Understanding your Tune Up

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Coils

Coil's are probably one of least understood items in the ignition system.

Coils are designed to operate just like a camshaft...or within a certain operating or load range.

The windings inside the coil are increased or decreased to operate within an RPM range depending on the RPM and resistance demands within the combustion chamber.

A stock coil is designed to operate best at let's say between 400 and 4500 RPM whereas a Blaster 2 or race type CD coil operates best between say 3000 and 7500.  When demands are placed on this component that exceed the operating band, the efficiency decreases, as does the spark quality. The end result is a poor running engine once you get out of the power band.

So...your coil needs to be matched to your specific demands much the same way as a camshaft.  Installing a Blaster or Accell coil on a stock engine will probably not be an improvement, rather a backwards step.  If we take a Blaster 2 coil for example we find that it is very inefficient at low RPM, the multi spark box (CD Ignition) cloaks this by firing multiple sparks at the plug to make it operate efficiently…kind of a band aid or trade off…but it works.

In running these different types of coils on our distributor machine we have found, just as in proper cam selection there are variables to be considered when choosing the correct coil.

DO NOT be fooled by Claims of 30,000-50,000-80,000V coils, the coil will only produce the voltage it takes to jump the spark from the electrode of the plug to the ground strap. If you plugs are gapped at .028 and your cylinder pressure maxes at day 2500PSI it may only take 20,000V to jump the spark.  If you widen the gap to .050 it may take 30,000V to jump the spark, if the coil can produce the voltage it will fire, if it's not up to the task the engine may mis-fire.  Ignition systems work by creating a resistance, plug gaps, fuel density within the combustion chamber as well as the gap between the rotor and cap contact point all contribute  The higher those resistance points the harder the coil has to work to get that spark to jump and ignite the fuel load.  If the coil is not up to the task it will overheat, drop voltage, skip cylinders and eventually fail.  On the other hand installing a 80KV coil in a stock motor will probably show no improvement other than the coil will run very cool.

On older cars the stock systems leave lots of room for improvement Mopars and Fords run a ballasted power wire to the ignition coil which reduces the voltage to the coil to 5.5-6V, this is great for longevity in a stock application but once you start changing the fuel from 98-105 octane to 93 at best, add 10 to 20% alcohol, fuel pump lubricants, injector cleaner and then remove the lead you better make some changes if you expect it to run properly. Today's fuels require a hotter spark and more initial timing to accomplish the task at hand.  Coil technology is a big part of being able to burn modern fuels and I think you'll agree that they aren't going to get any better in the next 5-10-20 years.  Take a look at a late model vehicle, most have a coil for each cylinder or 1 coil for each pair of cylinders, the computer controls the delay in the firing of each coil to build huge voltage numbers to fire the hottest spark possible in the combustion chamber to light off about any fuel you can pour into the tank. All that combined with a massive quantity of sensors telling the computer everything from O2 readings, engine temp, throttle position, barometric pressures and a whole list of other engine details allows the computer to tune the engine on the fly.  In our classics we have a timing light, a jet pack and a screwdriver.

Just a couple of notes....

*These new square epoxy coils by MSD, Crane, Daytona and a variety of others are not some new engineering feat, they are simple coil designs taken from modern Fords, Dodges and GM cars.

*Most aftermarket coils are Made in China, just because they look like an Andover USA design don't be fooled read the label and know what your buying.  MSD Blaster coils for example were designed and built by Andover for many years, MSD took their technology to China to have them copied and made cheaper...They are NOT an Andover USA Coil.  Pertronix is just as guilty, READ the Label.

*Andover Coils is a USA company that has been involved in the manufacturing and design of high quality Coils for nearly 100 years.  They still produce the Best coils in this industry OEM or Aftermarket all Made by American workers with American raw materials.  Read the label if it says Made in USA it was probably made by Andover.  We use nothing but Andover Coils in all our ignition kits.


Multi spark systems

So now lets look at what happens at the extremes of the MS (multi spark) systems.

Without getting into the intricate circuitry of these performance boxes I'll try to clarify what they do and why.

I've overheard people proclaim that the MS (Multi-spark) boxes put out anywhere from 3 to 50 signals to the plug every time the pickup coil sends a signal.

The facts are....below 3000 RPM the MS units fire the plug about 12 times, over 3000 they simply send one shot to the plug.  When we run these boxes on our Buzz Box with a plug attached you can actually hear the tone of the spark change and it's not subtle, it's a definite change of pitch.


Cap/Rotor Phasing:

This is another part of the ignition tune-up that is almost never addressed.  So called Super Tune shops are aware of this inherent problem with all distributors and it's one of the first things they check.

Phasing is getting the spark signal to the plug correctly by aligning the rotor, cap contacts and reluctor peaks so the spark travels to the plug at the instant that the components are aligned or phased.

Stock distributors almost never do this, the rotor is either not yet at the contact or it has long since past it's optimum point.  What this does is cause the spark to have to jump or arch to the cap, this causes heat energy and overloading of the coil.  If we go back to basic physics we know that  "Energy cannot be created or destroyed it can only change form" so what we do here is convert electrical energy to heat energy and effectively reduce the power of the spark as RPM increases and pressure in the cylinder increases causing a higher resistance for the plugs the spark efficiency is drastically reduced...result....poor performance...lost HP and tork.

