Our friends at Performance Distributors, led by company president Steve Davis, have been a great source for our ignition issues and education for several years at Power Automedia. We decided to start the new year out right with hitting the performance ignition company up with three basic questions for our personal edification.
What’s the difference between mechanical and vacuum advance?
Performance Distributors: Mechanical advance is a centrifugal weight and spring assembly, located underneath the rotor, that provides the largest part of the
timing curve. The vacuum advance canister, located on the side of the distributor, provides additional timing only when the engine is pulling
vacuum. The mechanical advance along with the initial advance – base timing set at idle – is what gives you total timing.
An example of total timing would be 12° initial, plus 24° of mechanical which equals to 36° total. Vacuum advance is not figured into total timing because it will only function when the engine is not under a load. Example: If your vehicle is cruising on a flat stretch of road or going downhill, vacuum advance will come in and add as much as 15° to the total timing for increased fuel mileage and cleaner spark plugs.
It is not uncommon to see as much as 50° – 52° of timing with vacuum advance. But don’t be alarmed by this being too much timing as the vacuum advance will decrease once you accelerate and put the engine under a load. Under hard acceleration the timing will go back to the original total with no vacuum advance.
Do I time the engine with or without vacuum advance?
PD: Always set your timing with the vacuum advance disconnected. The vacuum line must be plugged.
Where is number one on the DUI distributor?
PD: No particular terminal on the DUI is designated as number 1. When you bring your number one cylinder up to top dead center on the compression stroke, you can drop the distributor in and point the rotor toward any terminal that you want to make number one. Once you establish which terminal on the cap is number one, follow your firing order from that point.
For more information on the DUI distributor from Performance Distributors, make sure to look them up online at performancedistributors.com