Engines produce power by igniting an air-fuel mixture. This requires some serious voltage, with coils or distributors in gas engines stepping up the low battery voltage thousands of times to get spark plugs humming. For optimal performance at varying engine loads, all ignition system parts need to overcome high heat, electrical issues, general wear and tear, and external contaminants.
An often overlooked part are the spark plug wires. They’re some of the last parts car owners and mechanics alike suspect of damage in instances of misfires, rough idling, and power loss. Unless of course they’re cracked, melted, discolored, or have corroded or missing conductors.
Older model vehicles using conventional distributors or separate coils for each cylinder still rely on quality plug wires. Newer cars with the coil on the plug ignition systems do without wires, but any issues will be more costly to resolve.
Spark Plug Wire Basics
Whether they’re called high tension leads, ignition wires, or spark plug wires, these are the car parts that conduct high voltage current from distributors or coils to each separate spark plug. Wires run from the coil pack or individual coils down the side of the engine to the spark plugs.
In older cars, current is sent from distributors near the top of the engine. While some cars have coil-on-plug leads for each cylinder bank. Vehicle and engine age, layout and cylinder count determine how many (if any) wires there are and where they’re located.
Construction and Working Principles
A plug coil wire has a layered construction, with a central conductive core, a suppression and conductive layer to minimize electrical resistance or reduce electromagnetic interference as well as insulative materials and inner and outer jackets to aid in performance and protection. Here’s a more detailed breakdown:
Core
This dictates spark plug wire performance and is engineered either to offer lower resistance to current flow, or limit and prevent EMI and radio frequency interference (RFI) that can interfere with sensitive vehicle electronics, such as radios or sensors. Carbon cores are common in OEM wires, and offer good EMI and RFI suppression, but can struggle with higher resistance, negatively impacting engine performance.
Solid steel or copper cores are more conductive, meaning lower voltage resistance for bigger bangs and improved engine efficiency. They do however have higher interference not suitable for cars with sensitive electronics. A hybrid take is spiral cores. These have alloyed wires wrapped around a core to reduce EMI and RFI while keeping resistance low.
Conductive and Suppression Layers
These wrap around the plug wire core. They come either as carbon layers in OEM applications, or conductive silicone, facilitating high-voltage transfer and suppressing interference signals.
Outer Jackets, Braiding and Insulation
Outer jackets are the visible parts of spark plug wires, usually made of high-strength silicone that protects the inner layers from engine heat, oil, and abrasions.
Below this is a braided fiberglass layer to further increase protection and suppress EMI. An insulative silicone layer under this minimizes heat transfer and keeps the electrical charge within the core, thereby preventing current leakage.
Common Spark Plug Wire Problems
Damaged plug wires reduce combustion efficiency by altering the voltage that reaches spark plugs. Check for visible damage such as cracks, melting, and broken connections caused by aging, constant exposure to high temperatures, engine chemicals, and vehicle vibrations.
Heat damage results in burnt outer jackets, insulative layers, or boots (the insulative sleeves connecting wires and plugs), leading to erratic plug performance and incorrect seating. Excessive vibrations are the main cause for loosening in the electrical connections, meaning more voltage from coils to supply the correct voltage. An increase in abrasions scratches the outer protective spark plug wire layers, grounding the current.
Common engine and vehicle issues include:
- Rough idling – defective wires lead to voltage inconsistency and unstable engine performance commonly seen as engine stuttering and shaking at idle.
- Misfires – misfires happen due to incomplete ignition and combustion, in one or more cylinders. Melted, frayed and damaged wires can’t deliver enough voltage to the plug.
- Hard starts – drained batteries and frayed spark wires are interconnected. Bad wires are a key cause of parasitic battery drain, often resulting in hard starts.
- Surging – this is uneven power delivery manifested as abrupt changes in engine RPMs. While fouled plugs and contaminated fuel are more often the cause, faulty plug wires result in the same issue.
- Power loss and hesitancy – degraded wires cause irregular acceleration and hesitation when working the gas pedal. Low-power sparks also reduce the engine’s potential.
Diagnosing Bad Wires
Ascertain whether you need working wire replacements with basic testing. A digital multimeter attached to each end of the spark cables reads resistance levels. OEM wires with carbon cores should display readouts between 10000 and 12000 Ohms per foot, while some imported vehicles with newer spiral core wires can have resistance levels as low as 500 Ohms per foot.
Additional tools like 12V light testers test the quality of the insulation to rule out current leakage, while inline and inductive spark testers register spark strength and electromagnetic fields respectively to determine the electrical continuity and the condition of the wires.
Key Buying Factors
Vehicle Compatibility
Wires compatible with your vehicle make and model and the engine displacement and cylinder count ensure easy installation, a clean fit, and proper functioning.
OE vs Performance Wire Sets
Restore factory engine performance with compatible OE wire sets. All wires are changed, along with new plugs at the same time. Performance wire sets gravitate toward thicker spiral core wires with lower resistance and controlled electromagnetic fields. These are preferred in engine modifications and rebuilds.
Thickness
Common diameters are 7, 7.8, 8, 8.5, and 9mm with differences largely pointing to thicker insulation layers for improved heat and wear resistance or the inclusion of spiral cores which add 0.5mm to standard core thicknesses. For modified vehicles with high-capacity coils also consider larger insulation thickness to prevent melting and misfires.
Brands and Pricing
Most top-shelf spark plug brands also produce their own lines of corresponding wires. Popular names are NGK, Denso, Delphi, MSD, Accel, and Scott Performance Wire among others. While the parts are relatively affordable for what they are, price differences are determined by the build quality, design specifics (locking terminals, ceramic boots, etc.), heat and resistance ranges, and materials in outer jackets.
