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Old 05-03-2011, 07:03 PM
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Vac advance vac source

I have my vacuum advance hooked up to a timed vac source on my carb right now, but i keep reading people say that it needs to be on a full vac source. What is the purpose or difference in hooking it up different? Also should i set my initial timing with the vac advance hooked up at idle? how do you set total timing? with the vac hooked up or unhooked?

thanx

also this is a sbc 350

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Old 05-03-2011, 07:54 PM
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Vacuum advance

Always adjust timing with the vacuum advance DISCONNECTED.

Tell me more about the engine then I'll step you thru accurately setting the timing.

Ron
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Old 05-03-2011, 07:56 PM
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roller 350 sbc, .030 over, flat top pistons, vortec heads, comp cams xtreme energy 276hr cam, about 500 lift, rpm air gap intake, holley 670 carb, headman shorty headers, hei dizzy, with external msd blaster coil, with a streetfire ignition box, msd wires delco plugs etc anything else? also should the vac advance see full manifold vacuum or should it be hooked to a timed vac source on the carb?

thanx
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Old 05-03-2011, 07:58 PM
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Start with the vacuum advance blocked off and set your initial and total mechanical. Run the engine under load at various cruising speeds with a vacuum gauge on a manifold source. Make note of the maximum reading.

Vacuum advance should use the manifold vacuum on carbureted applications where the application of vacuum increases the spark advance (any source below the throttle blades). With the engine at cruising rpm (mechanical advance all in), apply the maximum amount of vacuum from your gauge test above to the distributor with a mighty vac or cheap hand pump and gauge. If it detonates or reads higher than you would like, most distributors can be adjusted with a hex key in the vacuum port. Look up yours.

Dial it in until you get your ideal total vacuum advance # and that's it. If you're detonating at cruising speeds, you need to reduce the vacuum advance. My experience is mild builds will be just fine with 50 degrees mechanical plus vacuum at cruising.

The alternative is to set mechanical, drive it, and dial back the vacuum if it detonates. I need more info for that.
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Old 05-03-2011, 08:25 PM
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Vacuum advance

Sounds like a good engine combination. Does the harmonic balancer have have degree marks on it? If not then we need to degree it about every 10 degrees so you can set the timing events.
1. Measure the diameter of the balancer
2. Use the following formula to calculate where the timing marks need to be.
(diameter x 3.1416/360 x degrees)

Example: 8" diameter x 3.1416 =25.13"
25.13/360=.0698
.0698 x 10 degrees=.698"
.0698 x 20 degrees=1.396"
.0698 x 30 degrees=2.094"
.0698 x 40 degrees=2.792"
Take a strip of masking tape and make marks at 0,.698,1.396,2.094 and 2.792".

Align the "0" mark with the TDC groove on the balancer and stick the tape on the balancer going in a CW direction. Make marks with white paint at each of the timing events. (10,20,30,40 degrees).

Now you have an accurate way to measure timing.

With the vacuum advance disconnected set your initial timing at 15 Degrees.
While you watch the white marks raise the engine rpm until the marks stop moving. That's your total timing.

Let me know what the total timing is and at what rpm.

Ron
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Old 05-03-2011, 08:30 PM
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yes its a 8" professional products balancer...ill report back in a few days what my timing is as i lost my timing tab and had to order another...i think my brother said my initial was set at 7....would that affect proformance very much being set that low?
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Old 05-03-2011, 08:45 PM
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Vacuum advance

7 degrees is too low. start at 15* but leave the vacuum advance disconnected until we talk again.

Ron
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Old 05-03-2011, 09:01 PM
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Quote:
Originally Posted by Darlantan
My experience is mild builds will be just fine with 50 degrees mechanical plus vacuum at cruising.
Hopefully you meant a total of both the mechanical AND vacuum advance. The 'fast burn' Vortec heads the OP has needs less timing than most SBC heads- on the order of 32-34 mechanical advance, max.

With a performance cam like the OP has, 10-12 of vacuum advance is all that's needed- this will give a total of around 42 to 46 under light throttle cruise conditions.

Last edited by cobalt327; 05-03-2011 at 09:03 PM. Reason: Correction.
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Old 05-03-2011, 10:13 PM
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Wow, pardon me. Yes, that's exactly what I meant. 50 degrees total with both mechanical and vacuum maxed.

