Nitrous on Chevy Small Block - Dyno - 'Round The Outside

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. The kick 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 PlateAs 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 CamsI 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.