Mustang Misfires....

Chances are that 98 percent of you, like me, are driving pre-owned 5.0's. This being the case, many Mustang owners could be the proud possessors of a newly acquired car long overdue for an ignition system service. This is exactly what happened to me with a couple of my MM&FF magazine project cars. The first was obtained significantly under the going price because as soon as it was given any right foot the motor missed badly. A look under the hood revealed plug cables in sad shape, a sure sign the rest of the ignition system could also be doubtful. The owner suspected an "expensive-to-fix" problem with the fuel injection; I betted a cheap ignition service was more like it. I refrained from voicing an opinion and the car changed hands at almost a grand under blue book.

As a first move toward replacing all the ignition's serviceable items, the distributor cap was removed. This revealed a rotor arm eroded almost beyond belief and a cap with posts in like condition. A new Pep Boys-sourced cap and rotor were on hand and these were installed. Although I had yet to hit the dyno, a reasonable guess was that the cap/rotor replacement was worth 20-30 hp but this was an ultra-extreme case.

With most of the misfire cured, the next move was to check the plug cables and plugs. The plug cables were the aftermarket carbon string type and a little past their prime. When new, these normally have around 5000 ohms per foot resistance and an ohmmeter check on mine showed that they were above this but not by any great margin.

Next the plugs were pulled, which is always easier said than done, especially with the back cylinders. With gaps eroded to .065 to .070, this looked to be a problem source. Sure enough, a set of new ACCEL plugs (part No. 574 for an '87-91 5.0) fixed the remaining miss. Next I threw in an oil change for good measure then zipped over to Tom Lesperance's Precision Dyno in Belmont just outside Charlotte N.C. A baseline run was done with stock timing, which tends to be too retarded, and then the timing was progressively advanced (12 degrees static in this instance) to give best output.

Cable Clean-up
Back at my shop, I decided to clean up the grubby looking plug leads and to shorten up those that were overly long. The intent here was to make them look tidy for photos and to replace them when conveniently possible. After spending an hour or so cleaning and shortening, my efforts were rewarded with the return of the misfire.

A check with the ohmmeter showed the resistance of a couple of the cables had escalated to better than 200,000 ohms. Obviously, handling the aged cables during cleaning had broken up some of the carbon string cores to the point it rendered them almost open circuit. So new ACCEL wires were acquired and installed.

Back on the dyno the new cables showed up to almost 5 hp increase over the then still functional (though possibly only marginally) carbon string cables of the baseline test (see the Plug Cable Test chart above).

Resistance--How Low Do We Go?
From the foregoing tests it is obvious that very high resistance in the plug cables reduces the plugs' ability to produce an effective spark. So does this mean zero resistance is best? Tests show two factors come into play here. First the law of diminishing returns and second a plug cable with near zero resistance seems to slightly shorten the spark duration. Radio suppression aside and assuming no inbuilt resistance in the plug, the bottom line looks like at least 100 ohms of resistance is needed in the wire (or cap/boot) to get best results.That now leaves us with the question of how much resistance is too much. Tests I ran some years ago on a hopped-up distributor machine indicated that dropping the total resistance in the entire length of the plug cable below about 1500 ohms showed no measurable increase in the rpm to misfire. Looking at the plug cable industry's high-performance offerings indicates they mostly fall between the 45 to 300 ohms per foot range. This means any plug cable made up of any high-quality cable will be between the 100 and 1500 ohm limits that are required.

Running tests on a distributor machine is one thing and on a dyno test motor another. With 40 years and thousands of dyno pulls under my belt I have burned up more plug leads than I care to count. I have back-to-back tested carbon string leads versus expensive low resistance, wire wound leads on numerous occasions and here is what I have found. First, as most of you are already aware, the life of a low-cost carbon string cable is limited. Figure within a year it will cause some moderate but measurable loss in power.When new, a high resistance carbon string wire will, with an otherwise perfect ignition system, deliver within a very small percentage the same power as a much more expensive, low resistance, wire wound cable.

Where the more expensive high performance cables start to pay off is when the rest of the system is less than as new. Also, they do not age and if not subjected to physical abuse will be good for a hundred years. All this means that for a street machine, if you cannot afford anything but the cheap carbon string leads then that is what you must run, but figure on replacing them in about a year. If performance is the goal and the budget allows it, then even the least expensive wire wound-type performance wire is the way to go.

Heat Stress
Dyno testing produces a lot of header heat that, due to lack of the cooling effect of underhood airflow, simply cooks plug boots and cables. In some cases, a set of $100 plug cables may only last a day.

