Last month, we talked about using the Zex Perimeter Plate nitrous-oxide system on a '72 Stingray, and covered the basic installation. Since the Zex system requires 6 psi of fuel running to the fuel solenoid (lest you risk engine damage), I added a low-fuel-pressure warning light and a kill switch to make sure the system would shut down if pressure got too low. In this installment, we'll cover making sure the system gets enough fuel, adding a safety blown-down and purge, and upgrading the nitrous controls in the cockpit.
One of the things I learned early on was that the fuel pump in my car simply couldn't deliver the amount of fuel needed to safely run both the nitrous and the Holley Race Series carb that feeds the small-block. It also had a nasty habit of losing pressure as the car warmed up, dropping from more than 6 psi to around 3 psi once the car reached operating temperature.
Although Zex does offer "booster" fuel pumps, the company doesn't make a standalone pump for my application, so my tech consultant there referred me to Holley. Long a performance mainstay, Holley's Blue electric fuel pump comes in 90- and 110-gallon-per-hour (gph) configurations, either of which is plenty to feed a carbureted 350. These pumps even include a regulator to adjust the pressure to meet your needs.
Holley supplied us with a 110-gph Blue pump, as well as a wiring harness and a new billet regulator. Although I opted not to completely re-plumb the fuel system with braided-steel AN line, Earl's supplied 90-degree NPT fittings with its Ano-Tuff hard anodized finish to get the fuel from the regulator to the nitrous system. While most aluminum fittings undergo anodizing, a process designed to harden the surface of this relatively soft material, hard anodizing produces a tougher, more durable finish of the sort found on tactical flashlights for law-enforcement or military use.
Installing the fuel pump was more involved than it might seem, requiring that the spare-tire assembly be removed and the fuel tank dropped. Although it's possible to mount the pump to the frame forward of the tank without removing the latter, we decided to place ours on top of the framerail beside the tank, where it was more protected. Using a bandsaw, we cut a piece of sheet aluminum and made a bracket to hold the pump, then drilled a pair of holes in the frame to mount it. Holley includes a rubber spacer to help isolate the vibration of the pump, and you'll want to make sure you install it.
Wiring the pump was much simpler, thanks in part to the diagram included with the harness. You'll need switched power, a ground, and a power source coming into the included relay. I pulled power from the positive terminal of the alternator and grounded the relay to the engine block, then ran that power from the relay to the pump at the rear of the car. Basically, I followed the frame-mounted fuel lines, getting above or behind them where I could, and zip-tied everything into place. Once I got power to the pump, Dave Emanuel, who did the heavy lifting on the pump installation, wired in a weather-pack connector for both the power and the ground, so the pump can be unplugged if it ever needs to be removed.
We re-ran all the rubber fuel lines with ethanol-compatible 3?8-inch hose from O'Reilly. When it came time to install the regulator, we found that a 3/8-inch NPT close union made it possible to literally screw the regulator and the dual-feed directly together. We used a 3/8-inch hose barb on the inlet side, then screwed the Earl's 90-degree fittings together to make a 180-degree turn back to the nitrous system on the other outlet.
With everything plumbed, it was time to set the fuel pressure. Zex recommends setting the pressure before the solenoid at 6 psi when flowing. The tool Zex sells for this purpose is a precision pressure gauge with a selection of fittings on the inlet side and a braided-steel line on the other side that holds one of a series of fuel jets. Using the included chart, select the jet that corresponds with the setup you've got; it will approximate the amount of fuel passing through the system when you're running nitrous. Hook it up--in my case, by screwing it into the inline fitting containing the low-pressure sending unit and kill switch--put the end of the braided hose into a catch can, and turn on the fuel pump. Use a wrench to loosen the jam nut on the regulator, and adjust it with a hex key until you have the right amount of pressure. ("In" means higher pressure, "out" means lower.) I set the pressure at a little more than 6 psi, just to provide a margin of error.
Another safety consideration is that of a blow-down tube. Frequently required by racetracks, a blow-down consists of a fitting that attaches to the burst disc fitting on the tank, and connects to a hose or pipe that routes to the outside of the car. If the pressure should get too high in your nitrous tank, generally as a result of heat in the car, the disc ruptures before the tank would. The tube contains and directs the gas out of the car, so you don't wind up with a car full of noxious fumes and/or shrapnel.
Zex offers two blow-down kits: one a hard aluminum tube, and the other an AN-based hose, which is the one I requested. It consists of a pair of hose ends, a length of black (non-steel) braided AN line, a bulwark fitting, and the fittings and burst discs. The instructions only direct you to ensure that the burst-disc fitting is torqued to 30 pounds, leaving the rest of the assembly up to your imagination.