Forum » EFI Wiring Fundamentals » Wire size

Wire size

EFI Wiring Fundamentals

Forum Posts



Tech Articles

Discussion and questions related to the course EFI Wiring Fundamentals

= Resolved threads

Page 1

What are the effects of using a larger size wire then required? Eg. using say a 18awg wire for say a fuel injector which requires a 22awg? I understand cost/weight and having issues pining it in the connector but would their be any other side effects?


Slightly less resistance in the wire and slightly more power delivered to the device being powered, perhaps a very slight improvement in opening time but I would expect that to be very small.

G'day Brendan.

Just the size and weight, you wont have any other ill effects. Make sure the terminals you have are capable of accepting the larger wire size though.

On the subject of wire size...

I'm in the early stages of moving an old Lotus Twin Cam (Lotus Seven) over to crank-fired ignition and fuel injection. The plan is to get the ignition side (Link Atom) working first (for which I have all the parts other than ignition leads) and then a few months later fuel-inject it. I already have the throttle bodies (Jenvey) and many of the other bits but there is more to collect as money allows.

The first stage - to take place over the next few weeks, is replacing the fuse box and associated wiring with something a bit more appropriate. I have previously done a safe but agricultural-looking wiring job to get the car running. Unfortunately most of the wire used was Narva PVC-coated and sourced from Repco. Budget is still a big consideration but I'm doing the fusebox to relays and switches again (and obviously the new wiring required for injectors/sensors etc) and want something a bit smaller and more flexible. I've settled on Spec 44 wire as it seems to be affordable with most of the performance of the flasher stuff - especially given it is only being used in a wheezy old classic. My question (finally!) is whether AWG12 will be big enough to power the relays, given that some are for the lights, horn and fan? Nothing is fused over 15A so I'm assuming it should be okay. AWG10 just seems to be a bit harder to find from the cheaper/easier to use retailers.

Continuing with the budget theme, I'll be taking most of the wires from the Link harness to a Deutsch HDP20 bulkhead connector so that I wont have to get back under the dash when the fuel injection goes in. Any feedback on whether I'd be better following a different route?

Thanks for any input.

G'day Paul, Love me a Lotus Seven, and a Lotus Twin-cam, hope she's got a nasty cam, and a stainless exhaust so the note is really 'zingy' :-).

12AWG is mostly likely going to be okay, but it'll depend on what is being powered from each relay. But, for example, a couple of 55w headlights will draw around 8 amps, so 12AWG wire supplying a relay switching these will be fine, infact you could run a couple more form that same relay and still be fine :-).

I haven't used an HDP20 connector yet, but am looking at one for a project of my own i the near future, they look like a really great budget orientated solution. I'd say its a really good idea to break out your link harness to one on the bulkhead so you don't need to disturb the under dash wiring when you go EFI.

When you're building the engine side of the harness, I'd put another connector in, most likely close to the intake manifold that breaks out the injector power supplies and signals, and possibly the throttle position signal, sensor supply and ground also. Then when you do install the injected throttlebodies, you only need to build a small sub-harness from this connector to the injectors and TPS. Makes any injector updates down the line much easier too. I'd still put in a coolant temp and intake air temp sensor for your spark only initial stage, as it'll give you some good trimming options and let you log the engine operation :-).

With the headlight wiring, when ever i do a re-wire, i always use individual fuse, relay and circuit for each side (and even hi/lo if applicable). Only the switching feed will be common, and where possoble, the switching feed will be negative. This means a short wont disable the lights, instead bring rhem on, only an open circuit can disable them. I do this as it means if there are any problems, it will only affect one side. Nothing worse than driving or even racing at night and losing all your lights at once.

And if its an old model vehicle, i will do all light switching via relays instead of using the standard light/dip switch for the main current switching. This fixes alot of volt drop issues with the age-ing switches if re-using them. And is generally the better way of doing it in my opinion.

As far as wire size. For headlights i will generally use 1.8mm^2. It is plenty sufficient for the current requirements without beint to big to work neatly. It also fits perfectly with Deutsch green band terminals.

I would agree with Bram but go a bit further, I would normally* run a rather heavy cable from the battery to two banks of relays - one left, one right, and use individually fused relays for each filamant with oversized wiring from the relays to bulbs, and to earth - normally I would have around 0.1V less across the filaments than the battery.

For most folks, this may be overkill but the light output is the cube of the voltage and I was doing a lot of 6ookm dives through the night, so every little bit helped. Note, that was with halogen bulbs, I believe HID systems are more tolerant of lower working voltages.

*haven't bothered with the shopping basket as, despite it being the one I dr4ive most, it is around town and on lite motorways, so not needed - and I drive it much slower.

Thanks for the help guys.

Thanks for the kind words Zac, this Seven is a bit of an acquired taste though as it's a S4 with a big wing on the back. It'd be a lot better looking without the wing but it's factory and the only one. Yep, nasty cam(s) but no stainless exhaust unfortunately. No matter, I can't hear much over the straight-cut gears anyway. The engine's dry-sumped and the car's great fun to drive with a LSD. The distributor ignition is a pain in the arse and the 45mm DCOEs probably need servicing hence the EFI 'project'.

Run from relays at the front of the car of the car will be the ECU, Coil with internal igniters, Injectors, Fan, Horn, High Beam, Low Beam. Pump relays will be in the boot and switched from the ECU relay. Coil and Injector relays will be also switched by the ECU relay but their switch's power supply will also run via toggle switched on the dash so that they can be switched off independently - say if I don't want gas while the ignition is being played with or don't want anything else while the ECU is being played with. Maybe I'll hunt down some 10AWG just to be sure - fibreglass just burns so nicely!

Cheers too Bram. I've got a variation of your set up with High Beam and Low Beam on separate fuses and relays just in case!

Attached Files

With the full front wings and rear spoiler, must be interesting at speed, especially with cross winds...

If you're definitely going throttle bodies and not just 3D ignition, have you checked out the Jenvey Heritage line of throttle bodies; they are designed to look like Weber carbies with the injectors mounted underneath. - how they look on a Mk2 Jag :-) - jump to 6:50 to go straight to the unboxing, looking over and dummy fit on manifold - he seems to have the linkage messed up as, IIRC, the flat lever goes between the clip and adjustment screw.

Cheers Gord. I looked seriously at the Heritage range but ended up going for the normal Jenvey TBP45's. My reasoning was that I wanted to own the fact I'd gone to fuel injection rather than hide it, it was cheaper, the range of injectors is better (and easier to change) and the fuel rail set-up is better etc. So far (as well as the ignition stuff) I have the bodies, trumpets, fuel rail, injectors, TPS, throttle linkage and ITG JC50 carbon tray. Still need to get fuel pump, regulator, temp sensors, filters, hoses and clips etc.... I'm doing the car in stages because it has an ignition problem and I want to use the car without spending any more money on the distributor. My plan is to install the Atom with simple 2D ignition, mimicking the current ignition curve and enjoy the car over summer. Although I do have a Webcon TPS for the DCOEs, I don't want to spend money going 3D as the tune will effectively be thrown away when the 'bodies go on. Then, I'll get the injection done, hopefully early next year. It's quite hard finding places that are happy working on classics and doing injection though.

Sweet - got a set of their SF taper bodies, nice bit of kit.