This should explain why I have diligently explored the possibilities of using cheap cables to the full. Being nearest the header, the plug boot itself is usually the first item to electrically break-down from a thermal overload. When it does, the spark can short- cut the plug by jumping to the header. This is especially prominent when bigger race gaps are used on the plugs. The most elementary precaution here is to use cable separators and supports that both minimize unwanted movement and keep the boots as far as possible from the hot headers.The next step, especially for endurance-type situations, is to use plug boots and cables with high heat resistance. Some examples are the Series 9000 cables from ACCEL, the glass-sheathed Livewires from Performance Distributors or the Jacobs ceramic boot high-temp items.

Spark Plugs
When it comes to plug selection, the 5.0 is electrode tip-form sensitive. I cannot pretend to have done extensive research here but a couple of things have come to light that are worth noting. First is the fact that you can install expensive platinum tip plugs and see no better output than with quality conventional ones. While testing on Mark Biddle's dyno at Panhandle Performance in Lynn Haven, Fla., we did find a big gain in output by doing a nitrous Prep on a set of plugs. The form required is as per Fig 3 (page 174). Cutting the electrode this way reduces the temperature of the side electrode, which gets much hotter when nitrous is being used. Especially important, in view of the 5.0's low chamber swirl, is that this move also produces a more exposed spark, so the plug proved easier to fire.

The results of this are a little variable but can be pretty substantial as our test results indicate. The fact that the big gains occur in the lower rpm range is especially important for an AOD car as the 20 or so lbs.-ft. increase right off idle helps considerably with the initial launch.

Module Woes
It seems as though the spark starts to measurably degrade at about 4500-5000 rpm when the stock distributor module is retained. Also, from time to time they have a habit of going bad. Just such an incident happened during the test session that resulted in the plug test just discussed. Mark made the run, gets out of the car and announces the module is on the way out. All I noticed was a drop of some 10-12 hp for some, to me, inexplicable reason. I heard no misfire. Mark installed a new module and, presto--the power came back. The reason it seems that it is not so easy to detect some module maladies is that the spark failure is random among the cylinders, so producing results is like a soft-touch rev limiter.The best fix for this problem and the decaying spark intensity is to install a high-performance module such as offered by Performance Distributors. Their unit, with the resistor in place for extended street usage plug life, runs the system to a solid 7500 rpm. That's about the limit for a stock bottom end. In race trim, which is without the resistor, this module has 10,000-rpm capability. Another module well worth looking at is the one produced by Holley. Part no. 891-105 fits the '84-93 5.0 and will run the ignition up way past the point at which the bottom end becomes self dis-assembling.

Coils
As has just been mentioned, the stock ignition starts laying down on the job at about 4500-5000 rpm. Although best used in conjunction with an improved module, another route to hop up spark output at higher rpm on an otherwise fully functional system is to use one of the aftermarket high output coils. A good coil appears, with all else stock, to push the useful rpm limit to about 5800-6000 with a stock .0050 plug gap.

If you are looking for a straight replacement, then Crane's offering (part No. 730-091) is a well-respected piece within the industry. Also, knowing how much they test on their test rig, another coil worthy of consideration is the Inferno coil from Performance Distributors. Still, these two brands are not your only options. MSD, ACCEL, and Mallory also have high performance coils to meet the needs of both stock and aftermarket ignition systems.

Distributors
We have talked about the negative effects of bad rotors and caps, wayward modules, near open circuit plug cables, burned cables and worn plugs. Is there anything else to go wrong? Yes there is. I saved this for last because it is usually the most expensive to fix.

On a high mileage motor, the distributor shaft bearings and the gear can wear to such an extent that it allows spark-scatter. It is an easy deal to check; all that is needed is a timing light and a clear mark on the crank pulley. First insert the timing check plug (located by the distributor) that bypasses the computer advance curve, then turn the motor up to about 3500-4000 rpm. If the spark jumps around by more than 3-4 degrees, something is amiss and it's most likely worn distributor shaft bearings and/or gear.

The cost of all this may be more than you may think if your motor is power-timed. First understand that with stock iron heads, a 5.0 makes the best power just shy of detonation. If the spark scatters, you will have to back out the timing to deal with the most advanced situation--which means that when the spark scatter is toward the retarded end of the scale it will cost power. The chart on the lower left of page 175 shows what happened when a 4 degree scatter was reduced to just a tad over 1 degree by substituting a selected new distributor body. Other than the body itself all the other components such as module, rotor and cap remained unchanged. The most static timing that could be used before trace detonation was heard was an average (remember it jumped around 4 degrees) just shy of 10 degrees.

After the bearing problem was fixed the timing held constant within just over 1 degree and timing could be bumped to what the motor liked best, i.e., 12 degrees, with detonation just being audible at 13 degrees. If your ignition is scattered then a new distributor is possibly on the cards.Some units come with an upgraded module already installed so if that's a feature you require, check with the company of your choice and see if that is an option.