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Discussion Starter #1
I am installing my new crane distributer tonight and don't know if I should connect the vacume advance or not. Also how much should I allow it to advance?

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Not sure how you change it, but as I understand it (just reading) a hose is conected to the vaccumme canister on the distributor from the intake. As the vaccumme rises (you put your foot into the gas) the canister adds timming to the distributor. As opposed to mechanical advance that uses weights to change timming based on the RPM of the motor causing the weights to fly further out. Unless you want a static timming curve, I would hook it up. I'd also wait for a responce from someone who knows really knows what they are talking about though.
 

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When throttle is closed you have high vaccum in the intake mainfold, thus the vaccum advance at the distributor (through the tube from intake)translates that to littel or zero advance. When you are full throttle you have the lowest level of vaccum in the intake, which translates to the maximun about of vaccum advance to the distributor. This works separate from the mechanical advance and your total advance is the sum of the vaccum and mechanical advance. didin't your distributor come with connection instructions and recommended timing settings? I can't imagine why you would not connect the vaccum advance hose.

Bill
 

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Discussion Starter #4
it did come with instructions but did not say how mechanical and vacume operate together. I thought that vacume was for fuel economy and advaces timing while crusing, but not at WOT Mechanical works all the time. But you set the mechanical and the vacume still will not exceed the set mechanical limit. This is also one of those new crane electronic distributers

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Vacuum advance is controlled by the air flow through the venturies of the carb. When the engine is at high air flow you have the most vacuum advance. It is not controlled by manifold vacuum. If it was you would have maximum advance at idle, not a good thing. To set timing at idle you remove the line and plug it. This allows you to only see the timing without any outside influence.

Bill G
 

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Vacuum advance is load dependant. Mechanical is not. Vacuum advance supplements for needed advance when the dizzy (engine) isn't spinning fast enough to supply advance for high load or part-throttle, low RPM conditions which translate into better fuel economy and lower emissions. You really can't measure it as far as I know. I always understood it opposite from what Bill32 described.

part-throttle or high load=high vacuum and low rpm=more vacuum advance but less mechanical advance
WO throttle=low vacuum=low vacuum advance (all mechanical in at lower RPM, vacuum advance has little to no effect)
 

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Most (all?) carbs have a port for manifold or timed vacuum. Either way Todd has it correct. Under load or WOT the vacuum advance will not have any effect. Under little load it will kick in some advance. At idle it may or may not depending on the carb port you connect the vacuum line to.

This article explains some of it. The basic principles are the same.
 

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just another builder
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traditionally the vacuum advance was used to help with emissions and the such....on my 71 olds, it is used (not limited to) to raise the idle a little when the engine gets hot to try and spin the fan a little faster to help it cool by advancing the timimg...of course this requires other sensors etc...for performance you get all you timimg advance mechanically from the weights and springs...the vacuum is for finer adjustments at idle and the like...i think it is helpful with a bumper cam to help the streetable side...
i could be wrong, but that's my understanding and i'm sure someone here will correct me
 

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As I understand vacuum advance, it provides more advance to provide additional time for the fuel to burn when the engine has high vacuum (low volumetric efficiency). When there is high vacuum, cylinder pressures are lower and this slows down the speed of combustion. It is completely independent of mechanical (or centrifical) which is only a factor of RPM.

Racers don't usually bother with vacuum advance because they spend little time at part throttle conditions. On a street car, it will help emissions and economy. Stock vacuum advance canisters provide quite a bit of advance and may combine with the mechanical and static advance to give too much total advance.

Fordmuscle had a pretty good explanation. Fordmuscle

Crane makes an adjustable unit that limits the vacuum advance.

Crane Vacuum Advance
 

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Advancing the timing during light throttle and part throttle cruise provides smoother operation and better fuel mileage. More advance during idle and slow speed operation helps emissions a bit.

However, having it that far advanced under a load will cause detonation. SO you can't just bump up the timing curve. It would ping every time you stepped on the gas.

The vacume advance mechanism is simply a rubber diaphragm can, with a vacume source on one side and an advance mechanism on the other side. When vacume is high, the diaphragm moves towards the vacume source and advances the timing. When vacume is low, the diaphragm is neutral and there's no change in timing.

