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Discussion Starter #1
Hi Guys I have a technical question for you guys.
I have been told that once you fit sides to a car you no longer need the vacuum advance. I find it hard to believe. Is this true??? If so. how the hell does the dizzy know what kinda graph to use or do you use the same graph thoughout???? help please as I'm currently sitting with @ roads infront of me. Get a new Dizzy for my race car (R6100) or fit side (R2000 with manifold). Which one should I go for????
 

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The weights in a distributor swing out as the engine speed increase and in the process advance the spark on your plugs to a certain maximum. I don't think the type of carburettor has any influence. It has to do with the burning rate of the fuel. The burning start slowly and then increase exponentially. You need the greater expansion of the heat to be after the TDC of the piston. If it is before you will have knocking or detonation and start breaking things The time the fuel takes to start burning stays the same, but because the speed of the engine change the spark must advance to allow for this period. Many cars came out with distributors without vacuum advances. The reason for the vacuum advance is to help the advance of the spark during open throttle periods. In most vehicles the distributor advance typically increase at a steady rate till 3000 rpm and stay there up to peak revs. This is where electronic systems are much more effective than an old distributor.

I hope I can manage to attach the file.The pressure curves are from a slow speed diesel. But the same principals apply, in a diesel the flame start with injection. This engine turns at 100 rpm and injection can be seen on the TDC line. Up to this stage the pressure increased due to compression. After TDC the pressure increase due to burning fuel. The faster the engine turn the further the injection/spark must advance before TDC to time this pressure curve to be in exactly this position.

Interesting is that at peak pressure less than 20% of the fuel burned. The pressure drop is due to the piston going down. The mean effective pressure determine the engine's power and can be calculated from the area below the graph. But that is another story.
 

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"The burning start slowly and then increase exponentially"..well, actually the time taken for the fuel/air charge to burn is set, it does not change. its this time by which the ignition is advanced so that the max pressure from combustion is reached as the piston starts its downward travels. trouble is, the speed of the engine is not fixed, so the faster it spins, the shorter the length of time for the piston to complete the final few mm's of stroke to get these two events to tie up, meaning you have to trigger the ignition even sooner than at a lower rpm. Hence the need for some form of advance.

As Corrie stated, electronic advance is so much much better than vacuum, not only is it more tuneable, but much less (some cases none) moving parts to sieze, break jam etc.

Save your money for an electronic dizzy....should keep u going dizzy then :)
 

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Discussion Starter #6
it is electric...
I think I made a mistake with my explanation somehow... the vacuum advance on the dizzy doesn't control the curve you guys refer to but the butterflies inside the intake manifold...
 

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it is electric...
I think I made a mistake with my explanation somehow... the vacuum advance on the dizzy doesn't control the curve you guys refer to but the butterflies inside the intake manifold...
It is the other way around. With a larger air speed through the carb, it "suck" more on the advance if the distributor and in the process get the spark a little faster on the advance curve.

An standard test after tuning the '70 Toyotas were to put it in 4th gear at 60 km/h and floor the accelerator. If you could hear it ping the timing was good.

This is also the time when it is easiest to brake a turbo car due to detonation. Often tuners allow to much advance at say 2500rpm and if the turbo already spooled up and the advance is too much, the pressure will be to much before TDC. The pressure will kick back on the piston braking the first ring land on the piston, the conrod, the crank and in the case of an Uno turbo the block.

But all this can happen in an NA engine as well with high compression and low octane fuel.
 

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You can prolly plug the vacuum attachment and lose slightly on part-throttle fuel savings. If you have a vacuum distributor, you can suck on it with the cap off to see how it works.
 

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You can live with only a centrifugal advance if you want to.
Vacuum advance does exactly what the name says advance the timing when there is vacuum in the manifold. Normally when driving with a small throttle opening.
Depending on the position of the vacuum nosily obviously.
The Uno fire use the vacuum advance to advance timing at idle and you set the static timing at 1 deg.
With most other cars the pickup is situated to allow timing advance as soon as you depress the pedal.
Normally at wide open throttle the vacuum in the system falls away and only the centrifugal advance works.
With side draughts you normally have e to work from only one cylinder which tends to pulsate so you do not get a steady vacuum which is why people tend to disable the vacuum advance.
 

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Discussion Starter #10
I can't explain it without showing you a photo...
I'm going to the car this weekend and will take pics.
 

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:thumbs:
 
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