Understanding your Tune Up


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 range.

The windings inside the coil are increased or decreased to operate within an RPM range depending on the RPM demands of the engine.

A stock coil is designed to operate best at let's say between 400 and 4500 RPM whereas a Blaster 2 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 efficiency. 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.  Blaster or Accell coil on a stock engine will 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 cloaks this by firing multiple sparks at the plug to make it operate efficiently…kind of a band aid or trade off…but a good one.

In running these different types of coils on our distributor machine we have found for most street/strip warmed up motors the Blaster 2 and Accell coils seem to have the best power band for what most of us do, so just as in proper cam selection there are variables to be considered when choosing the correct coil.

One more interesting note here….when we run anyone’s distributor and coil up to 7500 we have never found one that actually works….the plugs become glow plugs like in a diesel and  the combustion chamber becomes a self igniting atmosphere.  We have not yet found a distributor company that will give us the specs on their ignition above 7500.

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 20 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 Mopar distributors (and others) and it's one of the first things they check.

Simply this is getting the spark signal to the plug by aligning the rotor and cap contacts so the spark travels to the plug at the instant that the two components are aligned or phased.

The Mopar 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.  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 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".....
Of course the investment in a high dollar MSD or similar type distributor is a solution and after we run them up on our machine, calibrate them to your specific engine they are excellent, but....most of us here are using our cars as daily drivers and go blast off a few 1/4 passes on the weekend.  This is why we work over the stock distributors and or the Mopar Performance distributors (which are better for more performance orientated cars) and make them work to spec or "Blueprint" them for a fraction of the cost.


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

INITIAL TIMING - Is the point where the distributor is set in the
engine prior to start up. This timing can be advanced or retarded
by moving the distributor.

CENTRIFUGAL ADVANCE - Is a function of the engine RPM, and
will increase as the engine's RPM increases. The centrifugal
advance is controlled by the weights and springs inside the distributor.

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.

TOTAL ADVANCE - Is the total of initial advance, centrifugal
advance, and vacuum advance added together.

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 wherever it's set 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 power range. 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 10� 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

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