From schematic to 1:1 formboard without leaving the browser: twisted pairs for CAN, boots and transitions on the drawing, Deutsch AS and Souriau 8STA connectors with their cavities and a BOM ready to order. Below, a technical guide to how a competition harness is built.
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A competition harness lives centimetres from the exhaust manifold, soaked in oil and fuel, vibrating for hours, on a calendar that changes it every few weeks.
The PVC on a road-car wire softens at 105 °C and absorbs hydrocarbons. Near the exhaust or the sump the harness needs ETFE, a sealed elastomeric covering and boots bonded with adhesive, or the first long test session strips it bare.
Pressure sensors, thermocouples, lambda and a 1 Mbit/s CAN bus share the harness with injectors and coils switching amps in microseconds. Without twisted pairs, properly referenced shields and physical separation, noise becomes faults nobody can reproduce in the workshop.
Every engine, sensor or PDM change changes the harness. You need to disconnect the engine in minutes, replace one branch without rebuilding the whole thing and know which version each car carries. Without living documentation, the harness is the part nobody dares to touch.
Materials, connectors and build practices that separate a motorsport harness from a conventional automotive one. Written for the person who is going to design it, order it or build it.
The reference wire in competition is M22759/32: tin-plated copper with thin-wall cross-linked ETFE insulation (DuPont's Tefzel, hence the everyday name), rated 150 °C. The insulation is hard, does not melt under the hot-air gun when you recover a covering over it, shrugs off oil, fuel and brake fluid, and its thin wall is what lets sixty wires fit through a size 16 shell. M22759/16 is the same conductor with a normal wall: heavier and bulkier, but more abrasion-resistant and sometimes the only way to reach the minimum diameter a cavity's rear seal demands. For zones above 150 °C the same family offers silver-plated 200 °C variants.
Signals that need a shield (CAN, inductive crank sensors, wideband lambda, low-level analogue inputs) run in M27500: a multi-conductor cable built on M22759 conductors, with a copper braid and an ETFE jacket. The part number encodes gauge, conductor count, inner conductor and shield type, so it is worth storing the full reference in the parts library rather than "22 AWG shielded".
Why not automotive wire? Road-car PVC is rated 85-105 °C, absorbs hydrocarbons and has a wall two or three times thicker; TXL (cross-linked polyethylene, 125 °C) improves the temperature rating but is still bulky and does not resist fluids the way ETFE does. On an engine harness that means 30-40 % more diameter, more weight and connector cavities that no longer seal on the wire.
The outer covering is Raychem DR-25, a 2:1 elastomeric heat-shrink that works continuously between −75 and +150 °C, resists the same fluids as the wire and does not crack with vibration or cold. It recovers at around 175 °C, so you need a temperature-controlled gun: heat it too fast and the surface scorches before the inside has shrunk. With the moulded boots and adhesive it forms a watertight system from connector to connector.
Underneath sit tubings with specific roles. The adhesive-lined family (SCL semi-rigid, ATUM flexible, HTAT for higher temperature) carries an inner adhesive that melts and fills voids: it seals a branch entering a transition, encapsulates a splice or joins two different diameters. Its temperature range is lower than DR-25's, so it always goes underneath, never as the outer covering. RT-375 (PVDF, sold as Kynar) is semi-rigid and transparent: it gathers the wires on the last stretch under the boot and keeps the labels visible.
The braided textile sleeve and polyamide corrugated conduit of road cars are 105-125 °C materials: cheap, repairable and tool-free, but they do not seal, they collect dirt and water and they fray at the cuts. For a trackday car they will do; for a season in a competition engine bay, they will not.
The harness-to-connector joint is closed with a moulded boot of the same elastomer as DR-25, recovering onto the backshell on one side and onto the covering on the other. The usual families are 202K (straight, lipped for the backshell) and 222K/222D (90°, when the harness leaves parallel to a panel) from Raychem/TE, with equivalents from HellermannTyton. Each reference covers a range of backshell and bundle diameters: knowing the connector's shell size and the finished bundle diameter, the boot comes off a table without trial and error.
The boot seals with adhesive: S1125, pre-coated on the "/225" versions or applied as a bead on the lip and covering, or V9500 when more fluid resistance is wanted. Without adhesive it is just a sleeve that moves with vibration and lets oil in by capillary action. Branches are handled with moulded transitions in Y or T form (3xx families, such as 342A), ordered by the diameter of each leg and sealed with the same adhesive.
The two dominant families are Deutsch Autosport (TE Connectivity) and Souriau 8STA (now Eaton). Both derive from MIL-DTL-38999 series I in its lightweight version (series 1.5): triple-start quick-coupling thread, aluminium shell, interfacial seal, rear seals per cavity and removable crimp contacts. Deutsch splits its catalogue into AS (base series), ASL MicroLITE (lighter, with size 23 contacts), ASX and ASDD (double density for ECUs and data units) and ASHD (power contacts for battery, alternator and PDM).
