I think I've answered my own question and that would be to "Retard the distributer to 19 degrees" after reconnecting the vacuum advance; however, I would very much like to here your opinions: Thanks.

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Last edited by Z15CAM; Apr 15th, 08 at 08:53 PM.
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post #2 of 49 (permalink) Old Apr 15th, 08, 06:51 PM
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Re: Initial timing and Source Vacuum

Quote:
Originally Posted by Z15CAM View Post
Ok say you have just set up initial timing at 19 degrees on a BBC with carb adjusted to idle at 800 rpm using a vacuum advance distributer. The vacuum line to the distributer was disconnected and the ported vacuum source on the base of the carb is plugged in this particular example for the purpose of accurate Carb setting and establishing what the engine wants as initial advance (or advance at idle). When you reconnect the distributer vacuum advance the timing will advance and the rpm will increase somewhat. What do you do? Retard the distributer to 19 degrees or turn out the throttle adjusting screw with the anticipation that the vacuum will drop enough to retard the timing back to 19 degrees? I want the engine to Idle at 800 rpm with an initial timing of 19 degrees.

I understand that a vacuum distributer say idling at say 19 degrees will start the engine at a lower timing to make easier starting and less work for the starter in which case this factor could be considered as the initial timing.

Also, is there any benefit, when running long duration cams, to tap a vacuum advance distributer from the manifold vacuum rather then the carbís ported source? Another thing to consider some Performance Carbs do not have a Ported Source.

Iíve run Mechanical Distributors for so long that Iíve lost some of my neurons concerning Vacuum Distributors - LOL

I think I've answered my own question and that would be to "Retard the distributer to 19 degrees" after reconnecting the vacuum advance; however, I would very much like to here your opinions: Thanks.
If the distributor is advancing at your 800 RPM idle, then you are not hooked up to ported vacuum. You are at manifold vacuum. You don't want your timing to move around at idle. It will be impossible to adjust the carb within reason like this. Either run ported vacuum, or get an adjustable vacuum advance that you can tune to start advancing safely above your idle vacuum.

Do not do your retard to 19o with connected vacuum idea. Vacuum changes under different temps. etc. You will never have a stable idle with things adjusted like this.
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post #3 of 49 (permalink) Old Apr 15th, 08, 07:15 PM
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Re: Initial timing and Source Vacuum

I prefer manifold vacuum on the vacuum advance. My engine loves lots of timing. My idle timing now with EFI is 27 degrees.

With the old carb setup, idle timing was over 30. The motor loved it.

But my car's a stick, and I don't have to worry about converter load at idle.

With an automatic, and either or both of "lots of cam" and "not enough converter" you have to fight it a little bit, and sometimes fall back to a ported vacuum source simply to stabilize the idle timing.

Nuttin worse the lumping along at 900 rpm and 10" of vacuum in neutral, put her in gear, the r's drop to 700, vacuum drops to 6", 110 degrees of timing gets pulled out and the motor falls on it's face.





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post #4 of 49 (permalink) Old Apr 15th, 08, 09:49 PM Thread Starter
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Re: Initial timing and Source Vacuum

It is my understanding that an engine idles under a constant Vacuum which drops with throttle movement and becomes constant again once the plates have stopped.

When the throttle opens the vacuum drops releasing the distributer plate which rotates under spring pressure retarding the timing to say initial timing (assume the weights are tied - )) to compensate for ping, Auto-Trans shifts, Vacuum secondaries to open ... Etc. I would say you have to retard the distributer back to 19 Degrees rather then close the throttle plates to reduce rpm as the purpose of disconnecting the advance was to establish throttle Plate position and Timing at Idle; such that, when the engine is starting there is no vacuum and uses the more retarded Initial Timing till it fires creating vacuum which pulls the distributer plate Full position to establish the Idle Timing of 19 Degrees.

Does this make any sense?

