vacuum retard dizzy question

vacuum retard dizzy question

Tom Angle
Tom Angle

July 22nd, 2008, 6:42 pm #1

I'm looking for some more info on the rumored vacuum retard dizzy. Does anyone have one? If so, what is the model number? Is it a different vacuum motor or a different dizzy setup all together? and WHERE CAN I GET ONE.

The theory that I've heard is that the dizzy in 78 and earlier have vacuum retard at lower RPM and idle and 79 and later have vacuum advance at higher RMP / full throtle. If this is true, someone who had a 78 or earlier X and removed the vacuum line on the dizzy at idle and pluged the line would have an increase in RPM because the vacuum retard would not be working.

I'm interested because I could use this vacuum retard on my dual carb setup by runing a vacuum line off of the intake to the dizzy so the idle will slow down at a stop sign to an acceptable level.

Thanks for any info
Tom Anlge
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Andrew Patterson
Andrew Patterson

July 22nd, 2008, 7:15 pm #2

The 74-78 NA Spec X1/9s came with a Ducellier distributor which had vacuum retard. I wouldn't think the vacuum can would be able to physically hook up to the Bosch, but who knows. I have spares of both so I might take a quick look.
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Tom Angle
Tom Angle

July 22nd, 2008, 7:16 pm #3

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Mr Vegetarian Liberal Environmentalist
Mr Vegetarian Liberal Environmentalist

July 23rd, 2008, 11:24 am #4

I'm looking for some more info on the rumored vacuum retard dizzy. Does anyone have one? If so, what is the model number? Is it a different vacuum motor or a different dizzy setup all together? and WHERE CAN I GET ONE.

The theory that I've heard is that the dizzy in 78 and earlier have vacuum retard at lower RPM and idle and 79 and later have vacuum advance at higher RMP / full throtle. If this is true, someone who had a 78 or earlier X and removed the vacuum line on the dizzy at idle and pluged the line would have an increase in RPM because the vacuum retard would not be working.

I'm interested because I could use this vacuum retard on my dual carb setup by runing a vacuum line off of the intake to the dizzy so the idle will slow down at a stop sign to an acceptable level.

Thanks for any info
Tom Anlge
Slightly off topic but...

Whenever the term "vacuum retard" is used, someone always pipes-up and tells people to disconnect it and that it's worthless etc. So before that happens /again/:

There's lots of misinformation orbiting the dizzy vacuum-advance/retard subject. Some are even of the belief that because the term "retard" is used in its description it /must/ be bad and make the car go slower, this - as most know - is utter rubbish. The vacuum advance/retard system is an important component in the ignition system and allows the ignition map to better fit the characteristics of the engine (the distributor is - amongst other things - an analogue computer).

The two separate advance/retard mechanisms in the distributor (centrifugal and vacuum) adjust the ignition timing in response to varying engine conditions. The centrifugal mechanism adjusts the timing in response to changes in engine speed, the vacuum mechanism adjusts the timing in response to changes in engine load.

Comparing the "analogue" distributor to the distributor-less "digital" set-up found in most modern FI cars; the centrifugal mechanism serves a similar purpose to the crank-sensor (in that it can monitor engine speed), and the vacuum mechanism is analogous to the AFM/MAP-sensor (which monitors engine load). Some people would advocate disabling the vacuum mechanism on a distributor (for no discernible reason), but would baulk if you suggested they should disconnect the AFM or MAP-sensor in their lovely shiny new FI car. In general these people should be pointed at and ridiculed (for their own good - tough love).

There is one situation where /not/ using an available vacuum mechanism may be appropriate; and that is under race conditions where an engine spends +95% of its time up above 8k RPM. Under these extreme - and limited - conditions the diaphragm in the vacuum mechanism can flex and vibrate causing the ignition timing to stutter, resulting in a loss of power. In these situations it is common for the vacuum mechanism to be removed entirely and the centrifugal mechanism to be re-profiled to account for the loss of the vacuum mechanism. This only works because the adaptive effect of the vacuum mechanism is less prevalent at very-high RPM. Removal of the vacuum mechanism results in loss of lower RPM torque (which is not as important in high RPM racing situations).

Simply disconnecting the vacuum-hose from the vacuum mechanism of the dizzy is not the same as removing the vacuum-mechanisim and re-profiling the centrifugal mechanism. There is still the same potential for high RPM timing stutter (which should /never/ occur on the road, or even on the track unless the engine is very-highly tuned/stressed - 12k RPM red-line etc.), but you /do/ loose low RPM torque.
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Mike Richmond
Mike Richmond

July 23rd, 2008, 1:15 pm #5

The emission controlled SOHC motors that had vac retard didn't have it 'to match ignition to the characteristics of the motor'. It had vac retard to make the motor run worse, specifically, to reduce combustion temperatures in a crude attempt to meet standards for oxides of nitrogen emissions, NOx being a component of photochemical smog. The distributor without vac would be timed at 10 degrees static, then at idle the vac would produce 0 degrees. This was the same effect produced in the Spider engines of that era by using dual points.

Both systems, not to pun, suck, and both are easily and usefully defeated.

Mike Richmond (having an opinion on this since 1974 when I unplugged my vac hose on a new 128).

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Tom Angle
Tom Angle

July 23rd, 2008, 2:23 pm #6

I currently have a vacume advance dizzy, but no where to hook it up (dual carbs) therefore I am not getting the max amount of curve I could be getting. The mechanical advance weights only give a limited amount of advance. To maintain peak performance at driving speeds and at full throtle the timeing at idle must be set very high so that the advance at speed is correct.