An 8STA part number reads: 8STA + style digit + shell size + arrangement + contact type + keyway. The style digit identifies the body: 8STA0 oval-flange receptacle for panel or bulkhead (the classic firewall pass-through), 8STA1 in-line receptacle for joining two harnesses, 8STA6 straight plug with coupling nut and 8STA7 jam-nut receptacle, according to the manufacturer's catalogue. Shell sizes run from the ultra-miniature 01 (unique to 8STA) to 24, through 02, 06, 08, 10, 12, 14, 16, 18, 20 and 22; Deutsch AS covers 08 to 24. Contacts are the MIL M39029 series: 22D (5 A, 22-26 AWG), 20 (7.5 A), 16 (13 A) and 12 (23 A), plus the sizes 26 and 23 of the high-density arrangements.
The keyway (position of the shell keys: N, A, B, C, D, plus a universal option on some plugs) stops two connectors of the same size being mated in the wrong place. Because they share a parent standard, an AS and an 8STA of the same size and keyway mate with each other; the caveat is the high-density contacts, size 26 in 8STA and 23 in ASL, which are neither interchangeable nor crimped with the same positioner.
For what does not justify a 38999 (fans, pumps, auxiliary PDM outputs) the budget option is Deutsch DTM (size 20, 7.5 A) and DT (size 16, 13 A): sealed and compatible with M22759 given the right seal. The reference crimp tool is the DMC M22520/2-01 (AFM8) with the positioner for each contact; to extract without damaging the retention clip, the M81969 extraction tools for that size.
| Series | Origin | Shell sizes | Contacts | Sealing | Typical use |
|---|---|---|---|---|---|
| Deutsch AS | MIL-DTL-38999 series I / 1.5 | 08 to 24 | 22D, 20, 16, 12 (M39029) | Interfacial seal and rear seals | ECU, sensors, bulkheads |
| Deutsch ASL MicroLITE | Derived from AS, lightened | 06 to 24 | Size 23, high density | Yes | Weight and space saving |
| Deutsch ASX / ASDD | Double density | Per arrangement | Many small contacts | Yes | ECU, data logger, displays |
| Deutsch ASHD | High current | Per arrangement | Power contacts | Yes | Battery, alternator, PDM |
| Souriau 8STA | MIL-DTL-38999 series I / 1.5 | 01 to 24 | 26, 22D, 20, 16, 12 and power | Yes | Same field as AS; 01 for sensors |
| Deutsch DTM / DT | Industrial automotive | 2 to 12 way | 20 (7.5 A) / 16 (13 A) | IP68 with seals | Accessories, auxiliary outputs |
A competition harness is not bundled: it is concentrically twisted. The wires are laid in layers around a core following the 1-6-12-18 sequence, each layer twisted in the opposite direction to the previous one. The result is a round bundle at the minimum diameter for that conductor count, bending equally in any direction with no wire under tension in the bends; if the count does not complete a layer, filler wires are added. It is slower than parallel bundling, but it is what makes DR-25 sit evenly and lets the harness survive vibration.
Lengths are taken on the car or on a 1:1 formboard drawn from it: a pilot wire along the real route, with its cable ties and bulkhead passes, and one measurement per segment between branch points. Each branch gets a service loop to re-terminate the connector without remaking the branch and strain relief at the entry, so the load is taken by the boot and not the crimp. The void between boot and cavities can be filled with flexible potting epoxy that locks the contacts and blocks fluids, at the cost of the connector no longer being repairable.
Marking uses printed heat-shrink sleeves (TMS or equivalent) fitted before the covering is closed, or laser printing on the wire where weight justifies it. Before the DR-25 is closed, continuity is tested pin to pin against the connection table along with insulation between circuits and to the shell, because afterwards there is no way to reach a splice. If a contact has to be changed on a finished harness, backpinning with the correct extraction tool is the only clean way.
The harness is built around an ECU (MoTeC, Life Racing, Bosch Motorsport, Emtron or Haltech, to name manufacturers with public connector documentation) and a PDM that replaces relays and fuses with solid-state outputs controlled over CAN. Display, data logger and expansion modules talk over CAN bus: a 120 Ω twisted pair with a 120 Ω termination resistor at each physical end, short stubs and the shield, if there is one, grounded at a single point. It pays to draw those resistors as components in the schematic: a bus with three terminations or only one fails intermittently.
