Petrol engines of a certain age, have an vacuum advance on the distributor (the device creating & distributing the spark) but to understand what this actually does we need to discuss engine timing…. briefly, I promise.
The spark that ignites the fuel air mixture does so at a given point during the compression stroke of the piston. This is measured in degrees below top dead centre (BTDC)… why degrees? Because engine timing is measured from the perspective of the crank, which is rotating… hence measurement in degrees. For example: LGLs timing is set to 6 degrees BTDC (for 90-96 octane fuel)
The need to be able to change the point at which the spark is created BTDC is important, as not all the fuel air mixture is burnt immediately. A faster revving engine, has less time to burn all of the mixture before the piston reaches top dead centre (TDC). Conversely a slower revving engine has more time to burn everything before TDC is reached. Both scenarios have their own issues. We won’t go into them, but they’re both suboptimal for engine longevity.
So, for optimal engine performance, the timing of the spark needs to be advanced or retarded in relation to the speed of the engine…. meaning the car needs a mechanism to do this.
This is achieved in 2 ways… with the counterweights in the bottom of the distributor (a black art in its own right) and a vacuum advance.
The vacuum advance is integral to the distributor body and takes a vacuum feed from the carburettor. The higher the air flow through the carburettor (higher revs) the higher the vacuum created. This is directly proportional (or should be) to the advancement / retardation of the timing…. thus the spark and combustion commences at the correct position BTDC no matter what the speed of the engine happens to be. Lesson ends….
The modern replacement to the D25 Lucas distributor I installed when I put the engine back together, meant I couldn’t reuse the copper vacuum pipe. The original copper pipe screws into the carb at one end and the distributor at the other…
The modern distributor has a push fit connector which meant the copper pipe was redundant. Instead I had to use a plastic replacement designed for an MG with a push on grommet each end. It did the job but I wasn’t all that aesthetic.
Recently I’d noticed the plastic pipe had come into contact with the exhaust manifold and had started to melt. As the whole idea of the pipe is to provide a good vacuum, having it melt wasn’t going to help things…. time to find either a better route or a better solution. The second option is why you’re reading this…
I never discarded the original copper pipe as they are almost irreplaceable so I used that as a starting point. I had to consider that if I ever get round to restoring the original Lucas distributor, I’d need the original copper pipe. Anything I do in terms of modification had to be reversible. On on…
Some time later….
Even more time later… and after about a hundred offering ups to the engine bay, I had a new pipe that connected to the advance unit and the carburettor perfectly, whilst avoiding any clashes and opportunities to rub.
…. so that’s why a vacuum advance is important on old school, “suck, squash, bang, blow” engines. Interestingly, a drag racing car needs no advance unit. It’s set to only run either at idle or with throttle wide open….
This is a really good article and helped me a lot. I struggled to find any information on the function of the Vacuum hose. Thanks!
You’re most welcome Jack.