Ignition/Distributors

ok i just read some stuff on-line about how to set an ignition system and set timing, so here are my opinions and techniques based on my experience and training. for arguement sake we will just refer to the GM HEI coil-in-cap distributors, but the same can be applied to an older ford duraspark or older CEI chrysler ignition systems(not lean burn they are junk).

Anyways, lets start with the coil. your ignition coil is what power’s the spark and does most of the work. It’s basically a transformer that has been modified to convert low voltage/ high amperage to high voltage/low amperage electrical current in order to just the gap at the spark plug(resistance/ohms). it contains two windings internally, a primary winding and a secondary. the primary winding is made of many small windings while the secondary winding is a larger winding with fewer coils. the primary is provided 12 volts to charge up. The primary circuit is then broken and reconnected rapidy via the distributor. this induces a high voltage in the secondary which follows the path to ground across the spark plug gap. obviously now we have to charge and break the primary electrical circuit rapidly for each cylinder- this is where your distributor comes in play. it’s job is to trigger the coil at the right time, distribute the spark, and alter the timing at which the coil fires according to engine load and rpm.

Older distributors did this pretty straight forward, they simply had a set of points based off 8 bumps on a cam(for a V-8) which openned and closed the primary circuit to the coil. the amount of charge time is referred to as dwell, and by varying your gap at the ignition points you effect the quality of spark and performance. these were fine but considered a high maintenance item. back in the day you were doing good to get 10,000 miles before needing a tune up.

In the early 70’s we saw the introduction of electronic ignition, which held your dwell time consistent without need for maintenance, as well as creating a more powerful spark. all of the big 3 did it differently but they all work on the same principles. HEI works via a pick up coil, reluctor wheel, and ignition module. The pick-up and reluctor create together make what is called a Hall Effect. This is used to trigger the spark. The hall effect is a slight voltage signal which is induced in a coil of wire as a magnet is passed infront of it. It works pretty much off the same principle as a magneto. On a GM HEI the pick-up is the piece with 8 points on it below the rotor in the same place as a set of points. you will also see the 8 pointed star on the dist shaft- this is the reluctor which replaces the cam. as the reluctor spins past the 8 points of the pick-up it induces a voltage signal 8 times per revolution. on a V-6 it would be only 6 points…get it?
This signal is sent to an ignition module which when takes battery voltage and dictates to the coil when to fire. on modern fuel injection systems the hall effect is still used in much the same way but usually uses crankshaft position as referrence and the on board computer dictates coil firing, along with fuel injection timing. now it is not hard to understand how having multiple ignition coils and shorter plug wires on modern engines to share the load and deliver a high quality spark is a no brainer on an electronic injection set up ….but i digress that is another arguement.

We have established how the distributor gets the spark going , but how does it alter it to make the engine run fine under all load/throttle positions. well, on most 79 and older electronic ignitions there are two advance circuits, a mechanical advance and a vacuum advance. after 1980 as computer controls were incorporated into vehicles the ignition modules were partially controlled by the computer and….it is a cluster fuck. for performance – again we are dealing with 79- older GM HEI distributors as well as aftermarket performance distributors which omit computer control of the ignition module.

Advancing or retarding timing by the distributor is done by altering the location of the pick-up coil or rotor in the distributor in relation to engine position- and we all know that we time an engine in relation to top dead center(TDC) of number 1 piston. the advance circuits work identical to how the governor and vacuum modulator work on a transmission. mechanical advance=governor/ vacuum modulator=vacuum advance. mechanical advance increase timing in relation to engine speed whereas the vacuum advance increases timing in relation to engine load.

Mechanical advance consists of two small flyweights under the dist. rotor held in by spring tension(2 small springs). as the speed of the engine increases the weights fly out and change the rotor position in relation to the dist cap terminals advancing timing. the amount of advance you get and at what rpm is referred to as advance curve. mechanical advance is altered by changing around weights and springs using aftermarket parts. this timing is a fixed timing once it is set up- it will only change with engine rpm. so once you get it where you want it, that’s where it’s at.

Vacuum advance is a low rpm load sensative advance for part throttle conditions and is hooked to the pick-up coil. A vacuum advance is suppose to be ported vacuum off the carb and gets no vacuum signal at idle. on some stock applications they also put in a vacuum heat switch to block off vacuum to the distributor until the engine reached operating temperature. just off idle, the engine vacuum is high and the canister of the vacuum advance will fully advance- full advance hovers around 52 deg before TDC. as you go to wide open throttle the vacuum advance has no effect cause it is dissengaging from lack of vacuum signal. if it did not your engine would detonate. upon decelleration the advance fully applies but does not detonate the engine as it is a no load condition. as the throttle opens up to compensate for heavy load-vacuum decreases. the vacuum advance does nothing during wide open throttle(WOT)or idle, and does very little above 2000 rpm other than save on gas. the primary job of the vacuum advance here again is to improve mileage and low rpm performance.

Base timing is set by simply unhooking the vacuum advance and using a timing light at idle. the mechanical advance is uneffected at it is not at a high rpm it is at idle. Stock application for timing on a chevy can vary between 4-12 degrees. i like to set base timing using and adjustable timing light so all i have to do is find the 0 deg TDC mark on the timing cover.

In all out drag racing/ serious performance application both the mechanical and vacuum advance are bypasses as they are unnecessary in most cases….or custom taylored with aftermarket ignition boxes. we then set base timing using an adjustable timing light. base timing of 28-35 deg is not uncommon. but here again these engines idle over 1100rpm.

For street performance, i like to run a full vacuum/mechanical advance but may taylor the mechanical advance to come into full timing at the point the powerband on my camshaft/engine hits……and now you can also follow how the right torque converter comes into play as well. everything has to work together and it mostly revolves around the cam you got. overlooking your ignition advance curves on a performance engine is like shooting yourself in the foot. it’s easy horsepower you just have to find it for your application.

For demo derby i prefer omitting the vacuum advance all together. Your not out for a sunday drive and usually it gets in the way of the distributor protector. i simply weld the actuating rod to the vacuum advance and cut the canister off. this keeps the pickup from flopping around. you can simply leave it unhooked as well. i then taylor the mechanical advance to the cam in the engine. i typically set base timing at 15-18 deg and may have a mechanical advance from 28-35 deg. on some engines, i set the distributor by setting it at final advance. i do this simply because the springs i use in some engines on the mechanical advance may vary the ignition timing at idle…….final timing is set by rev’n the engine to high rpm then adjusting the timing light and turning the distributor to set my final advance to where i want it. by doing this the mechanical advance has maxed out and your setting max timing.

if you unplug you vacuum advance and the engine stalls, chances are your base timing is messed up and the carb is jacked. by not having the correct base timing you are putting a halloween mask on the real problem and even though you may get it to work good enough to get around the block- in a derby or anywhere else it will rear it’s ugly head at the worst possible time. an engine should run properly at idle without any vacuum advance.

aftermarket ignition boxes like an MSD or mallory box come in all varietys and price ranges. basically they wire into the system in place of the stock ignition module and can alter just about everything to do with the ignition. i like MSD ignition coils and abuse them, other then that i have had a lot of problems with aftermarket ignition modules that fit in the distributor and either use the stock module or wire in an ignition box. i have ran both MSD and Mallory products as far as they go and they both have their plus and minus.

never weld on a distributor without removing the ignition module. as a matter a fact don’t weld anywhere on an engine without unplugging the ign module. even though the battery ios unhooked for some damn reason the current from the welder will find it and screw it all up!!

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