So I read quite a number of tunnel plate threads and drew some conclusions.

The C6 plate is 1/8" thick - stiffer but not as good as a heat shield as the C5 plate.

2 C6 plates can be made into one best by bolting them together.

The C5 plate while 1/16" thick has 7 ribs that add strength and it is sheet metal so it is stronger per thickness and it transmitts less heat

Finally air is the best know insulator and most insulation take advantage of the use of air to make it what it is.

So, I got a C6 plate and used it as the base layer and placed the C5 plate below it with the 7 ribs touching it.



I then bolted the two together making a quite stff combination.



The "ribs" are about 1/8" deep - so I took a 4' strip of 1" wide, 1/8" thick aluminum on each side and used it as a spacer for the sides of the two plates.



Turns out that the "ribs" are just slightly less than 1/8" so I just started the rib screws, then layed out the spacer and as I tightened down the rib screws it sort of locked the spacers in place. Note that I only made 2 cuts in the 4' piece - one because it was about an inch too long and the other where it changes direction and begins to widen out at the bell housing area.

I then drilled out the 7/16" big holes and the smaller #1 & #2 placement holes and as far as the replacement plate job - it was complete.

C6 1/8" aluminum, 1/8" air space for heat control, 1/16" steel ribbed plate screwed for additional strength and heat protection.



I then make a "blanket" out of two layers of Reflextic and some aluminum tape and attached that with screws and 4 cross plieces of aluminum and it was done.



Screwed the plate back in the car with out the blanket - then pushed the blanket on and secured it - wa la - making an inexpensive and quite well engineered (IMO) solution to increase both strength and heat resistance.



Some of the C6 guys just might be better off adding a C5 plate below their C6 plates for a best of both worlds solution.

I put the Reflextic up around the tunnel while the plate was out for yet additional heat / sound reduction.

Clearance - I have a Corsa exhaust and while the plate & blanket looks thick (less than 1" total) - nothing close to a clearance issue anywhere along the plate.

 

A vacuum is the best insulator, and the next best thing is argon gas I beleive - this is what they use between double paned windows. Air is a very good insulator, still air that is, however, keep in mind that is exactly what most insulating materials do - trap still air. Most heat insulating materials do this by encasing a porous material to trap air just like your Reflextic. Great in theory, but not sure if it's worth the effort when you could just use insulating heat wrap and use the extra thickness for additional stiffness.

 

KingTut,
I take it that you don't have long tube headers?

 

Brick on homes is not put up against the frame - they use wall ties to make an air space to increase insulation. Certainly no wind will be blowing through that air space with the ribs in there and the narrow opening on the end.

There was no extra effort - it is the only way you can bolt the two pieces together (increasing stiffness over either piece by itself) and once they are made to be one - that is the left over space. The spacer is so that you can bolt it to the car more securely.

And no - I do not have long tube headers - personally I think headers only add both noise and heat with only minor increase in HP. My 3.73 gears give me all the quickness and effective power I want.

So No - I can not comment on long tube header clearance.

I found in another post the interesting thoughts on the tunnel plate having such large holes and such small bolts and low torque holding it in place that it was designed to move and was more for heat shielding.