Frank H. Cicerale
September 1, 2008
We slapped Zex's new Perimeter Nitrous Kit on a carbureted '89 Mustang GT. The result was 430 rwhp and 500 lb-ft of rwtq at the press of a button. Nitrous is cool!

Whenever the MM&FF staff goes to a restaurant, we usually end up being served plates overflowing with food-lasagna, chicken parmesan; you name it. Once the dinner is over and the dessert is skipped (yeah, right), we head to our homes, loosen our belts, and sit in our chairs-fat, happy, and stuffed like a Thanksgiving turkey.

It was sort of the same thing after we finished wrenching on Chris Winter's '89 Mustang GT at Crazy Horse Racing, except this time it was the Fox-body that was feeling fat and happy from the Zex nitrous kit we installed and the subsequent influx of power.

You're probably saying, "C'mon, it's just another nitrous install." While that may be the case, this time we installed Zex's new Perimeter Plate nitrous system, made specifically for carbureted cars. Bet your ears just perked up, didn't they?

While superchargers and turbochargers aren't only great for power, but also for putting up a big mark on the "wow" scale, there's nothing that compares to the on-demand power of a nitrous hit. Nitrous oxide (N20) is a chemical makeup of two parts nitrogen and one part oxygen. Since the engine breathes in 78 percent nitrogen, 20 percent oxygen, and 2 percent other gasses, any additional oxygen that the engine can ingest can be countered with an increase in fuel, and, obviously, extra horsepower will result. Engines burn only the oxygen in the air (and fuel) so adding more oxygen is always a plus.

Most nitrous kits are uniform across the board, coming in either two styles (wet or dry), as well as being made up of a similar construction. A wet nitrous system is a kit that delivers both the nitrous and fuel directly into the intake tract together. A dry nitrous kit delivers just the nitrous, leaving the fuel system to supply the additional fuel to maintain the correct air/fuel ratio. This can be done by an increase in fuel pressure to, say, the fuel injectors, or it can be done by increasing the pulse width in EFI applications.

When pressurized, nitrous oxide becomes a liquid. When the liquid nitrous is moved from the bottle to the engine via a nitrous feed line, solenoid, and nozzle or carburetor plate, the nitrous is converted to a gas. When it reaches the combustion chamber, it's compressed, and when introduced to heat, the compound breaks down, releasing the lone oxygen molecule from the pair of nitrogen molecules.

With this extra oxygen molecule available to be burnt, extra fuel is added to compensate for the extra oxygen, and the resulting influx makes for a more powerful combustion process. When done right, this results in greater cylinder pressure, which forces the piston down the cylinder bore with greater thrust on the power stroke, thus making more power overall than the engine could make on its own.

When it comes to a carbureted application in particular, when running a plate-style system, the ice-cold nitrous also provides a cooling effect to the intake charge, in addition to the plate itself acting as an intake spacer. Having done numerous fuel-injected nitrous applications recently, we decided to tackle a carbureted nitrous install and see how much power we could make with the new kit Zex offers.

The Combination
Before a part was ordered, we needed to find ourselves a car worthy of being sauced up. Our chosen lamb was the '89 Mustang GT belonging to Chris Winter of Crazy Horse Racing, who would also perform the install. If the car looks familiar, it should, as Winter was a participant in our naturally aspirated shootout (August '08 issue). The Fox-body features a stock-block 5.0L that has been punched out to 306 ci. The poked small-block Ford showcases a Comp solid-roller cam featuring 0.660 lift on both the intake and exhaust, as well as a set of World Products Windsor Sr. iron heads, an Edelbrock Victor intake, and a worked-over 750-cfm Holley carburetor. A full-on MSD ignition lights things off; fuel is supplied via an electric fuel pump and accompanying regulator; and the potent small-block is backed by a C-4 automatic trans, a 3,800-stall converter, and an 8.8-inch rear stocked with 4.56 gears. With a 13:1 compression ratio, the engine combination is not only out of the realm of running on straight pump gas, but it's also more suitable for the race track than for the street. Nevertheless, while we would have to change a few things to allow the car to run on the sauce, for all intents and purposes, this Pony was perfect for a shot of laughing gas via the Zex kit.

