I have already posted this on the carb board, but only got one reply, so I am posting it on here. Ok, heres the deal. On your carb you have a vacuum line going to your distributer advance. Every post I have read on this board says your ported advance should go to this distributer advance(the one with no vacuum at idle and increasing vacuum as you press the throttle).right Mine was hooked up backwards, I just posted a post on this. I called the guy tonight that built the car to check and see if he set the timing with the vacuum tube on the distributer advance, or with it off, so I would know if my timing is completely out of whack. He said he meant to hook the distributer advance into the full vacuum (the one with all the vacuum at idle and cuts off as you press the gas) He said he hooked it up like this because he built it for racing and if it was hooked up the other way it would not run right. He said the weights would advance the timing. This contradicts everything I have read on the board. Is this guy full of or, is there any way your car can run right like this. This guy works for a chevrolet dealership and builds engines all the time, so I would think he knows what he is talking about, but I wanted to ask your guys opinions, since there are alot people who know this area very well. If it will work like this, can you please explain to me how. I am confused. Thanks, please reply

 

"normally" it would be hooked up like you say or thought, no vacumn at idle. you can check to see what you have with an advance light. most or all SBCs will want around 35 to 38 total.

 

I perfer hooking up to ported also. 10-12 degrees initial timing at idle. 34-38 degrees at 3000rpm with vacuum advance disconnected.

Lonnie

 

There is no way to get more vacuum as you open the throttle, unless something crazy is happening. The vacuum advances the timing at idle to help it start and run at part throttle. It should not be connected when checking base timing. At WOT there is little or no vacuum so the only thing advancing the timing is the weights and springs.

 

ATO MONKEY
This quote comes of of the carb board under the post that says welcome to the carb board please read. It is the third paragraph up from the bottom in the non computer controlled hei setup for performance section. Here it is "The vacuum advance can should be plugged into a "ported" vacuum source on the carb. This is a vacuum port on the carb that provides NO vacuum at curb idle. When the throttle is opened past idle the vacuum kicks in and starts your advance curve for cruising/part throttle." Besides, all you have to do is unplug your vacuum line on your carb and stick your finger on it. The one one the left has no vacuum at idle and the vacuum kicks in as you press the throttle.( this is ported vacuum). The right side has full vacuum at idle and when you press the gas the vacuum shuts off.

 

I'm going to go aginst the grain here:
I've seen vac advances hooked up both ways, but IMHO, the manifold vac method is correct.

If ported vac was used, why disconnect vac line when checking timing. At idle there's no vacuum there anyway.

If you use ported vac then you will have close to 52 degs of advance at WOT. (12* initial+20 degs mechanical adv+20 degs vac adv)
Isn't around 32* supposed to be optimum for WOT.
If manifold vac was used, at WOT you would be at 32* right??

At highway speed with a light load on eng, manifold vac would be relatively high. Under these conditions a lot of advance would be desireable for MPG. If you stomp on it, it will ping unless the adv backs off. If manifold vac was used, the vac advance would drop off. If ported vac was used, it would advance more.

At idle, a lot of advance will help eng run smoother.
Manifold vac will give you a lot of advance at idle.
If ported vac is used, the only advance you will have at idle is the initial advance. Too much initial advance will cause the eng to kick back aginst the starter.

What am I missing here??

 

ported is fine for a stock engine with a small duration camshaft. Its when you get into larger duration cams that manifold vacuum is required. Because the duration is larger the idle is more erratic. Why?? Because the vacuum at idle is much less. This is bacause the valves are held open for a much longer period of time causing the compression to bleed off. For this reason you must run vacuum at idle. This will allow the vacuum advance to keep the timing higher and better control idle.

 

WHAAAAA??!!!

Well, that's the first time I've heard of that. Is there a venturi somewhere in the carb that makes a vacuum while the throttle is open?(other than the main jet venturi's) I'm with ZZ28ZZ here. Unless that HEI only advances off of vacuum, which unless there are a special few weird ones in the world it advances off of weights, then you want no added advance under WOT. I only know holleys though, so that might be my problem. IMO the only place a Q-jet belongs is in the trash.

 

All non-computerized distributors - points, HEI, GM, F*rd, import, domestic- use weights to advance the timing based on RPM. That's not the issue here. Virtually all non-CC distributors also use vacuum advance also.

Total timing is the sum of base timing (what you set by twisting the distributor); centrifugal advance (RPM dependent - 0° above base at idle RPMS, usually 24° or so at 2500 RPM or so, starting at around 1200 or 1400 RPM); and vacuum advance, which is usually 10° or 12° at vacuum to the dist of above 15" or so.

Vacuum advance was created to add extra advance at high speed, high vacuum operation conditions. This corresponds to cruising conditions. It is not active at WOT. It is not an issue when racing. Its only purpose for existence is gas mileage, and improved tip-in throttle response while cruising.

Originally it was hooked straight to manifold vacuum. This caused problmes because it was active at idle; as engine compression rose after WW2, that metho dof hookup started making cars with the hotter motors ping every single time you step on the gas, until the dist had enough time to back the timing back off. Starting in the mid-late 50s, they added a port in the throttle bore of carbs, on the front side of the bore just above the throttle blades at idle. This port is exposed to atmospheric pressure at idle when the throtlle blade is below it, so there's no vacuum then. However, when the throttles are opened any kind of meaningful amount, the throttle blades move above the port, i.e. the port is now below the blades, so it gets exposed to manifold vacuum. This port method replaced sending raw manifold vacuum to the dist, and solved the pinging at off-idle problem. All cars with vacuum advance have been hooked up this way since at least 57 as far as I know, until vacuum advance disappeared because the ECM acquired the capability to make the necessary adjustments based on throttle position and RPM and vacuum instead of using the mechanical means.

Make sense to everybody now?

 

That makes much more better sense now. Thank you.

 

RB83L69, that does make sense. Here is my newby question. ZZ28ZZ mentioned it earlier. Why does every tune up manual I have ever seen have you disconnect the vacuum line to the distributor when setting timming at idle. What difference does it make if there is no vacuum?

 

Because there is no vacuum at idle as long as the throttle blades are closed , if you raise the idle speed high enough you will ipen the throttle blades and that port will have vacuum. You should disconnect it just to be perfectly sure that you arent getting any vacuum advance while you set your timing.

 

That is the best I have heard it explained. It makes more sense now. :hail: