So I'm getting close to getting everything together for my Megasquirt install.

I do have a question on wiring. I'm installing this on a '79, so I don't want to tap into the stock fuse panel with the old style fuses. Instead, I'm going to wire in a stand alone fuse panel dedicated to the EFI system.

Here is what is suggested on the Megasquirt website:



So here are my questions:

1) What gauge wire should I use off the positive battery post (the one with the 80amp inline fuse). In my search for an 80amp inline fuse, the range seems to be from 8 gauge to 0 gauge. (The fuse box sends power to the MS brain, narrow band 02, injectors, ignition coils, and the fuel pump relay)

2) Can I take the power for the above lead off the jumper post, or should I run it all the way back to the battery. I'm planning on using an enclosed fuse box and have it mounted in the engine compartment, so it would be nice to be able to run it off the post.

3) What kind of junction is used to run the small lead with the 10amp fuse to the ignition switch? Basically, what does that big red dot stand for?


Thanks for any advice.

 

You dont need 80 amps to feed the main board.

MS box itself requires a 3 amp fuse iirc,
fuel pump 10 amps,
fuel inj (high imp) with ign system is again less than 10 amps.

So if you run a 30 amp fuse, you'd be fine. As well, the bosch/hella 4/5 pin relays that are used are normally only rated for between 30-40 amps max.

Think of it this way, most cars have a 90 amp alt, if it required that much current to run the engine, you would turn on the headlights and shortly there after the car would run out of juice and die.

I would actually expect that if you put a, ampmeter on all the power to the MS box, WBO2, injectors, and ign system you would likely see only around 20 amps max, but more likely closer to 10-15.

 

For a 5% voltage drop, this table says that a #8 gauge wire will carry 30A for 11 feet in a 12V DC circuit. If 10% drop is OK, you can use #10.

That big red dot is a splice. Better to run the ignition switch wire all the way to the battery (or jumper post). For the relay panel location, know that current carrying capacities are reduced by heat.

 

I use a table that shows ohms/1000ft for wire gauges.

8AWG shows .6282ohms/1,000ft. So 11 feet s/b about 0.0069ohms.

30amps x .0069ohms = 0.21v drop

I see this chart popping up a few places. Maybe a standard?

AWG Chart

 

Following closely Adam. I got a small used Microsquirt and will jump into that.

 

Is this a roadcar or a race car? chassis & looms all from 79? , all other CE panel stuff staying the same?

I'd propose different for road car vs race car.

Use a Mega fuse for the big fuse - they bolt down - then you have an easy place to tap into.

However the key switch is basically the ignition line - you already have it - so I'd just use that. Its unfused - but that's the way the stock car is wired - I'd say don't mess with it.

Wire guage is all about distance - so depends on the install. I think Stan is right that the 80A is overkill here...

Get a bosch/tyco heavy duty SPST 75A relay (for the main at least). You want this one to last so even if its only supplying 30A get plenty of overhead capacity.

Alan