How Ignition Timing Works: Vacuum and Mechanical Advance Explained!

we've all heard the conversations before 12 degrees base timing 35 degrees total timing and have it all in by 2 500 rpms oh and don't forget to disconnect the vacuum advance just what does all this lingo mean i'll help you better understand why you need ignition timing and what mechanical and vacuum advance can do for your engine to understand ignition timing we first need to understand its role in the internal combustion engine as an engine reaches top dead center on the compression stroke the spark plug fires igniting the air fuel mixture within the cylinder in a perfect world all this would happen at the exact moment that the piston reaches top dead center in the cylinder but every action in the ignition process well it takes a small amount of time to complete time for the signal to travel from the distributor down to the coil even more time for the spark to make its way from the coil to the distributor cap then more time going from the terminal to the rotor and getting directed to the individual cylinder then time for it to travel through the spark plug wire and down to the spark plug itself all these actions take mere milliseconds but when combined with the time that it takes for the air fuel mixture to combust if we fired the spark plug at true top dead center well we'd be late for the party by the time the spark plug fires the piston will have already passed top dead center and it's well on its way down the cylinder leaving you with an incomplete combustion of the air fuel mixture and a significant loss in power by advancing the timing the spark happens earlier giving the signal ample time to reach the spark plug and adequate time for the air fuel mixture to completely burn most engines use 10 to 15 degrees before top dead center of initial or what we call base timing this is done by simply loosening the hold down clamp and rotating the distributor in the appropriate direction as engine rpms increase it takes even more time for the combustion process to complete mechanical advance or what is sometimes referred to as centrifugal advance offers a simple way to adjust ignition timing based on engine rpm a pair of weights and springs mounted to a plate on the distributor shaft control the rate of timing advancement that's applied the design is simple and the rate of advancement can be adjusted by simply swapping out the weights for lighter or heavier pieces or by simply replacing the springs with a different spring rate the total amount of mechanical advance achieved can be controlled by using stop bushings or by modifying the length of the slot that's found in the base plate for example if we started with a base timing of 15 degrees and our combination of weights and springs allowed us an additional 20 degrees mechanical advancement we'd get a total timing value of 35 degrees that is assuming though that the engine was at a high enough rpm to overcome the weights and springs and give us maximum advance since mechanical advance is based on engine rpm alone it won't take into account other variables like engine load this is where that little vacuum advance canister hanging off the side of your distributor comes into play some people may be surprised to know that while cruising down the highway your engine could have as much as 40 or even 50 degrees of total timing without ever having a hint of pre-detonation issues this may seem like an extreme amount of timing but at cruising speeds where the engine is under light load the air fuel ratio approaches the ideal stoichiometric ratio of 14.7 to one air fuel mixture this lean could cause engine damage if it was running wide on the throttle but since it happens at cruising speeds where the load on the engine is low it can actually be beneficial a fuel lean mixture burns slower than a fuel rich mixture because there's more air present than what is actually going to be needed to complete the burn process the additional timing provided by the vacuum advance gives the lean airfield mixture more time to achieve a complete burn thus optimizing efficiency and improving fuel mileage so how does this magical vacuum canister work well it's pretty simple as you're cruising down the highway and it takes less horsepower to move your vehicle the throttle blades only have to be slightly open to maintain the speed as the engine pulls against this restriction it creates a higher manifold vacuum reading this engine vacuum runs from a full vacuum port found on the engine or carburetor to your vacuum canister found on the distributor the vacuum created by the engine pulls on the diaphragm found inside the vacuum canister as the diaphragm moves it rotates a plate which advances or retards the ignition timing as you press on the accelerator and the throttle blades open the engine vacuum reading drops the diaphragm returns to its original position retarding the timing until it's back to its original spot timing is then based solely on initial timing and any mechanical advance that may be applied at that engine rpm some vacuum advanced canisters can be adjusted using an allen wrench through the vacuum line nipple you can increase or decrease the additional amount of timing that the vacuum canister provides with this adjustment a question i hear a lot is can i run vacuum advance in a boosted application in short the answer is yes you can it's perfectly okay to run a vacuum advance in a boosted application such as a turbo or a blower when your engine builds boost the vacuum canister reacts exactly the same the positive boost pressure pushes against the diaphragm preventing it from adding any additional timing once you reach cruising speeds the boost level will drop and the engine will begin to build vacuum pulling on the diaphragm and advancing the timing there are however some concerns when using a vacuum advance with a root style blower like this one in certain situations a slight vacuum can develop between the top of the blower and the carburetor or throttle body the last thing you need is a false vacuum signal adding timing while the engine's under boost to be safe you should always get your vacuum signal from a port that's located under the blower if you're running this type of setup if you spend some time down at the drag strip you'll probably notice that most drag racers don't use vacuum advance this isn't some kind of secret trick or anything it's simply because of the fact that most drag racers if they're any good are going to be a wide open throttle on the engine all the way down the track they wouldn't benefit from a vacuum advance canister simply because they're never going to be in a cruising scenario and they definitely don't care about fuel economy so if you plan on using your vehicle for anything besides a trip down the quarter mile you can definitely benefit from a vacuum advance canister if you enjoyed this video and would like to see more visit our website at msdperformance.com

Channel: MSD Performance Ignition

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Keywords: MSD, MSD PERFORMANCE, MSD Ignition, how ignition timing works, how does vacuum advance work, how does mechanical advance work, how to set ignition timing, msd performance, ignition timing, msd ignition, how to, ignition

Id: msHlcjrnjV0

Length: 6min 18sec (378 seconds)

Published: Fri Jul 31 2020