Until recently, I had absolutely no experience with nitrous. Scratch that, I did go for a ride in a squeezed C5 Corvette a while back, so I'd at least know what it felt like. Thekick was subtle, but it was cool; I could understand the attraction. On the other hand, I had yet to employ nitrous on a vehicle-or engine-of my own. That changed during our Sept. '06 3-Way Small-Block Showdown. With Henry involved, it was a sure thing that the juice would show up at some point, and I wasn't disappointed. Hearing my 383 bellow while under the influence, churning out almost 600 hp and gobs more torque, was intoxicating. Could I help but try it again?
The answer to that one, of course, is no. After looking over the options, I decided to give Zex's Perimeter Plate System a whirl. To be honest, the technology incorporated into this setup intrigued me. Rather than injecting nitrous into the manifold via spray bars, the Zex system, using something called Perimeter Injection Technology, injects the nitrous through 12 points surrounding the intake-manifold opening. It sounds good, but I called Zex's Matt Patrick for further explanation, and he was happy to oblige. "There's a tremendous amount of kinetic energy present when nitrous is injected, in the area of 900 to 950 psi," he began. "We figured, 'Let's aim it, let's focus that energy and utilize it.'"
This approach yields several benefits, theoretically, but how does it work in practice? Quite well, actually. First of all, I never intended that the 383 I built these several months ago be a nitrous motor. But as it turned out, the cam that had the specs I was looking for happened to be from Comp Cams' Nitrous HP line. These cams work well with any engine that has "good heads, and higher compression," according to Comp Cams engineer Billy Godbold. It worked great on this engine, helping it make 458 hp and 470 lb-ft of torque in our Sept. '06 test. With a single-plane intake installed and on the Zex squeeze, well, it worked even better.
Our first tests, in which we sprayed a 100hp shot through both dual-plane and a single-plane manifolds, showed gains of almost exactly 100 hp each time. When we worked our way up to a 150hp shot, however, the investment was returned to the tune of 648 hp and 694 lb-ft...which turned out to be more like a 180hp shot. Zex's Patrick was mildly surprised. "Our systems are usually very consistent," he said, "but some parts really work with it. Your particular combo just liked it." Indeed...and so did I. This could get addictive, to be sure.
AT THE PLATE
As we mentioned earlier, Zex's Perimeter Plate System seeks to harness-and focus-the energy created in the very act of injecting nitrous into an engine. According to Zex's Matt Patrick, this approach provides a number of benefits. "On a typical 425hp small-block," he explained, "We'll see an exhaust gas temperature (egt) variation of up to 60 degrees. It's inevitable that some cylinders get more air/fuel mixture than others." Perimeter Injection, he told us, evens out this distribution-so much so that the maximum temperature variation comes down to 30 degrees.
There are two other benefits to this setup, Patrick explained. One is that the nitrous' trip through the system helps "super-chill" the plate, a process Zex calls Cryo-Sync Technology. This process also cools the carburetor and intake manifold, he explained. And as you know, a colder charge is denser, and carries the potential for more power production.
The last feature, Airflow Enhancement Technology, refers to the low-pressure area created by the Perimeter Plate spray pattern. The nitrous is injected at a carefully designed angle, toward the center of the intake plenum. This pressure differential, we're told, helps pull more air/fuel mixture into the engine, creating even more power. We can't say if the Perimeter Plate System works better than a traditional spray-bar setup, since we haven't tried one on this particular engine. We can say, on the other hand, that this particular nitrous setup was a great match for our engine combo.
A FORTUNATE CHOICE OF CAMS
I wasn't looking to fit my Small-Block Showdown 383 with a nitrous-friendly cam-it just worked out that way. Nitrous HP camshaft number 12-419-8 is ground with the numbers I wanted. It's got lots of lift and pretty good duration for a street cam, and as I observed back in Sept. '06, the lobe separation angle of 113 degrees would allow for a decent idle and adequate vacuum My idea was that this cam would give this engine a fighting chance of passing a smog test (at the pipe, of course) if I decided to sneak it into a smog-era car. This isn't really what Comp Cams had in mind.
Nitrous HP cams are designed to work well without nitrous, but also to deliver "incredible gains" when on the squeeze. Comp engineer Billy Godbold was only too happy to elaborate for me. "The Nitrous HP uses the same intake lobe as an Xtreme Energy 288 cam," he began. "This makes it a good midrange cam." The difference is on the exhaust side. "We use a bigger exhaust lobe," he explained. "We also use a 113-degree LSA and a 5-degree advance for earlier exhaust valve opening. This cam works well with nitrous, but also with anything that has a faster burn rate." Higher-compression engines fitted with good heads make good use of a Nitrous HP cam.
The idea, he elaborated, is to reduce pumping losses. The exhaust valve opens early, and for a long duration, meaning less burnt gas needs to be pushed out on the piston's upstroke. "We get more out for free," Godbold summed up. "It does give up some low-end, but it carries better past peak power." We didn't notice any loss of low-end power, but we did notice the aggressive note created by the long exhaust duration. "The power curve is moved up," Godbold told us, "but the sound might be worth it." We'll go along with that.
Zex's Perimeter Nitrous Plate Nitrous System comes ready to install. Except for the addition of Westech's bottle-heating apparatus, what you see here is what we ran. |  | 
The big difference between this setup and other nitrous plates is Zex's Perimeter Injection technology. As you can see, traditional spray bars have been replaced by strategically placed injection points, which are fed by an internal delivery channel. Even the angle of injection has been considered in the plate design. |
During our previous nitrous runs on this engine, Westech's Steve Brul cautioned against running anything higher than a 125hp shot on a dual-plane manifold, due to fuel distribution issues. It's a conservative route, he admits, but one that keeps engines from going boom. Accordingly, we limited ourselves to a 100-horse shot before moving on to this Dart single-plane intake. | 
The Dart manifold moves lots of air down these wide ports. At the time of our Small-Block Showdown, we discussed what this engine might be like with a single-plane intake. Given the chance to try it, peak torque dropped by 8 lb-ft, but stayed above 400 lb-ft from 3,100 rpm all the way to 6,100 rpm. Horsepower increased by 8, giving us a 469-lb-ft/468-horse package. And we could safely spray more nitrous. | 
Since we'd be making multiple timing changes during our testing, we upgraded our improvised timing pointer to this slick, easy-to-read TCI piece. |
Now that's what we're talking about...before we even touched the nitrous, Brul increased the secondary jet size in our Holley Street HP to 85. This quick change increased our baseline torque to 476 (an increase of almost 6 lb-ft) and 460 hp (just shy of 2 more horsepower). It was a different day and a different dyno, but hey, we'll take it. The 80 jets we started with were reinstalled for the Dart single-plane runs. | 
Despite its fundamental differences from other systems, changing jets in the Zex Perimeter Plate System is done the same way as most other plate setups. | 
Given that our subject engine has forged pistons and rods, but a cast crank, we figured we could safely go as high as a 200hp shot. It sounded anything but safe when the engine started missing on that particular run, so we shut it down and broke out the leakdown tester. Everything checked out OK, but our test time was over. The culprit turned out to be a loose rocker-arm adjusting nut. We'll have to try the big shot another time. |