vacuum advance or no vacuum advance?

[mgtony]

Can anyone explain the difference, in simple terms 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]

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]

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]

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]

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.

[Poledriver]

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]

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]

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]

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]

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]

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

[MG Mark]

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

[irelandp]

Vacuum advance or no vacuum advance - it depends on what you are using your car for. If you are racing (full throttle or no throttle) then it makes no difference. If you are driving on the road then the Manchester XPAG tests showed a vacuum advance can mitigate the damaging effects of modern petrol.

It is worth reading the book Classic Engines, Modern fuel. Also see the website. https://classicenginesmodernfuel.org.uk/ It has a super set of pages you can use to plot and compare your centrifugal advance curve.