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mgtony

Original Poster:

2,006 posts

76 months

[news] 
Sunday 19th April 2009 quote quote all
Can anyone explain the difference, in simple termswobble between vacuum and no vacuum advance distributors. Having just seen the new 123 distributors from the mgoc,I'm thinking of going for one, they look pretty good.

GreenV8S

23,471 posts

170 months

[news] 
Sunday 19th April 2009 quote quote all
The ones without vacuum advance don't adjust the timing as the load varies, so you get worse fuel economy off load and they run much hotter at idle. For a road car, you want vacuum advance.

Robert060379

12,234 posts

69 months

[news] 
Sunday 19th April 2009 quote quote all
Non vacume advanced dizzy will make your car lumpy and overheat at anything more than half revs. Only use non-advanced distributers on track day and competition engines that don't need a slow idle.

Poledriver

21,531 posts

80 months

[news] 
Sunday 19th April 2009 quote quote all
They might be that bad if you just take the vacuum parts off a standard dizzy. I've set up 'A' and 'B' series engines in the past with the Aldon vacuumless distributors and they've been perfectly OK for road use. Slightly higher fuel consumption, pops and bangs on over-run but everything else OK. My TVR Wedge has no vacuum advance either and she runs fine at low revs and doesn't overheat!

GreenV8S

23,471 posts

170 months

[news] 
Sunday 19th April 2009 quote quote all
Poledriver said:
My TVR Wedge has no vacuum advance either and she runs fine at low revs and doesn't overheat!
You *should* have a vacuum advance and there is no good reason not to. The engine might not overheat, but it will run hotter. Fuel consumption might not be something you bother about, but it will be worse. Wear and tear on the engine generally will be higher. All for no benefit. The ignition requirements vary with load. With the vacuum advance system the timing is adjusted to suit the load. Without it, the timing will be wrong.
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Poledriver

21,531 posts

80 months

[news] 
Sunday 19th April 2009 quote quote all
GreenV8S said:
Poledriver said:
My TVR Wedge has no vacuum advance either and she runs fine at low revs and doesn't overheat!
You *should* have a vacuum advance and there is no good reason not to. The engine might not overheat, but it will run hotter. Fuel consumption might not be something you bother about, but it will be worse. Wear and tear on the engine generally will be higher. All for no benefit. The ignition requirements vary with load. With the vacuum advance system the timing is adjusted to suit the load. Without it, the timing will be wrong.
Fitting the correct advance weights in a dizzy which had been designed to run without vacuum advance will achieve the correct values of advance for this engine and makes it run far smoother and more predictable, the max advance shold be reached before 3,000 RPM, which is lower than for many small engines,
It was a standard mod for 'A' and 'B' series performance engines and is used often with the RV8 when extracting higher outputs. The reason that manufacturers frown on it is that vacuum advance was primarily an economy device.
Again, sucess with this is reliant upon setting up the distributor correctly, NOT just by removing the vacuum pipe.

Edited by Poledriver on Sunday 19th April 21:03

GreenV8S

23,471 posts

170 months

[news] 
Sunday 19th April 2009 quote quote all
Poledriver said:
It was a standard mod for 'A' and 'B' series performance engines and is used often with the RV8 when extracting higher outputs. The reason that manufacturers frown on it is that vacuum advance was primarily an economy device.
Again, sucess with this is reliant upon setting up the distributor correctly, NOT just by removing the vacuum pipe.
I believe you're missing the point. The mech advance curve ONLY gives you the full load timing. If you don't have a vac advance or any other way to adjust the timing according to the load, then the ignition will be far too retarded under part load which means you use far more fuel than you need to and the engine is very inefficient and runs hotter. If you're talking about a race engine where full load is the only thing you're concerned about then that may be OK, although even there you might still be interested in fuel efficiency and overheating.

For a road car there is every reason to fit a vac advance and no good reason not to; it's a no-brainer.

99hjhm

203 posts

72 months

[news] 
Monday 20th April 2009 quote quote all
My advice is don't buy a MGB 123 dizzy... The advance curves are a load of rubbish!

Get this 123....

http://www.123ignition.nl/id/28.html

I got mine from SC Parts, it comes with vac advance, and the curves are much beter. Forget what MGOC say!


99hjhm

203 posts

72 months

[news] 
Monday 20th April 2009 quote quote all
Vac advance is a simple emission control device, I bet many people are driving their cars around with the vac module broken or disconnected, and don't even know!

It makes the car idle much beter, by raising the rpm and allowing the carb butterfly to be further shut. Saves fuel, but only at idle and small throttle openings.

GreenV8S

23,471 posts

170 months

[news] 
Monday 20th April 2009 quote quote all
99hjhm said:
Vac advance is a simple emission control device
No, it absolutely isn't. The engine's ignition timing requirements vary with load, and if you have the timing optimised for full load at a given rpm then it will be far too retarded for part load operation at that rpm. As a result the engine will run inefficiently, it will use more fuel, it will suffer more wear and it will run hotter. Those are the disadvantages of running without vac advance. There is NO advantage under any conditions, from running without vacuum advance. It's a no-brainer; road cars should have vacuum advance fitted and working.

99hjhm

203 posts

72 months

[news] 
Monday 20th April 2009 quote quote all
rolleyes

eeeeer.... I didn't say remove it! There won't be a issue of overheating either.

byebye

MG Mark

489 posts

104 months

[news] 
Tuesday 21st April 2009 quote quote all

For road cars the vacuum advance gives improved idle cooling and quality, fuel economy, throttle response and driveability, and it enables both spark knock control under full throttle accelerations, and leaner fuelling for light loads.

This is because the vacuum advance allows the distributor to supply optimum spark timing, proportional to both the load and the speed output. Without it, the distributor can only vary spark timing in proportion to engine speed and ignores the need for around 20 degrees of timing advance at light loads. The change in optimum timing at light loads is that when operating at light loads, the mixture is leaner and less dense, which cause the combustion charge to burn slower, which means that to reach peak pressure at the optimum point in the cycle, the spark must be initiated earlier. Failure to do so results in "retarded" spark timing with all the associated issues of higher fuel consumptionand emissions but also, importantly, less power.

All engines are different, and have different spark timing requirements, but they are all the same in that as load is decreased, additional spark timing is required for optimum combustion.
There are only a few applications where vacuum advance is not a benefit - Racing engines, Heavy duty large trucks, and other engines where there is a relatively constant speed and load.

MG Mark
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