Perimeter Plate Nitrous system - Go, Baby, Go!

Don't tell me you haven't seen it. I know Corvette guys aren't supposed to like Mustangs, but if you're into car culture, you've definitely seen the part in Gone in 60 Seconds where Nicholas Cage flips the nitrous-arming switch on the '67 GT500 he's stealing, hits the little red button marked "go-baby-go," and loses the police helicopter that's closing in on him. Thanks in part to that scene, and the Fast and Furious franchise, nitrous oxide has steadily increased in popularity over the past several years.

Among the newer nitrous products is the adjustable Perimeter Plate system offered by Zex, which adds between 100 and 300 horsepower and is designed to overcome some of the weaknesses of traditional "plate"-style nitrous systems. Here, we'll look briefly at what nitrous is and does, and then take a more in-depth look at how to install the Zex kit on my red '72 small-block coupe, AKA "Scarlett."

Though still frequently considered a somewhat naughty addition to a car (note how many of the kits have "sneaky" or "cheater" in the name), using nitrous oxide to increase engine power is nothing new: As far back as World War II, the Nazi Luftwaffe used it to help airplane performance at high altitude. Not actually a fuel, nitrous oxide is composed of two parts nitrogen and one of oxygen; the latter element is released as the compound breaks down at high temperature. This aids the process of oxidizing fuel so that more can be burned, thus increasing the engine's power output.

The reason there's "no replacement for displacement" is that more room allows for more fuel to be burned. Nitrous essentially accomplishes the same thing in a smaller space, with little modification required to the engine. And unlike stroker kits, turbos, or superchargers, the extra power is only produced on demand, leaving fuel economy and general driveability unaffected.

Nitrous systems are generally divided into two types. In a "wet" system, the nitrous is mixed with additional fuel and sprayed into the intake tract. In a "dry" setup, the nitrous is sprayed by itself, and the fuel delivery system is calibrated or modified to deliver the additional fuel needed. One of the advantages of a wet system is that, since the fuel and nitrous are sprayed together, it's simpler than a dry system. This reduced complexity somewhat lessens the possibility that the nitrous will be sprayed in without the extra fuel.

Should the engine run lean (i.e., without sufficient fuel) while nitrous is being added, the result will be very high combustion temperatures, coupled with really good odds of engine damage. You've heard of melting pistons, and burning holes in them? Yep. That's why you'll need to be careful.

With a little caution, however, there's nothing to fear. Estimates vary, but assuming your engine is in good shape and unmodified, anything between 100 and 150 horsepower should be safe to add. Much over that, and you're going to need to beef some things up.

In this case, the engine we'll spray is a low-mileage ZZ2, a GM crate motor producing somewhere in the neighborhood of 345 hp and 400 lb-ft of torque. The kit Zex sent contained a Perimeter Plate for a standard Holley application (the company also has one that's Dominator compatible), fuel and nitrous solenoids, an assortment of jets, all the braided-steel line and electrical components required for installation, and a 10-pound nitrous bottle and mount. While mine came in Zex's trademark purple color, the kit is also available in a stealthy "Blackout" version. The bottle comes empty: Expect to pay around $4 per pound to fill it.

The first part of installing nitrous is the actual plumbing, starting with mounting the bottle in the car. While putting it in the passenger compartment is generally frowned upon, if you've got a Corvette, that's pretty much where it's gotta go. From the bottle, the nitrous passes through a braided-steel line to a solenoid that mounts near the intake manifold. When the solenoid receives an electrical signal, it opens a valve, releasing the pressurized nitrous through another braided line into the perimeter plate, where it's mixed with fuel and sprayed into the intake manifold. Where the line meets the plate, it screws onto a fitting that accepts one of several different jets, or "pills," that meter how much nitrous gets in. These are marked with different numbers, and the kit specifies which one is required, and which fuel jet should accompany it in order to deliver a certain power level.

Traditional "plate" nitrous systems use a roughly half-inch-thick plate that fits between the manifold and the carburetor or throttle body. Generally, the plate has a perforated tube that runs across its middle, and the nitrous/fuel mix is sprayed from that tube. The Zex perimeter plate, however, uses a series of channels machined into the plate itself (actually two pieces screwed firmly together) to mix the fuel and nitrous and then spray the mixture through 12 different ports. The result is more-even distribution. (For those who already have a conventional plate system, the Zex perimeter plate is available separately.)

