I decided I HAD to have dual exhausts, not just for performance, but to give the subtle look of performance to the rear end of the car. The problem is, the stock gas tank starts at the passenger side, and extends under the spare tire well, all the way over to the driver's side stock exhaust.
I'm not the only person that feels strongly about dual exhaust I guess. A local Z-shop which has done some Chevy conversions told me that one customer required them to weld in a dummy right-side exhaust, right down to painting the inside black, so it would look like it was active.
My brother (who had done a Chevy Z-conversion) had discovered that a late eighties, early nineties Mazda RX-7 fuel tank was a good shape for a Z-car, and allowed room on each side for mufflers, so I took advantage of his research and scrounged one up at the local junk yard. Unfortunately, the junk yard has to drain all the fuel tanks, so rather than pull out a handy plug to drain it, they drilled a ¾” hole not two inches away. I don't like the idea of welding a gas tank, so I took it to a local radiator shop where they hot-tanked it, and soldered up the hole of my $10 fuel tank for $40.
To use this tank (which is 16.6 gallons – and I’m sure a Wankel RX-7 motor will drain it quickly), it is necessary to dispense with the spare tire well in the back of the car. I didn't mind this since the car had a neat speaker box back there anyway which limited access to the spare.
So I got out my trusty saber saw and dispensed with the spare tire well, then cut a big disk of aluminum to replace it:
Once that was installed, I made some brackets for the fuel tank out of thin-wall 1" square stock steel.
The straight part bolts to the luggage floor, and the tank is sandwiched between the loops with some foam padding to protect the tank. I didn't want to be too chintzy about the strength of these brackets, they have to hold about 140 lbs of bouncing weight if the tank is full.
In retrospect, it would have been much easier to use an off-the-shelf fuel cell, maybe one would have even fit into the spare tire well.
Once I figured out how to mount the tank, I turned my attention to getting fuel out of it, and sensing the fuel level. The RX-7's are fuel-injected, so their native fuel pump would have been too much pressure. So I decided to machine up a new cover for the fuel tank entry. Then I extended a post down into the tank. On one part if it, I bolted the original Datsun 240Z fuel level sender. This was a great spot for it, since it was previously leaking fuel around the wires when mounted on the outside of the Datsun tank. Then, I bought a fuel pump from a '80-'89 Ford that pumps 50 GPH at 5-8 PSI, and mounted that on the same post. Looks like this:
Then I mounted the whole assembly into the Z:
Later on, I routed that vent line you see to come out the back and not be near any exhaust system components. The Z originally has a complicated fuel vent system that involves a number of hoses to a collection tank over by the fuel filler, then going forward to the motor where it is routed to the intake or crankcase, and the fuel line also has a return line so fuel can recirculate. I deep-sixed all this stuff and ended up with a simple vent line going out back, and a simple fuel line going forward to the engine.
Now it was time to address getting gas into the tank - the fuel filler. With the tank inboard, and a muffler to be residing outboard between the fuel filler and frame, it was necessary to go topsides with the filler. I bought some stainless steel tubing and welded up these parts to start at the fuel filler door, and end up coupling to the rubber filler hose at the Mazda tank. The middle section is shown reversed top to bottom in this picture (oops):
A local muffler shop was nice enough to slightly swage one end of the tubing to match the RX-7 filler hose size. No charge.
I incorporated a filler vent hose from the Mazda tank going to the filler:
With the side panel back in place, and the filler tube carpeted over, and the speaker box, the topside filler isn't very noticeable.
None of the Datsun and Mazda vent or filler hoses matched sizes, adding some complication to the works. As you see, it took a lot of effort in the fuel system area for the simple luxury of dual exhausts. However, time in the shop is mostly fun rather than toil, and I'm glad I went to the extra effort. Every time I walk by the rear of the car, it makes me feel good to see those dual pipes.