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Nitrous Optimization Part 2 - Go, Baby, Go!

Part 2: Optimizing the performance of our zex perimeter plate nitrous system.

Jeremy D. Clough Apr 9, 2012
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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.

Prior to assembling the hose, I'd watched instructional videos online, as well as read Mark Stielow's excellent book, Pro Touring: Engineered Performance, where he describes the process. The short version is, insert the hose into the outer fitting, then screw in the inner fitting. In my experience, assembly lube and the proper vise jaws make a world of difference. Thread sealant and a proper aluminum wrench don't hurt either.

Intent on keeping the blow-down as low-profile as possible, I drilled a hole on the rear deck between the storage tray and the front lip, so the hose could pass down into the storage area. I then bored another hole in the bottom of the storage area for the bulwark fitting. Since I get nervous anytime I'm bolting metal through fiberglass, I trimmed down a pair of 1/8-inch-thick rubber washers and used them on either side when I screwed down the bulkhead fitting.

Similarly, although I wanted to install a purge system to get the air out of the lines before hitting the "go" switch, I had no interest in festooning the outside of the car with pipes and lights to announce it. Instead, I mounted the purge solenoid to a vacuum canister located behind the driver-side front wheelwell, just inside the fender vent. Being careful to mock it up so that I got the right dimensions, I carefully bent the hard tube and cut it so that it would exit the fender vent flush with the outside. I also painted the tube flat black to deter rust, sleeved it with vacuum hose so it wouldn't scratch the paint where it bears against the inside of the fender vent, and used a slit piece of fuel line over that to locate the tube.

Wiring it up was straightforward: I spliced into the power going to the arming switch and ran that through the included mini-fuse to the solenoid, then grounded the solenoid to the engine block. While the purge kit came with a perfectly good momentary pushbutton switch, I ordered one from Vehicle Wiring Products, out of England, that looked more period-correct for the car.

By now, you've figured out there are a fair number of switches, buttons, and lights involved in this process, and not a lot of places to put them. I was talking with Tray Walden of Street Shop, Inc., about this when we were at the Sevierville (Tennessee) Corvette show last fall, when he casually mentioned he could do it on his Haas CNC mill. "How long would it take to draw something like that up?" I asked. He simply turned around his laptop and showed me the one he'd been designing while we talked. After I got him the dimensions, he made the plate and cut in the labels for the switches, leaving them silver against the black anodizing. While I didn't intend for the plate to look "correct," the goal was to make something that would still look right for a chrome-bumper shark, and Walden did an excellent job of turning that idea into a piece of machined metal.

With the switch plate in hand, it was time to measure, then slit, file, and grind out the center console and its metal supports to fit the switch plate in place. I could then wire up all the switches, some of which required ground wires. Installed, the plate has a switch and indicator for the driving lights (which are coming soon), as well as the purge button, low-fuel-pressure light, and a covered toggle switch to arm the nitrous system. Another item I ordered from the UK (this one from Stafford Vehicle Components), the toggle has a small red LED in the end that lights up when the switch is on. If you somehow arm the system by mistake, you'll know it--especially at night, when the LED makes the red plastic cover glow red.

One of the other controls I chose to install was a nitrous-pressure gauge in the cockpit. While I'd previously had one on the bottle--which is the preferred location--you can't always see that from the driver seat. Instead, I removed the ashtray assembly and measured the opening in which the removable tray fits, then machined a plate from UHMW, an industrial plastic. This plate would fit into the opening and hold a liquid-filled nitrous pressure gauge from Zex. While the gauge screwed nicely into an inline fitting, the 15 or so feet of -4 AN line that Zex sends with the kit uses a smaller outside diameter than regular AN line, and therefore won't accept a standard two-piece hose end. While I could have ordered pre-made AN lines from Zex to go from the bottle to the gauge--and from the gauge to the solenoid in the engine compartment--the company didn't offer the lengths I needed.

As an alternative, I bought a coil of -4 AN line from Summit, measured to length, and installed Earl's straight swivel hose ends with the appropriate adaptor that let me screw them into the fitting for the nitrous-pressure gauge. Cutting to length also removes any "dead air" in the system, but be forewarned that cutting braided-steel hose so it will go in the fittings later is devilishly hard. When you measure, figure in some extra length, because you may well have to make that cut twice.

While getting it all bolted together inside the console is a real challenge, the finished gauge hides nicely underneath the sliding door for the ashtray. It'd be downright low-key if it weren't for the switchplate and the bottle bolted to the back deck.

With everything installed, it was time for a trip to Balanced Performance, to put the car on the chassis dyno and see what sort of numbers the engine produced with and without the nitrous. Unfortunately, the initial dyno pull revealed a significant engine problem, which put further testing on hold until we can get it resolved. Stay tuned.

Special thanks to Jimmy from O'Reilly Auto Parts; Dave Emanuel of Random Technology; Tray Walden of Street Shop, Inc.; L.W. Precision Tools; and N. Ga. Hydraulic.


Memphis, TN 38118



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