If you really are doing a full bare bones race car build then a DIY harness would be fine. even easy.

But I'm talking, no lights, bare bones nothing.

Buy an MS3pro with a long harness and that has everything you need. Run it all to the engine and fire it up.

Still more work than a stock harness though.

I would prune the existing harness. It took probably a 100 hours of work, but I yanked 13 lbs of wiring from my Mazdaspeed6.

Nope. We did this to our race car. Took a stock VVT harness down to just the essentials for the engine, then added only our "accessories". Light bar, tablet charger, etc. It was still a decent amount of wiring to add. I think we have a half dozen extra power wires and half dozen extra ground wires.

Starting with a stock harness and shortening is definitely the faster way to go.

Ok, maybe easy wasn't the right word. But I know I've wanted to replace my 25 year old harness many times. Its nasty, and I'm suprised its still holding together. For 100% reliability I would want a new clean harness.

I would run a stand alone, I run an electromotive in my f prod car. I think there are maybe 15 wires coming out of the ecm. Water temp, AIT, Crank sensor, throttle position sensor, and fuel injectors, are all you need to run the engine.

I cornered Ben and Kevin from DIY while I was at PRI on Friday. I talked to them about various things, but a majority of the conversation revolved around wiring options. Long story short, I'm NOT going with a PNP. I'm going to use the MS3 Pro with the 8' harness and use the connectors from the MSM harness.

I really appreciated those guys taking the time to answer my questions and show me some of the features that I was interested in.

You might have already seen this, but Ballenger carries most of the OEM miata connector housings and pins. They also have a couple reasonably priced crimpers that work with the OEM pins.

Ballenger Motorsports - High Performance Electronics

I did a chassis harness once, and am about to do it a second time. I hated it so much, that if I had to put an engine harness together on top of it, I would have farmed it all out, regardless of cost. I find the most manageable is the DIY chassis harness with the prefab engine harness (another Holley EFI user.) Went the DIY route with a board from Leash Electronics to cover the street functions (lights, turn signals, brakes, etc.) It's amazing how quickly it all adds up, even when bare bones. "Oh, add another switch for the data logger. Oops, forgot the one for the two step. Oh, and the fuel pump override switch..."

I love making racecar spec cables. Though mostly what I do are simple cables from a DAQ box to a sensor, on an electronics bench with all the proper tools, with someone elses money. I would hate to do that to my personal car..