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Practical Wiring - Professional Motorsport: Step 1 - Specifications and Circuit Design

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Step 1 - Specifications and Circuit Design


00:00 - In this section of the course we're going to show you the 10 step process that we use to construct professional motorsport wiring harnesses.
00:07 By following along with this process, the large harness construction and design task is broken down into smaller more manageable steps that when completed in order will result in a high quality reliable wiring harness that is fit for the harshest of conditions.
00:23 The vehicle we're going to be using to demonstrate this process in our first worked example is a Mazda FD3S RX7 that's undergoing substantial modification.
00:33 We do need to address the fact that this is the same vehicle as shown in the club spec harness course.
00:39 As is often the case in the performance automotive world, the plans for this vehicle and the scope of modifications have evolved.
00:46 And a motorsport orientated harness is now going to be the right choice for the vehicle.
00:51 This car is going to be used for generating rotary tuning content for our other courses here at HPA, along with occasional webinar topics specific to the rotary engine.
01:01 As it will be seeing substantial time on our dyno as well as time on the racetrack competing in a street car sprint series, the choice to build a professional level motorsport harness for the vehicle has been made.
01:13 The first step of the HPA 10 step motorsport harness construction process is the circuit design and specification step.
01:21 In this step we'll look at all the electronic components planned for the vehicle that will form the EFI system, determine their power requirements and current draw, how they need to be connected, and when the use of subharnesses is going to make sense.
01:35 Linked below is the circuit specification document for this project.
01:39 But we'll have a look at the design process now and discuss some of the decisions made.
01:43 As mentioned, this vehicle is going to be used as a training tool to generate tuning lessons specific to the rotary engine.
01:51 For this reason, the EFI system needs to be designed to allow the usage of different ECUs.
01:57 To achieve this, we will use two bulkhead mounted interface connectors, which will pass the wiring harness through to the interior of the vehicle.
02:06 This means that we can consider the engine bay side of the harness independently and deal with the ECU interfacing within a small connection harness inside the vehicle.
02:16 The two bulkhead connectors will be separated into one for sensor supply, sensor ground, sensor signals and network communications.
02:24 And one for power supply, power ground, and actuator commands.
02:28 The vehicle will initially be going together with the original cable throttle control and idle speed control solenoid arrangement.
02:35 But there is a planned upgrade to a drive by wire throttle system in the future.
02:39 To accomodate for this, the wiring that connects to the sensors and actuators on the upper intake manifold will be terminated to a single connector, from which a sub harness will be made to connect to the original throttle position sensor, idle speed control solenoid and intake air temp sensor.
02:57 This single connector will also be populated with the required wiring for a second throttle position sensor and the drive by wire throttle motor.
03:06 This will allow just the recreation of this smaller sub harness at a later date when those future modifications take place.
03:14 Another issue we have and one that is often struck is that to meet our timeline for this project, the work on the EFI system wiring is required to begin before all the mechanical parts of the system are in place.
03:26 This includes the single turbo system, the fuel injector set up and the ignition coils.
03:31 These three elements will also be broken out into distinct sub harnesses.
03:36 The turbo system sub harness will need to cater for quite a few sensor measurements, including a wideband O2 sensor, exhaust gas temperature thermocouples, exhaust gas pressure and turbine shaft speed.
03:49 Additional to this there is a future possibility of a requirement for wastegate position to be measured.
03:56 So we'll provide connetors for this also.
03:59 The fuel system sub harness will include four injectors in a staged configuration, having a smaller primary and a larger secondary injector per rotor.
04:07 As well as a fuel pressure sensor.
04:10 The ignition system sub harness will cater for four direct fire ignition coils with a separate connection to the engine block for that secondary winding current.
04:20 Breaking up the harness construction like this may seem overly complicated, but it adds a lot of flexibility for future upgrades.
04:27 It allows us to progress with our harness construction process to meet our deadlines as well as simplifying the harness construction overall.
04:35 This might seem somewhat counterintuitive but it's often easier to build a main EFI harness which has many sub harness break outs and then build the more detailed branch point heavy sub harness sections of the system separately.