How much to advance it depends on a lot of stuff: compression ratio, cam timing, ambient temps, fuel quality, etc. Total Timing = initial+mechanical+vacume.

Here's what I would do. Set 10* initial at the crank. Add 24* in the distributer. Leave the vacume unplugged for now. Once you get the engine dialed in, you can figure out how much total advance you need under load. 34* total is usually about right. Some engines will do better at 36*, and some at 32*. See how much vacume you'll actually get during idle, cruise, and light throttle.

Once that's done, now add the vacume advance. Start by adding 4-6*. Drive it around a while and listen to how it sounds under light throttle changes. If you get pinging, back it off. If not, add another 1-2* and repeat.

I didn't put a vacume advance on mine because I knew the cam would be too radical. Not enough of a vacume signal to bother with it.
 

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I want carpet in my garage
 

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I am running the new Crane Street/Strip distributor. I hooked up the vacum advance and ran it for a while, but shut it off in the end. It did keep the plugs nice an clean but I think it was a little too much advance at cruise and low throttle. I am glad I tried it. It won't hurt to hook it up and keep the dial at 0 so its off.
Sean
 

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Discussion Starter #14
I was reading the manual again and the vacume advance settings only allow 20, 16, or 12 deg advance. I think I will leave it off for a while also the soonest I can get full advance is 2800 rpm. With the initial set at 12 I will have a total of 36 deg at 2800.

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The vacuum advance works off of Bernoulli's principle; where the speed is greatest the pressure is least (vacuum). Therefore, at a high rate of flow (open throttle) the highest vacuum would give the most vacuum advance of the distributor. At higher RPM's (usually above 2500-2800) the mechanical advance kicks in and you will get a combined advance effect.
Hope this helps,
Joe
 

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Originally posted by COBRAORTHO:
The vacuum advance works off of Bernoulli's principle; where the speed is greatest the pressure is least (vacuum). Therefore, at a high rate of flow (open throttle) the highest vacuum would give the most vacuum advance of the distributor. At higher RPM's (usually above 2500-2800) the mechanical advance kicks in and you will get a combined advance effect.
Hope this helps,
Joe
Joe,

I hate to disagree with you but I think you are confusing things a little. What you say is true about how the carb draws fuel through the mains, it's not the same as manifold vacuum. Manifold vacuum is the vacuum generated by the fact that the engine wants to draw air in but the carb restricts the air flow. Close the throttle, lots of mainfold vacuum. the following article should clear things up for you better than I can explain it.Article
 

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I think most vacuum advance mechanisms are plumbed above the throttle plates, ie. you only get vaccum when the tthrottle is opened ( opposed to manifold vacuum where you have vacuum at idle).
This is evidenced by an OEM Boss 351 dizzy with a dual input advance diaphragm: one side to advance under low rpm load, the other to retard under closed throttle/ high vacuum conditions.
 

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I think most vacuum advance mechanisms are plumbed above the throttle plates, ie. you only get vacuum when the tthrottle is opened ( opposed to manifold vacuum where you have vacuum at idle).
This is evidenced by an OEM Boss 351 dizzy with a dual input advance diaphragm: one side to advance under low rpm load, the other to retard under closed throttle/ high vacuum conditions.
 

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Dist vacuum advance goes above the throttle plates; i.e. vacuum under load. Plumbing it below the throttle plates would advance the timing at idle and retard it under load.

Total timing = initial + (vacuum and mechanical @ given rpm/load)

Vacuum advance arose out of the need to have advanvced spark timing before the dizzy was spinning fast enough to advance mechanically. This is why race carbs have no provision for ported (above the throttle plate) vacuum; they have no need for low speed adavance, the engine doesn't operate there.
-Matt
 

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I just asked my wife about this one.
Vacuum advance is typically accomplished though the use of a belt driven brush in the head of the vacuum. When engaged, this brush helps remove dirt and debris from the carpet while providing forward propulsion IE "advance" at the same time.

It's a very fascinating priciple, however my dog does NOT care for it.

Hope this helps
 
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