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Had to do some major thinking and measuring to figure out exactly what clearance I needed for the PPF. First was to locate where the rear differential needed to be then measure how much I needed to take out of the PPF. Looks like 11" will be about right. I cut the aluminum PPF and then bolted it together for further refinements. I will weld it later

To keep the tunnel as narrow as possible, I had to redo the lower tubes and extend them beyound the original intersection with "B2". In addition, I had to narrow the tubes to clear part of the PPF. I really did not like having to do this but after contemplating the tunnel design and bracing, I am OK with it. Plus, I plan to use steel on the sides of the tunnel versus aluminum.

Forgot to post the notching of the tubes I explained above

That last picture looks a little frightening. I am certainly no engineer so full steam ahead. Are you going to attach/weld the PPF to the frame or will it just be connected to the drive train parts? Just my noob self wondering.

It does look a little scary all by itself but doing a few quick calcs actually shows there is minimal difference in actual strength based on how I made the modification. Plus, you have to remember, the transmission tunnel will be framed and boxed in. I am also using 14 ga. 1" square tube versus 16 ga. Below is a little comparison of the difference in moment of inertia which is directly related to strength of the tube.

Started to build out the transmission tunnel. Had to revise tube "Q" to clear the bell housing on the transmission. This lets me set the engine back in the frame.....and should help with weight distribution. My oil pan will be 1" below the frame rails with the current motor mounts. I can live with that.

Well, I've been practicing with my new TIG welder getting ready to tackle the aluminum PPF. I'm teaching myself to weld aluminum and so far so good. You do have to be very careful and use good techniques as you can get a crack right down the middle of what appears to be a good weld. If you do not feed enough filler metal into the joint, the shrinkage will cause a crack every time. After welding, I dye penetrate tested the joint to see if there were any cracks. None found so I am good to go.

Installed the PPF to check alignment.

You do realize that the PPF needs some room to move around, right?
I wouldn't butt it tight against the frame, the drivetrain moves.

Moti

I also have a bit of concern about your tunnel design. By not boxing in the bottom of your tunnel, you are sacrificing chassis strength. The slightly curved bottom tunnel rail (as opposed to one straight line) technically makes the chassis even weaker when not properly triangulated. Not sure if any of that will matter in the real world, but it's not what I would build.

The pictures do not show it but the PPF does not touch the tunnel. It is tight and I may have to do a little adjustments. I have actually considered tieing the PPF to the tunnel but I prefer to keep it isolated. On the second question about boxing in the bottom of the tunnel. I'm not too concerned, as most all car's that I can afford have open bottoms for the transmission tunnel, including the Miata. I've got some more triangulation to do but I expect once I get the full cage installed, the tunnel will be much less critical. I do appreciate your commeents.

Just as a side note -
Take a look at the bottom of the 04-05 Mazdaspeed miata, the stiffest miata of the NA-NB production.

Moti

Well....I've not made too much progress on the frame these last 2 months. I have started to weld out the frame. I know a lot of guys wait later in the build but I am struggling with the rear control arm designs so while I am waiting for parts to build the control arms, I decided to weld. Amazing how much time it takes to weld all the joints. I could sure use a canvas welding hood to get closer to some of the interior tubes. My eye sight is not what is use to be and since I am TIG welding everything now, I need to get closer.

Needed to get back into taking apart the Miata donar to make some room in my garage. It took me quite a while to get the wiring harness free, making sure I tagged all the connectors. Amazing how much wire is in this car and to think what current cars have with all the electonics.

Well....I finally got to a point that I could chop up the donor Miata and make some more room in my garage. It took me longer than expected but with a plasma cutter and sawsall, I managed to cut up the the shell into small enough pieces to load all the parts in the back of my pickup, minus the doors, fenders and hood, of which I will keep as spares for the Spec Miata. I would have never thought I could roll up a windshield...