04:49 Part of the design process at this stage is specifying the size and type of wire to be used for our harness, as well as the connectors required.
04:57 Getting these decisions made now is essential as we can order our parts earlier, hopefully avoiding delays later on while we wait for parts to arrive.
05:06 For the individual wires within our harness, they will all meet the M22759/32 standard, often what is referred to as Tefzel wiring.
05:17 As well as the excellent temperature rating that this wire gives us, its low coefficient of friction on its insulation will make our concentric twisting much easier and give us the flexibility we require in our harness.
05:30 Also as we're going to be using circular motorsport connectors for many of our connections, this wiring specification is required for the tight dimensional tolerances it gives, making pinning and depinning those connectors possible.
05:44 The critical engine position sensor signals and low level knock sensor signal, will be run in shielded wiring meeting the M27500 standard.
05:53 We will be using two exhaust gas temperature sensors in our design which are a K type thermocouple.
06:00 We need to run dedicated thermocouple wiring for these sensors back to the sensing device which will be in the vehicle interior.
06:08 The wire selected for this is the Omega Engineering TTK24STWSH.
06:14 Which is a twisted shielded pair thermocouple wiring with stranded conductors.
06:20 There are three important factors in the decision to use this wiring.
06:23 The first is that it is stranded so it will be flexible and resistant to cracking from vibration.
06:30 The second is that it is shielded.
06:32 Thermocouple signals are very small and need to run in shielded cable for the best results.
06:38 The third is the PFA insulation material used on these cables.
06:42 It has a very high 260 degree celcius upper working temperature, and a low friction surface that will work well with our concentrically twisted construction.
06:53 An important note about using thermocouples in your EFI system design is that it is best practice to keep an unbroken chain of thermocouple wire material from the sensor to the sensing device.
07:04 Typically this means between the EGT probe in the exhaust and your ECU if it has thermocouple inputs.
07:12 If the thermocouple wiring has to pass through any connectors, you should use pins made of the same material as the wire, being chromel and alumel in the case of the K type thermocouples we're using here.
07:25 Chromel and alumel pins area available for the circular motorsport connectors through which these wires are going to need to pass, so we can keep that unbroken chain back to our bulkhead connectors in the cabin of the vehicle.
07:37 Looking at the sensor section of our EFI design first, with the exception of our thermocouple wiring which will have individual conductors of 24 gauge, everything else will be 22 gauge, including the conductors within our shielded wiring.
07:52 None of the sensors in our system have a high current draw and the wires will be more than adequate in that regard.
07:59 Keeping all this wiring the same size will make our concentric lay up design much easier and is a good fit with the motorsport connectors we're going to be using for our bulkhead and sub harness connections.
08:10 That 22 gauge wiring is going to be able to be reliably crimped into either size 22 or size 20 pins and sockets.
08:19 There will be some variance from this in the size of the wiring used for our actuators.
08:23 Our injectors, idle speed, and boost control solenoids, will all have relatively low current draws and will be run using 22 gauge wire.
08:33 Whereas our ignition coils and future drive by wire throttle motor upgrade will have larger current draws and will be run with 20 gauge wire.
08:42 The stock cooling system on the FD3S RX7 is somewhat of a concern, and we are planning for modifications in the future here to an aftermarket radiator and to aftermarket radiator cooling fans.
08:54 Initially we will use the original fan wiring and have our ECU control the fans via the original relays.
09:01 In the future if these are changed to more powerful cooling fans, we may want the PDU or power distribution unit in the vehicle to be able to control these directly, giving us soft start and speed control features.
09:14 Cooling fans have quite large power requirements and to this end, we will run three parallel 20 gauge wires to each fan for handling this load.
09:25 Keeping this wiring 20 gauge as opposed to stepping up to a larger size, allows us to pass the wires through the actuator bulkhead connector which can only accept up to a 20 gauge wire size.
09:37 Linked below is the circuit design documentation for this project.
09:41 And we recommend you become familiar with it as a good guide to the level of detail required when building a professional motorsport wiring harness.
09:49 This documentation will be built upon and detailed further in step three of the 10 step process.
09:56 But to undertake that task, we need a clear idea of how the harness will physically be routed through the vehicle.
10:01 Which we'll go through the design of next in step two.

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