By the way Jim what insane compression and cam are you running that likes 27 or 30 degrees of timing at idle? You're communicating with a good old 10.25:1/[email protected] BBC Holley/Muncie 12 Bolt 3.31 Boy? The only thing Vacuum on my ride are the PCV and Power Brakes - LOL

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post #5 of 49 (permalink) Old Apr 16th, 08, 05:03 AM
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Re: Initial timing and Source Vacuum

Quote:
Originally Posted by Z15CAM View Post
Ok say you have just set up initial timing at 19 degrees on a BBC with carb adjusted to idle at 800 rpm using a vacuum advance distributer. The vacuum line to the distributer was disconnected and the ported vacuum source on the base of the carb is plugged in this particular example for the purpose of accurate Carb setting and establishing what the engine wants as initial advance (or advance at idle). When you reconnect the distributer vacuum advance the timing will advance and the rpm will increase somewhat. What do you do? Retard the distributer to 19 degrees or turn out the throttle adjusting screw with the anticipation that the vacuum will drop enough to retard the timing back to 19 degrees? I want the engine to Idle at 800 rpm with an Initial Timing (n the case of a Vacuum distributer - Idle Timing) of of 19 degrees.
Turn the throttle down - too much rpm, throttle open too far and the effect is atmospheric pressure rushing in through the venturi to "fill' the vacuum, the ported nipple, above the throttle plate(s), has an induced vacuum, as in an exhaust header crankcase evacuation system.

Quote:
I understand that a vacuum distributer say idling at say 19 degrees will start the engine at a lower timing to make easier starting and less work for the starter in which case this factor could be considered as the initial timing.

Also, is there any benefit, when running long duration cams, to tap a vacuum advance distributer from the manifold vacuum rather then the carb’s ported source? Another thing to consider some Performance Carbs do not have a Ported Source.

I’ve run Mechanical Distributors for so long that I’ve lost some of my neurons concerning Vacuum Distributors - LOL

I think I've answered my own question and that would be to "Retard the distributer to 19 degrees" after reconnecting the vacuum advance; however, I would very much like to here your opinions: Thanks.
Performance carbs and vacuum distributors don't mix. Its an either/or situation, street carb & vacuum distributor or not.

Long duration camshafts like lots of ignition lead at idle, and this can be gotten from using manifold vacuum. The total amount of ignition, #°, can be adjusted by using an adjustable vacuum cannister. Kind of an oxymoron statement, long duration camshaft and idle, two don't mix very well.

Point I'm trying to make is the ported nipple should have no vacuum at idle, throttle plate(s) open too much.

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post #6 of 49 (permalink) Old Apr 16th, 08, 05:36 AM
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Re: Initial timing and Source Vacuum

Quote:
Originally Posted by Z15CAM View Post
By the way Jim what insane compression and cam are you running that likes 27 or 30 degrees of timing at idle? You're communicating with a good old 10.25:1/[email protected] BBC Holley/Muncie 12 Bolt 3.31 Boy? The only thing Vacuum on my ride are the PCV and Power Brakes - LOL
I don't run anything insane at all...lol. Milder than yours in fact.

Current combo is 232/[email protected], 10.5:1.
Previous was [email protected], 9.5:1





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post #7 of 49 (permalink) Old Apr 16th, 08, 05:45 AM
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Re: Initial timing and Source Vacuum

Quote:
Originally Posted by Z15CAM View Post
Ok say you have just set up initial timing at 19 degrees on a BBC with carb adjusted to idle at 800 rpm using a vacuum advance distributer. The vacuum line to the distributer was disconnected and the ported vacuum source on the base of the carb is plugged in this particular example for the purpose of accurate Carb setting and establishing what the engine wants as initial advance (or advance at idle). When you reconnect the distributer vacuum advance the timing will advance and the rpm will increase somewhat. What do you do? Retard the distributer to 19 degrees or turn out the throttle adjusting screw with the anticipation that the vacuum will drop enough to retard the timing back to 19 degrees? I want the engine to Idle at 800 rpm with an Initial Timing (n the case of a Vacuum distributer - Idle Timing) of of 19 degrees.