A Saucy Situation
What makes the Zex Perimeter Plate kit so different from the others on the market? It all revolves around the plate itself, which not only looks neat, but also showcases a different way of getting the nitrous and fuel into the engine.

The plate Zex developed for its carbureted nitrous system is a perimeter plate that injects the nitrous and fuel in a 360-degree ring instead of using spray bar(s). "This is the only plate that Zex makes," says the company's Matt Patrick. "When we developed it four years ago, we wanted to do something different than the rest of our competitors. The plate has no spray bars. Instead, the nitrous and fuel is sprayed around the perimeter of the plate. This helps to improve the distribution of the nitrous as the 360-degree ring allows it to be sprayed evenly to each cylinder. A spray bar-type plate only aims at the runners, which can lead to certain cylinders getting more nitrous, and others less."

In addition, the manner and angle in which the nitrous is sprayed also helps it to act as a carburetor spacer. "The inverted cone-shape in which the nitrous is sprayed creates a low-pressure area between the bottom of the carburetor and the area, right before the air, fuel, and nitrous make their way into the intake manifold runners," Patrick says. "This low-pressure area allows the engine to pull in more air effectively. We've seen the plate alone boost power numbers by up to 30 hp."

The other interesting feature when it comes to the plate is Zex's trademarked Cryo-Sync technology. "We designed the plate not only to inject the nitrous in a different manner, but also, using our Cryo-Sync technology, to be a heat sink as well," Patrick says. "The super-chilled nitrous cools down the plate, as well as the carburetor and the intake manifold, leading to a denser intake charge and more power."

With Zex investing so much time and ingenuity into the carbureted system, we were curious if there was a similar setup for fuel-injected engines. "Currently, we don't have anything similar to this [carb] system for fuel injected applications," Patrick says. "To develop a system like this for the fuel-injected crowd that would utilize this sort of setup would mean catering to a small and select group of enthusiasts. If the need arises, then we'll look into it."

So how does this system stack up against a similarly constructed fuel-injected kit? "Both the carb systems and the fuel-injected systems do the same thing but operate on different principals," Patrick says. "Thus, the tune-up for each application is different."

Speaking of the tune-up, Patrick offers some pearls of wisdom when it comes to setting up the car for the hit of the sauce. "Most of the time, a change in the jets of the carb isn't needed, as the wet kit will feed the extra fuel into the engine via the plate," he says. "For starters, you should go with a spark plug that is two heat ranges colder than stock. Most of the plug gaps we see are between 0.040 and 0.045 inch, so make sure the gap is closed up to between 0.032 and 0.034 inch. Tightening up the gap allows the spark to jump the gap. Nearly 9 out of 10 times, the gap is too wide, and the spark gets blown out under the increased cylinder pressure.

"A good rule of thumb is that when you're using a 150hp shot, only a spark-plug upgrade is required. When you go with a bigger shot, something like 300 hp and up, an aftermarket ignition system is highly recommended. When it comes to ignition timing, a good thing to remember is that for every 50 hp of nitrous, retard the timing 2 degrees. I recommend this for anyone using a nitrous setting of more than 50-75 hp."

One thing Patrick was adamant about was setting the fuel pressure for the carbureted system. "To properly set the fuel pressure with any carb kit, you need to have the fuel source for the plate in a flowing condition. This means that if you have an electric fuel pump, turn the key on and get the fuel pump to flow fuel. Without flow, you can't properly and accurately set the fuel pressure, as you can be off between 3 and 4 psi. We like to see 6 psi of flowing fuel pressure for the inlet side of the nitrous kit, and highly recommend our fuel flow check tool to properly set and monitor the fuel pressure."

