Understanding your Tune Up

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 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. A Blaster or Accel 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 Accel 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 Torque.

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 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.

Review

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
 

Spark Plug Reading    Carb Sizing    Hydraulic Cam Installation and Braking    Mopar Rear Ends    Remove/Install your Distributor

Cross Reference Demon to Holley    Mopar Valve Geometry    Torsion Bar Removal    Mopar Ignition Wiring Diagrams

What size Fuel Pump do you Need    Basics of Cam Specs    Mopar Distributors    Demon Carburetion Sales Policy

Valve Adjustment Chart    Ignition Tune Up Help    Mighty Demon Q&A    Secrets of Cam Design    Voltage Regulators

4 Pin/2 Pin Ballast Resistor    A688 Drop Cylinder Test    A688 Q&A    Computer Controlled Ignition Swap    GM HEI Distributors

Dual Pickup Conversion Instructions