I understand that a vacuum distributer say idling at say 19 degrees will start the engine at a lower timing to make easier starting and less work for the starter in which case this factor could be considered as the initial timing.

Also, is there any benefit, when running long duration cams, to tap a vacuum advance distributer from the manifold vacuum rather then the carbís ported source? Another thing to consider some Performance Carbs do not have a Ported Source.

Iíve run Mechanical Distributors for so long that Iíve lost some of my neurons concerning Vacuum Distributors - LOL

I think I've answered my own question and that would be to "Retard the distributer to 19 degrees" after reconnecting the vacuum advance; however, I would very much like to here your opinions: Thanks.

I've got a 224į cam in my 10.3:1 406 and my idle timing is over 30į with the vacuum advance.
If your idle speed is too high, close the throttle blades.
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post #8 of 49 (permalink) Old Apr 16th, 08, 06:25 AM
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Re: Initial timing and Source Vacuum

I have an 11:1 SBC. Cam is 234o/244o @.050",.576"/.592"lift, 112 lobe. Sportsman II heads, Victor Jr., etc. Vacuum advance made a huge difference on my car. It got way more streetable and efficient. My baseline timing is 20o initial with 14o mechanical. I run ported vacuum on a BG carb (which has the port higher in the bore).

You do not want your timing moving around at idle. This is why you would also choose the spring in a curve kit that won't start advancing until after idle. The place where the problems will be differences between warm up and cool down. The carb will never bet set right unless you can lock the advance at a consitent number.

Think about it. What happens when you add advance from say 10o to 18o initial? The idle RPM goes up. Did the throttle blades move in the carb? No. So what happens in the idle circuit of the carb to make up for more air being drawn with the same throttle blade position?
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post #9 of 49 (permalink) Old Apr 16th, 08, 06:31 AM
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Re: Initial timing and Source Vacuum

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Originally Posted by lluciano77 View Post
I have an 11:1 SBC. Cam is 234o/244o @.050",.576"/.592"lift, 112 lobe. Sportsman II heads, Victor Jr., etc. Vacuum advance made a huge difference on my car. It got way more streetable and efficient. My baseline timing is 20o initial with 14o mechanical. I run ported vacuum on a BG carb (which has the port higher in the bore).

You do not want your timing moving around at idle. This is why you would also choose the spring in a curve kit that won't start advancing until after idle. The place where the problems will be differences between warm up and cool down. The carb will never bet set right unless you can lock the advance at a consitent number.

Think about it. What happens when you add advance from say 10o to 18o initial? The idle RPM goes up. Did the throttle blades move in the carb? No. So what happens in the idle circuit of the carb to make up for more air being drawn with the same throttle blade position?
You can also select a vacuum can that is fully deployed at idle, as I did. My VA is all in by about 10"hg, and my idle vacuum is over 14"

If you advance the timing and the idle increases, it means that you are running more efficiently. You can then close the throttle blades, which further increases the vacuum.
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post #10 of 49 (permalink) Old Apr 16th, 08, 07:01 AM
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Re: Initial timing and Source Vacuum

I am in agreement with what I'm reading here, especially if your ported vac port has negative pressure the only inference is that the butterfly is too open.

Jim hit on the conundrum dilemna catch22 snafu fly-in-the-ointment. Which is that it will PO the pope and others too when it comes in and out. I've had way too many of em where I got sick of the fight when various components were mismatched. And my solution is no vac advance. High base timing plus centrifugal works fine for me if I am running an HEI. On MSD I run locked out. But I'm not into street manners etcetera like you guys and I readily acknowledge there are gains to be had with vac advance if it can be attained. My zz4 up until I yanked it Sunday thought 20ish and 35ish was pretty fun.