Doing The Deed
With all of our questions answered, it came time to do the deed. We cruised to Crazy Horse Racing in South Amboy, New Jersey, where Winter and his Mustang waited patiently for us to arrive with the kit and our camera. Before we even popped the Mustang's bonnet, we put the Fox-body on a lift and yanked the 4.56 gears out of the 8.8-inch rearend. We replaced them with a set of Ford Racing Performance Parts 4.10s that we sourced from Down's Ford in Toms River, New Jersey. The 4.56 gear ratio was perfect for allowing the 306ci engine to scream through the lights naturally aspirated, but once the nitrous was activated, chances were good the car would run out of gear well before the finish line. The 4.10s would hurt the Stang a bit when running sans sauce, but would help it use all of its available power range when the laughing gas was activated.

We also switched the worked-over 750-cfm Holley carburetor for a plain 750-cfm Holley four-barrel. With the work that was performed on the first 750 carb, it was flowing in the neighborhood of 950 cfm, which was way more than what this little 306 needed. Even the straight-up 750 carb we replaced it with was probably too big, as we had to lean the jets out a few sizes to get the air/fuel ratio in the 12:1 range. If we could have slapped a 650-cfm carb on this puppy, the Mustang might've been a bit happier, but in the end, things worked out just fine.

Compared to an EFI kit, the Zex carb kit is fairly straightforward and easy to install. The kit comes without the bells and whistles such as a purge kit and a bottle heater, so the only wiring that needed to be done was the wiring of the solenoids to a power source, the crimping of the wires to the wide-open switch, and a button to arm and activate the system. As for plumbing, Zex supplied more than enough braided line to run from the bottle's mounting position in the rear hatch area of the Mustang up to the nitrous solenoid. We had to order a few other fuel-system pieces (second regulator, Y distribution block, and braided line) to siphon off fuel from the main fuel line to the fuel side of the system, but for all intents and purposes, the kit can be installed in a day.

Checked Out
According to Zex's Matt Patrick, getting the fuel pressure set correctly is crucial to optimal performance and longevity when it comes to running any carbureted nitrous-plate system. As to the proper way to set fuel pressure, Patrick recommends checking out Zex's fuel-flow check tool. The tool temporarily attaches to the fuel solenoid's feed line, so you can simulate the normal fuel flow the nitrous kit will see when it's activated. Once the fuel system is in the "flowing" mode, you can then check and set fuel pressure to the correct level.

Cracked Wide Open
Once we had the kit on and everything was double-checked for proper connections, power, and leaks, we strapped the Mustang to the DynoJet chassis dyno and let her sing. Our first dyno run was aborted due to the fact that the air/fuel ratio was in the toilet. With the meter reading 10:1, we knew the engine was running way rich. Wanting to stay on the conservative side, we made a small jet change, swapping out the 76 primary and 82 secondary jets for 72s in the front and 76s in the rear. The result was a 330-rwhp hit, with the torque figure coming in at 323 lb-ft-not bad for a small little 306ci naturally aspirated motor.

We then jetted the nitrous kit for a 100hp shot of the sauce, and pulled the timing back from the N/A setting of 40 degrees to 36. Once everything was in order, the Mustang was fired up, and we ran the engine up to 7,800 rpm, this time on the bottle. The result was 410 rwhp and 435 rwtq, but realizing the bottle pressure was low (900 psi), we knew there was more power to be had if we could get the bottle pressure up. After letting the bottle sit in a tub of hot water for a few minutes, the pressure was at an ideal 1,050 psi. Once the bottle was installed and the feed line was hooked back up, we cracked the bottle open, planted the right pedal to the firewall, and realized a 429-rwhp and 501-rwtq dyno pull.

Thanks to the Zex nitrous kit, the Mustang safely made almost 100 hp more than it did naturally aspirated, while cranking out a stump-pulling 500 lb-ft at the same time. Mission accomplished.