How deep does an engineer actually go compared to a mechanic who fixes these things daily?

Yeah, that gap is real and it goes both ways. I design suspension components for a living and I can tell you exactly why a control arm has that weird forged shape and where the loads flow through it. But hand me a car I've never touched and ask me to swap a fuel pump, and I'm the guy looking it up on my phone in the parking lot. Your buddy probably spends most of his working week deep inside one or two parts across several vehicle programs. You spend yours touching fifty different systems on fifty different cars. Neither one of you has the full picture, you just have different halves.

The shortest version I can give you: you know how it comes apart and goes back together, he knows why it's shaped that way, what it's made of, and how long it should last. When he talks about load paths, he's basically asking where the force enters a part, where it exits, and whether the material between those two points is thick enough. That's most of what he learned, just applied over and over.

If you want to close some of that gap without a degree, start with statics, then basic strength of materials. That's the foundation underneath almost everything he says. There are free lecture series online that cover both, and you don't need the calculus-heavy versions to get the concepts. A physics 101 level of comfort with forces and moments helps too.

Honestly though, the best teacher you already have is the work itself. Next time something fails, instead of just swapping it, ask why it failed there and not somewhere else. Why did the ball joint wear out but the boot looks fine. Why does that bracket crack every time. Bring those questions to your buddy. Explaining things to someone who actually wrenches is way more fun for an engineer than talking at someone who nods along, and you'll pick it up faster than any textbook.

Agreeing with what the suspension guy said about asking why instead of just swapping. I did exactly that a couple years back when my truck's leaf spring cracked and instead of replacing both sides like I normally would, I asked an engineer friend at work about it. Turns out one side had been carrying more weight for years because of how I loaded the bed, so the steel eventually gave up there first. Never would've thought about uneven loading on my own. Now I catch myself looking at broken parts way differently before tossing them in the scrap pile. The questions really do teach you more than the repair manuals do.

Forty years turning wrenches here, back when we set points and timed engines with a light instead of plugging in a laptop. I'll tell you what worked for me. The old factory service manuals had chapters up front explaining the theory behind each system, fuel, brakes, all of it. Read those and a lot of the engineering talk starts making sense. These days I just ask my nephew the engineer to draw it out on a napkin, same as a foreman did for me in 1978. A sketch of where the forces go beats a semester of math for a garage guy.

This thread is making me feel better about my own situation. I tried one of those free lecture series a while back after my water pump seized and I wanted to know why the bearing died instead of the impeller. Ten minutes in the guy was filling a whiteboard with equations and I bailed. What I acutally got more out of was a cheap used book with lots of diagrams showing wich parts of a component take the squeezing and which take the stretching.

So does the math actually matter for understanding this stuff, or can you get most of it just from pictures and your buddy drawing on a napkin?

Something that helped me bridge that gap a bit was realizing how much of the engineering talk comes down to being picky about units and directions, which as a torque wrench user I was already half doing without knowing it. When my control arm bolt sheared last spring I got out a pencil and drew the wheel, the arm, and arrows for where the road force enters and where it bolts to the subframe, and suddenly his rants about moment arms made actual sense. Sketching the part with arrows on it does more for me than any equation does, though I admit my diagrams look like a toddler drew them.