Not familiar with Vortec heads.
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Old 05-03-2011, 10:29 PM
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Quote:
Originally Posted by Darlantan
Wow, pardon me. Yes, that's exactly what I meant. 50 degrees total with both mechanical and vacuum maxed.

Not familiar with Vortec heads.
After re reading the post, I thought that was what you meant.

Besides flowing good, Vortec heads are considered to be "fast burn" heads- meaning they can make max power using less timing than 'traditional' Gen I SBC production heads. They use self aligning rockers from the factory and require a dedicated intake manifold because the bolt pattern is different than previous heads.

CLICK for some more info on Vortec heads and mods to them, etc.
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Old 05-12-2011, 07:01 PM
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ok set timing today, initial is 15 and total mechanical advance is 34, what should be the total with vac advance hooked up? cause with it hooked up my total timing is above 60 when i rev it...does this sound correct? my harmonic balancer only goes to 60 and it goes above that with it hooked up

thanx
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Old 05-12-2011, 07:27 PM
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Quote:
Originally Posted by JCurry
ok set timing today, initial is 15 and total mechanical advance is 34, what should be the total with vac advance hooked up? cause with it hooked up my total timing is above 60 when i rev it...does this sound correct? my harmonic balancer only goes to 60 and it goes above that with it hooked up

thanx
At what rpm did you check your total mechanical advance? Your total timing advance should not be over 53 degrees, 48 degrees is closer to optimum for your engine.
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Old 05-12-2011, 07:34 PM
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Timing

If I'm reading you correctly you set the initial timing at 15 degrees, then reved the engine until the timing stopped increasing and the total timing is 34*. At what rpm did you get total timing? You can increase the initial to 20*( 39* total) without any problem. Experiment some and let the car tell you what it likes.

Adjust the vacuum advance for 10* and try it. You want the total timing to be 45-50*(total plus vacuum advance) at cruise (high manifold vacuum).

Let us know where you end up and how it runs.

Ron
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Old 05-12-2011, 08:05 PM
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I think Ron ment total timing with the vacuum advance hooked up to manifold vacuum at idle, as I did. Cruise manifold vacuum levels will be a few inches higher than just at idle, just remember this. This will add to your total advance when you back out of the throttle, which in turn will burn the additional fuel charge in the cylinders better.

Initial (base) timing set around 15-18 degrees, with all in mechanical timing to 36-38 degrees, by 3000 rpm(vacuum advance disconnected), is really close to what your engine will need.

With the vacuum advance hooked to manifold vacuum, the total timing above the total maximum mechanical rpm, should reach close to 48-51 degrees at idle(hot in neutral).

Try these settings, and let us know how it performs.
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Old 05-13-2011, 01:22 AM
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Quote:
Originally Posted by JCurry
ok set timing today, initial is 15 and total mechanical advance is 34
You have 15 initial- w/o the vacuum advance hooked up, right? And the vacuum advance (when hooked up) is using manifold vacuum? If so, that sounds OK for your cam- it's going to want a good amount of timing at idle.

The total advance of 34 (w/o vac. adv. added in) is the MAX for Vortec heads- so do not go any higher w/the total timing.

Quote:
what should be the total with vac advance hooked up? cause with it hooked up my total timing is above 60 when i rev it...does this sound correct? my harmonic balancer only goes to 60 and it goes above that with it hooked up
60-plus is too much. Because the vacuum advance should add about 10-12 of timing, the total w/the vacuum advance hooked up should be 44-46 or so. You might be able to use more, but at some point you'll start to have a 'surge' at steady/light throttle throttle cruise- that's an indication of excessive timing from the vacuum advance.

Crane has an adjustable vacuum advance can kit that has both the adjustable vacuum advance and the limiter plate- Crane #99600-1, 99600-1 Instructions. This is for a GM HEI distributor.

If the vacuum advance you have comes in OK but gives too much advance (like yours appears to be doing), you can physically limit the vacuum advance can's travel w/a vacuum advance limiter plate like the Crane #99619-1, #99619 Instructions. Also for the GM HEI. Or you can make one.

You should verify that what you are seeing at the damper and timing tab for TDC is actually TDC. DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC.

Last edited by cobalt327; 05-13-2011 at 01:43 AM. Reason: Add links.
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