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post #11 of 49 (permalink) Old Apr 16th, 08, 07:16 AM
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Re: Initial timing and Source Vacuum

I'm with JimM. I like manifold vacuum at idle. So, you set base timing at say, 14 degrees (with vacuum advance disconnected), then connect the vacuum advance and timing will advance quite a bit more, usually speeding up the idle a bit too.

At WOT the vacuum advance all goes away, so then you're running solely on mechanical advance.

When I started tuning these types of cars for a living (about a hundred years ago, lol), they pretty much all had manifold vacuum advance at idle. Then, as emissions started to become a consideration, ported vacuum started to be used. Heck, I remember some Ford dizz's that actually had TWO vacuum lines on them, lol.

Here's a nice write-up (the author is the venerable John Z., a longtime member of this site) on timing and vacuum advance:
http://www.csgbenefits.org/Timing_an...dvance_101.pdf


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post #12 of 49 (permalink) Old Apr 16th, 08, 08:29 AM
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Re: Initial timing and Source Vacuum

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Originally Posted by Chevy-SS View Post

Here's a nice write-up (don't know who the author is) on timing and vacuum advance:
http://www.csgbenefits.org/Timing_an...dvance_101.pdf


-
The author is the venerable John Z., a longtime member of this site.

https://www.camaros.net/forums/member.php?u=3459
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post #13 of 49 (permalink) Old Apr 16th, 08, 10:16 AM
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Re: Initial timing and Source Vacuum

Credit given to John, thanks for the info.................

Dave F. in Rhode Island
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post #14 of 49 (permalink) Old Apr 16th, 08, 01:32 PM
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Re: Initial timing and Source Vacuum

John Z's last paragraph reminds me of a story.
Last year I tossed my old Accel distributor for a new MSD. I ordered it from Summit. Of course I ordered the model with vacuum advance. They sent a race model without the advance. I picked up the phone and screamed at them. They got it right.

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post #15 of 49 (permalink) Old Apr 16th, 08, 01:47 PM Thread Starter
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Re: Initial timing and Source Vacuum

Quote:
You can also select a vacuum can that is fully deployed at idle, as I did. My VA is all in by about 10"hg, and my idle vacuum is over 14"

If you advance the timing and the idle increases, it means that you are running more efficiently. You can then close the throttle blades, which further increases the vacuum.
Ah BINGO! That's what I was thinking but couldn't nail it. BEWARE that Longer duration cams = less vacuum and not enough to fully deploy the Vacuum Can at Idle. Nicely explained and I agree if you note an idle increase when advancing timing will indicate the Carb throttle plate position is pretty much right on. This is why I disconnect the Vacuum Advance when tunning a Carburetor especially if it has a Performance Cam.

Stands to reason if you only have say a 14 Hg Vacuum at Idle with throttle plates located at optimum position that the Vacuum canister requires less then that to fully deploy.

Running vacuum distributors are rather an enigma to me and why I personally choose to run a Mechanical. A lot of members are having problems associated with what we are discussing in this thread. However, I am also of the opinion that Vacuum distributors are optimum with vehicles running Automatic Transmissions and Hole Shots on the Street.

GEEZ! After reading JohnZ's "TIMING AND VACUUM ADVANCE 101" Doc makes me wonder if I should go back to a Vacuum Advance Distributer, even if I do run a Manual. I've kept an OLD GM Points Unit in the trunk as back for my Mallory HyFire VI CDI unit and used it with a 244 @ .050" Mechanical Cam, Holley 830Dp Annular Carb, BBC off Manifold Vacuum mainly because there is no Ported Vacuum on it and it blew my mind how good it worked and I'm now getting the PIC as to why. I can see the Mallory Mechanical setting in the trunk as backup in the future - LOL

I also note that he recommends using Manifold Vacuum for a Vacuum Distributer backed up with some very convincing argument.

THANKS Guys for Responses.

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Last edited by Z15CAM; Apr 16th, 08 at 04:46 PM.
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