The vacume retard would allow me to pull down the timing at idle and have a normal idle speed and more touque at takeoff. At takeoff, the carbs butterflies open, the vacuum drops, the timing would advance until the mechanical advance takes over and the RPMs increase. Tapping into the intake manifold will only give me vacuum at low speed or idle when the butterflies are closed.

In summary, who has a vacuum retard dizzy I can get?

Thanks
Tom Angle
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Eric Hamilton (Durham NC)
Eric Hamilton (Durham NC)

July 23rd, 2008, 4:12 pm #7

Slightly off topic but...

Whenever the term "vacuum retard" is used, someone always pipes-up and tells people to disconnect it and that it's worthless etc. So before that happens /again/:

There's lots of misinformation orbiting the dizzy vacuum-advance/retard subject. Some are even of the belief that because the term "retard" is used in its description it /must/ be bad and make the car go slower, this - as most know - is utter rubbish. The vacuum advance/retard system is an important component in the ignition system and allows the ignition map to better fit the characteristics of the engine (the distributor is - amongst other things - an analogue computer).

The two separate advance/retard mechanisms in the distributor (centrifugal and vacuum) adjust the ignition timing in response to varying engine conditions. The centrifugal mechanism adjusts the timing in response to changes in engine speed, the vacuum mechanism adjusts the timing in response to changes in engine load.

Comparing the "analogue" distributor to the distributor-less "digital" set-up found in most modern FI cars; the centrifugal mechanism serves a similar purpose to the crank-sensor (in that it can monitor engine speed), and the vacuum mechanism is analogous to the AFM/MAP-sensor (which monitors engine load). Some people would advocate disabling the vacuum mechanism on a distributor (for no discernible reason), but would baulk if you suggested they should disconnect the AFM or MAP-sensor in their lovely shiny new FI car. In general these people should be pointed at and ridiculed (for their own good - tough love).

There is one situation where /not/ using an available vacuum mechanism may be appropriate; and that is under race conditions where an engine spends +95% of its time up above 8k RPM. Under these extreme - and limited - conditions the diaphragm in the vacuum mechanism can flex and vibrate causing the ignition timing to stutter, resulting in a loss of power. In these situations it is common for the vacuum mechanism to be removed entirely and the centrifugal mechanism to be re-profiled to account for the loss of the vacuum mechanism. This only works because the adaptive effect of the vacuum mechanism is less prevalent at very-high RPM. Removal of the vacuum mechanism results in loss of lower RPM torque (which is not as important in high RPM racing situations).

Simply disconnecting the vacuum-hose from the vacuum mechanism of the dizzy is not the same as removing the vacuum-mechanisim and re-profiling the centrifugal mechanism. There is still the same potential for high RPM timing stutter (which should /never/ occur on the road, or even on the track unless the engine is very-highly tuned/stressed - 12k RPM red-line etc.), but you /do/ loose low RPM torque.
Generally I agree with VLE's observation about messing around with the manufacturer's tuning of the system. It's not easy to make a modern engine run better/cleaner/more efficiently across the range of street driving conditions by trying to "improve" the valve/ignition timing or air/fuel mixture, and without some dyno time you're probably kidding yourself. There's a reason why the thing was set up the way it was...

But in the case of the vacuum retard on the '74 1300, I don't think the general principle applies. There's a reason why the thing was set up the way it was, and that reason was to reduce NOx emissions at idle to meet American emissions standards while making minimal changes to the rest of the engine. It retards the timing at idle, producing less complete combustion, hence lower combustion chamber temperatures and less NOx production. The downside of less complete combustion is higher CO and HC emissions, and the belt-driven air-injection pump takes care of this by burning the additional pollutants in the exhaust manifold. That is, the vacuum retard is designed to sacrifice part-throttle power and fuel economy to cut down on the NOx emissions.

It's a question to work out between your conscience and your local smog laws whether you're going to improve part-throttle operation while increasing your NOx emissions, but it's not silly to think about it.

An interesting and somewhat related question is whether the european-spec 1300 uses a different centrifugal advance curve than the US-spec. If not, it's really hard to argue that the vacuum retard is an integral piece of the engine's design.

BTW, I'm not a VLE, but I am an LE
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Joined: January 25th, 2005, 3:33 pm

July 23rd, 2008, 5:28 pm #8

I currently have a vacume advance dizzy, but no where to hook it up (dual carbs) therefore I am not getting the max amount of curve I could be getting. The mechanical advance weights only give a limited amount of advance. To maintain peak performance at driving speeds and at full throtle the timeing at idle must be set very high so that the advance at speed is correct.

The vacume retard would allow me to pull down the timing at idle and have a normal idle speed and more touque at takeoff. At takeoff, the carbs butterflies open, the vacuum drops, the timing would advance until the mechanical advance takes over and the RPMs increase. Tapping into the intake manifold will only give me vacuum at low speed or idle when the butterflies are closed.

In summary, who has a vacuum retard dizzy I can get?

Thanks
Tom Angle
I've upgraded to electronic ignition from a later X with vac advance dizzy. Sorry, I already sent the old dizzy off to another X-Web'er.

Can you have a port added to one of your carb's? I did on my 32 DMTR...

Dayton Tom
1974 X1/9

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Tom Angle
Tom Angle

July 23rd, 2008, 6:30 pm #9

I don't think I can. I bought another style 40 DCNF and it had vacuum ports on it, but they were below the butterflies which would (I assume) only provide vacuum when the butterflies were closed.

I'm thinking that some tests need to be conducted on my part to confirm this theory.
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Joined: January 25th, 2005, 3:33 pm

July 23rd, 2008, 6:31 pm #10

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Dayton Tom
1974 X1/9

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