mil-spec v. auto-spec

[C. Ludwig]

We get a lot of guys asking about mil-spec wiring and connectors. My impression is that it's a buzz word a lot of people hear and they assume it's better. While mil-spec components certainly can be better, why are they better, do you need them, and is the added cost justified?

WIRE
First, let's look at wire and what the difference is between mil-spec and auto-spec (commonly referred to as hookup or primary wire). Mil-spec is actually a pretty generic term. For any given part (wire, heat shrink, connectors, etc.) there may be dozens of military specs outlining different characteristics for different purposes. A wire might have a mil rating for heat resistance. Another wire might have the same heat resistance but has a different mil rating because it has a different sheath that gives off a different chemical when it burns. Additionally, a wire that is not mil rated (auto-spec), may have similar characteristics in terms of heat resistance or load capacity, but it might not meet a mil-spec for vapor emissions when it burns.

What you generally get with a mil-spec wire versus an auto-spec, in terms of actually functionality, is a better conductor. Mil-spec wire, as used in performance auto applications, has tin coated copper strands that are better conductors than the copper standing of auto-spec wire. The individual strands of wire are usually finer, a given gauge of wire having more internal conductors, which results in a better conductor. What this means is that you can usually carry the same load with a smaller wire. For example, instead of using a 20 gauge auto-spec wire for an injector channel, you can use 22 gauge mil-spec. Additionally, the sheath, or outer coating of mil-spec wire is a different material than auto-spec. Usually Tefzel versus common PVC. Tefzel is a trademark of DuPont for ETFE. The Tefzel is tougher than PVC and this allows a thinner coating to provide similar, or better, protection of the wire from abrasion.

The downside to mil-spec wire is the cost. Mil-spec wire is usually 2 to 3 times the cost, per foot, of auto-spec wire for a given gauge. This adds up quickly when building a complete harness. Additionally, the hobbyist level ECUs (Haltech, Microtech, AEM, etc.) that come with auto-spec wire harnesses, generally don't have a mil-spec alternative available from the manufacturer. Therefore a mil-spec harness needs to be constructed from scratch. This negates any volume build discount associated with the manufacturer supplied auto-spec harness and creates an even larger price gap between an auto-spec harness and a mil-spec harness. Another bit of a downside is that the Tefzel coating, being thinner and tougher, requires a specialized stripper to properly strip the wire without damaging the conductors. If you're doing the work yourself, you'll need to invest in some new tooling.

So, essentially, what you gain with a mil-spec wire is a smaller, lighter wire over an auto-spec wire that will carry the same load. This is critical in cutting edge motorsports, where weight and weight distribution are highly critical. Formula car apps, where space is at a premium, are also an obvious application. For the hobbyist, auto-spec is good enough for auto manufacturers that guarantee their products for tens of thousands of miles, so it will work just fine, in terms of electrical performance, for anything you will want to do.

At LMS-EFI we consider value to be a key priority. We can build anything you'd like, but is your money being well spent? If you want mil-spec wire, shrink, and connectors, we can do it, have done it, and have many of those components on hand. Just don't be surprised if we try to talk you into something less expensive that will do the job just as well.

We'll talk about heat shrink tubing and connectors later.