We realized early on we were going to need a tubular subframe, and for a long time, we were convinced we should build one ourselves from scratch. V8 Roadsters already sells tubular Miata subframes, since stuffing an LS V8 in this hole, not surprisingly, also runs afoul of the stock subframe. In addition to their V8 conversion subframe, they also sell a naked version, devoid of engine mounts and unpainted, specifically for projects like this. Weld on your own mounts wherever they need to be and get on with your life.
We knew about this subframe, but looking at teeny tiny pictures of it on their website, we thought we could do better. We could eliminate some bends, improve the strength of the subframe, and give ourselves a little more clearance between the low point on the Hayabusa engine and the singe cross tube on V8 Roadsters' subframe.
We actually went so far as to spend an entire weekend building an elaborate subframe jig around a stock NB subframe. The deeper we got into the subframe fabrication project, the more respect we had for exactly how complicated a Miata subframe is. You have to hit 6 mounting points on the body (8 on an NB), four mounting points on the steering rack (with rather complex shapes to match up with the steering rack bushings), and 12 control arm mounting points. Each mounting point requires a different shape, a different sized hole, slot, or threaded fitting.
Finally, elbow deep in the subframe project, we realized we were spending entirely too much effort on what ultimately would be some very minor differences between V8 Roadsters' subframe and the one in our heads. So we just bought one.
In person, full sized, and with a full understanding of what it takes to make one of these things, we were actually pretty happy with how the V8 Roadsters subframe was made. High quality welds and perfect fit at all 22 mounting points. Can't ask for much more than that.
The V8 Roadsters subframe leaves tons of room for the engine, but structurally, it replaces a deeply gussetted crossmember with a single tube. When a noodly Miata body twists the frame rails will try to move in different directions. One going forward just a hair while the other goes backwards just a hair. The crossmember serves a critical role in holding those frame rails together.
Just eyeballing it, the stock crossmember looks much stronger at resisting this flex than the single tube spanning the V8 Roadster's subframe.
A little bit of 3/4″ chromoly tubing solves that problem pretty thoroughly. Moving the frame rails in opposite directions with the naked subframe just requires bending one tube. Now it requires bending that tube, stretching another, and compressing a third. This X-brace should dramatically improve rigidity. The “X” could have gone farther back, but we wanted to preserve the ability to remove the transmission from the bottom.
The triangular gussets aren't hugely important as gussets, but they do a great job of holding engine mounts.
Oh yeah! Engine mounts! How are we mounting this thing? We initially wanted to use Miata engine mounts, since anyone with a Miata already has Miata engine mounts, but in reality, anybody with a Miata already has BROKEN engine mounts, since they all tear in half eventually.
Besides being too big to fit on the passenger's side where the engine is crowding the frame rail, the Miata mounts were designed for mounting at a 45 degree angle, a packaging impossibility with the engine's new location.
We found a more compact, more conveniently horizontal solution in a very unexpected place:
A 1932 Ford Flathead V8 sitting in a corner of Tim's shop. Holding up this massive lump of cast iron are two little rubber mounts.
Surprisingly little mounts, considering the weight they carry. These mounts are still widely available and very cheap. Speedway motors sells this set of two mounts for only $20. Since they're designed to hold a cast iron V8 with cast iron heads, cast iron intake and exhaust manifolds and even two cast iron water pumps that double as engine mounts, they should actually be quite stiff when holding up an entire engine that probably weighs less than a Flathead's carburetor. We'll see…