Grounds are organised by function: analogue sensor references return to the ECU's sensor-ground pins, not to the engine block; injector and coil power grounds go to a solid point on the engine; and the ECU is tied to the chassis at a single point. Physically, injectors and coils run in separate branches from the sensors, and CAN never shares a twist with them.
To change the engine between sessions, the engine harness terminates in one or two bulkhead receptacles (8STA0 or flanged AS) on the firewall: the engine leaves with its wiring and the chassis side is untouched. Under FIA regulations, Appendix J requires a master switch operable from inside and outside that cuts all circuits; that wiring and the extinguisher's are documented and routed separately from the engine harness.
| Product | Function | Temperature range | Where it goes |
|---|---|---|---|
| M22759/32 | Thin-wall wire, cross-linked ETFE, tin-plated copper | −65 to +150 °C | Whole harness |
| M27500 | Shielded multi-conductor | Per inner conductor | CAN, inductive sensors, lambda |
| Raychem DR-25 | Elastomeric outer covering, 2:1 | −75 to +150 °C | Every run between boots |
| SCL / ATUM / HTAT | Adhesive-lined heat-shrink | Lower than DR-25 | Splices and entries, under the covering |
| RT-375 | Semi-rigid transparent tubing (PVDF) | Up to 175 °C | Last stretch under the boot |
| 202K / 222K boots | Sealing and strain relief | −75 to +150 °C | Every connector backshell |
| 3xx transitions | Moulded Y or T branch | −75 to +150 °C | Every branch point |
| S1125 / V9500 | Sealing adhesive | Per data sheet | Boot and transition lips |
| TMS sleeves | Heat-shrink identification | Per material | Every branch, before closing |
Each row is a real need from the guide and the specific feature that covers it. No simulation or automatic calculation: the tool documents what you decide.
Place ECU, PDM, sensors and actuators with their connectors and join the pins. Mark CAN H / CAN L as a twisted pair, add the terminations and the sensor-ground splices. Give every wire its gauge and colour.
Load the engine-bay photo as background, place the connectors where they sit on the car and route the harness with a branch point at every split. Orient the boots and record each segment's length as measured with the pilot wire.
Search the Digi-Key catalogue for the bulkhead 8STA0 or the ECU's AS connector and pull it in with its cavities. Add contacts, seals, boots, adhesive and DR-25 by the metre as parts, and export the BOM to Excel.
Generate the PDF with the 1:1 Layout, the connection table and the lengths per segment. Save a revision named after the car's evolution. When something changes, new revision.
M22759/32: tin-plated copper with thin-wall cross-linked ETFE (Tefzel) insulation, rated 150 °C and resistant to oils and fuels. M22759/16 is the same conductor with a normal wall, for more abrasion resistance or when the connector seal needs a larger diameter. Sensitive signals (CAN, sensors, lambda) run in M27500 shielded cable.
For competition, DR-25: an elastomeric heat-shrink rated −75 to +150 °C, resistant to fluids and abrasion, which with moulded boots and adhesive forms a system sealed from end to end. Braided sleeve and corrugated conduit are cheaper and repairable, but they do not seal, they collect dirt and they are designed for lower temperatures.
Both derive from MIL-DTL-38999 series I and share shell geometry, keyways and most contacts and tooling, so an AS and an 8STA of the same size and keyway mate with each other. 8STA goes down to the ultra-miniature 01 shell and uses size 26 contacts in some arrangements, while Deutsch's ASL MicroLITE series uses size 23; those contacts are not interchangeable.
The digit after 8STA is the body style: 0 oval-flange receptacle for panel or bulkhead, 1 in-line receptacle, 6 straight plug with coupling nut and 7 jam-nut receptacle, according to the manufacturer's catalogue. Then come shell size, contact arrangement, contact type (P pins, S sockets) and keyway letter.
It means laying the wires in ordered layers around a core (1-6-12-18), each layer twisted in the opposite direction to the one before. The bundle comes out round, at minimum diameter, flexible in any direction and with no wire under tension in the bends. It takes longer than a loose bundle, but it is what lets the DR-25 sit evenly and lets the harness survive vibration.
On the car, running a pilot wire along the real route with its fixings and service loops, or on a 1:1 formboard drawn from those measurements. They are documented per segment (branch point to branch point, and each branch point to its connector), not as a single total, because that is what the bench needs.
Yes. In the schematic you define connectors, splices, twisted pairs for CAN and termination resistors; in the Layout you draw the 1:1 formboard with branch points, orientable boots and the engine-bay photo as background; in Parts you bring Deutsch AS, 8STA or DTM connectors in from the Digi-Key catalogue with their cavities; and you export the BOM to CSV or Excel and PDF drawings with lengths and connection tables. harnessPRO does not simulate the circuit or calculate currents: you document gauges and lengths yourself.
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