For fuel delivery, a fuel line must be added to the existing one and run to the fuel solenoid. This was the only necessary component that wasn't included in the Zex kit, likely because fuel-delivery systems vary so much between vehicles. Since this car has a 3/8-inch fuel line, I added a 3/8-inch brass tee to the stock line and ran the additional one from there. When the solenoid is activated, fuel goes through a braided line to the opposite side of the plate; this line screws onto a fitting containing the appropriate jet.

The system is activated by a switch in the cockpit. To install it, you'll need to splice into a source of switched power at the fuse box. This wire will then run to the toggle switch, and from there to a relay mounted in the engine compartment. A relay is generally used when you need to run more power to something than it's safe to put through a simple 12-volt toggle switch. The power required to operate whatever it is--here, the nitrous solenoid--comes from a direct power source, such as the alternator or battery, to the relay. A separate arming switch then "turns on" the relay, which is essentially a heavy-duty switch.

In addition to the power coming in from the alternator and the arming switch in the cockpit, the relay also has a contact for a ground wire, which you'll want to mount to the intake. There's also a contact for the power going out to the solenoids, each of which also will need to be grounded. Although I'm a relative novice at electrical work, the diagram and instructions that come with the kit are quite clear, and I had no trouble putting it all together.

The only other electrical components required are an inline 30-amp fuse for the power coming into the relay, and a throttle switch. Since nitrous can damage the engine if it's run at low rpm, many systems use a microswitch, mounted on the carb or throttle body, that only activates when the engine is at wide-open throttle. This switch is wired into the arming circuit, so that the wire coming from the arming switch in the cockpit goes to one side of the throttle switch, and another wire leads from the other side to the relay. The circuit is only closed when the arming switch is on and the throttle switch is activated.

One of the things you learn quickly about nitrous is that it will reveal any weaknesses in your engine. For example, after I installed the throttle switch, I found out the gas pedal wouldn't open up the carb far enough to engage it. Seems I'd been driving around for six months with no more than 50 percent throttle. Adjusting the cable bracket and bending the pedal assembly outwards fixed that problem--and lo and behold, it drove like a different car, even without nitrous.

Another, more insidious problem, is that of fuel pressure. In the instructions, you'll find a warning that the fuel system must have no less than 6 psi of pressure right at the nitrous solenoid. Any less runs the risk of engine damage. In order to check the pressure, I installed an inline fitting with an integral fuel-pressure gauge just ahead of the nitrous solenoid. It showed between 6 and 9 psi initially but tended to drop below 6 when the engine was hot.

In an effort to ensure that didn't happen with the spray on, I looked for ways to make sure I knew exactly what was going on under the hood. Two items quickly surfaced. The first was a warning-light kit, calibrated to come on if pressure dropped below 6 psi. To wire it in, I joined another wire to the one coming from the arming switch and ran it to one terminal of a 12-volt warning light. From there, I ran the wire off the light's other terminal to the self-grounding pressure switch, then mounted the switch in a brass tee to the gauge fitting I'd already attached to the fuel solenoid. Since both the gauge and the switch had a 1?8-inch pipe thread (NPT), they both fit easily into the tee.

While you can build your own system like this, I was unable to locate the switch anywhere but Jegs. Most pressure switches operate at a range--say, 5-12 psi--which means that each switch will function somewhere in that range, but you won't know where without buying one and testing it. The Jegs switch is pre-calibrated to 6 psi.

The second item is a safety switch. Adjustable from 5 to 24 psi, it's wired into the same circuit as the arming and throttle switches. Should fuel pressure drop, it will kill the whole nitrous system. We installed it in the same fitting as the other switch by drilling an additional 21/64-inch hole and using a 1/8-inch NPT tap to thread it. This is a $30 part, and considering that it could save your engine, only a fool would omit it.

Two other things you'll need to do prior to running "the juice" are changing the spark plugs to a set two steps colder than standard, and retarding the top-end timing to reduce the potential for detonation. The exact amount of timing retard needed varies with the power level you select, and that information is included in the jet-selection chart that comes with the instructions. For adding 100 horsepower, we needed to pull it back by 4 degrees.

With the basic installation done and everything double-checked, I pointed the car in straight line, hit the switch, and laid into it. And when the bottle hit, and the car suddenly started laying down more than 400 horses at the wheels, believe me, Scarlett was gone with the wind.

In the next installment, we'll look at making the most of the nitrous installation: fine-tuning the fuel delivery, adding a purge (for quicker response) and a safety blow-down (to make the car legal for the track), and getting some numbers to show the horsepower gains.

Special thanks to Leon Arrowood, Dave Emanuel, Tim Faircloth, and Tray Walden.