The cause of this phasing problem is multiple, the biggest thing we find is the vacuum advance plate on a Mopar distributor is really a poor design allowing it to tip and wobble in the distributor housing....when you get your Stage 2 distributor back from us you'll find that plate welded solid and the advance curve is controlled by the weights and springs. Bad bushings, worn gears, poor factory tolerance, cheap caps and rotors can all contribute to or multiply the phasing problems.

It's very common to find a Mopar distributor with the phasing so far off that at around 5000 RPM and up the rotor will get confused and start arching to either the contact ahead or behind it's position....ever heard this "I have a high speed miss I just can't find".....
 
Buying an MSD or other high dollar distributor does not mean it's in phase, probably no better than a stock distributor.  They all need to be phased and curved to your specific application.

Review

Automotive ignition timing advance consists of three components:
Initial timing, centrifugal advance, and vacuum advance.

INITIAL TIMING - Is the amount of timing in the motor at idle with no vacuum advance connected.

CENTRIFUGAL ADVANCE - Is controlled by the engine RPM, and will increase as the engine's RPM increases. The centrifugal advance is controlled by the weights and springs and limited to a total mechanical advance by various types of stops inside the distributor. The RPM that the total mechanical comes all in at is determined by the engine specs, gear ratio, tire diameter, convertor stall and fuel octane level.

VACUUM ADVANCE - Is a function of the engine manifold vacuum.
As the engine vacuum decreases, the vacuum advance will
decrease. At full throttle, engine vacuum is zero and vacuum
advance is zero so it has no track effect on the timing.

TOTAL ADVANCE - Is the total of initial advance, centrifugal
advance under power. 18* initial plus 18* mechanical equals 36* total timing under power and at RPM.

When using our Stage 2 Distributor the total would be the sum of
the Initial and the Mechanical.

Stage 3 distributors have no advance mechanism they are locked
and the total timing is set and remains the same at any RPM.



The Tune-up

So now we're building a recipe for a performance tune-up and through these discussions we've determined that:

1. Carb selection is done by cam duration, compression ratio, gearing, RPM and CID.

2. Ignition systems need to have the correct coil based on RPM and combustion chamber pressure. A high RPM coil can be made to work well with the addition on a MS box to multiple fire the plug at low RPM and once it gets into the higher end of the RPM band it does the job well with a single spark pulse.

3. Initial Timing is determined by the ability of the carb to read the manifold signal at idle, combustion chamber efficiency and design, cam overlap and cylinder pressure.

4. Phasing of the cap and rotor is critical to ultimate performance and just as important as new plugs and wires.

5. Advance curve of the distributor is critical to overall performance and is also a major factor in selection of fuel octane level.

Although there are many other fine details that need to be looked at for each of the above, this is a simple list giving you a good general scope of the science of a performance tune-up.

How and Why Engine Modifications Affect Timing
Engine Parameter Volumetric Efficiency Flame Front Velocity Combustion  Time Ignition Advance Requirement
Engine RPM VE peaks near torque peak Increased at VE peak Reduced at VE peak Less relative advance at VE peak. However, predominant effect is that more advance is required as RPM increases due to less time for crank to sweep through a given angle - thus requiring spark initiation at a greater angle BTDC.
Increased compression ratio Minimal effect Increased Reduced Less advance
More radical camshaft (increased duration and overlap) Less at low RPM; greater at high RPM Less at low RPM; greater at high RPM Less at low RPM; greater at high RPM More advance at low RPM; less advance at high RPM
Improved exhaust  scavenging or less  back pressure Varies throughout RPM range Lower levels of  exhaust gas residuals in cylinder increases velocity Reduced Less advance within the RPM range where exhaust is most efficient
Improved intake system efficiency (bigger throttle body or low restriction air cleaner) Generally greater at high RPM for H-D� engines Increased Reduced Less advance
Increased fuel octane  No direct effect Reduced; less likely to reach knock limit Reduced More advance; increased knock limit
Air/fuel ratio No direct effect Optimum near stochiometric 14.7 A/F ratio Optimum near stochiometric 14.7 A/F ratio More advance required for rich mixtures
Improved fuel atomization  Minimal effect Small fuel droplets burn faster Reduced Less advance 
Increased intake air temperature Lower Increased; may reach knock limit where end gases ignite Reduced Less advance; lower knock limit as temperature increases
Increased humidity Slight reduction as water displaces air Reduced Increased More advance. Extreme example is water injection used to increase knock limit.
Increased cylinder head temperature   Minimal effect Increased; may reach knock limit where end gases ignite Reduced.  Less advance; lower knock limit as temperature increases
Spark plug position in head; number of spark plugs No direct effect Minimal effect Affected by distance from plug to farthest cylinder wall. Ideal location for single plug is center of squish area Less advance for centered spark plug or dual spark plug designs
Greater bore/stroke ratio Minimal effect unless valve shrouding occurs in large bore designs Short stroke increases rate of compression and results in higher  velocity Large bore requires more time to burn from spark plug to cylinder walls.   Very long stroke or large bore (over-square) engines may require up to 2* more  advance than an equivalent CID engine with optimum bore/stroke ratio 
Combustion chamber design with high squish and swirl  Minimal effect High swirl increases velocity High squish  designs take less time to burn to farthest reaches Less advance for efficient combustion chamber designs


Page 2 Mopar Ignition
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