Unnaturally Aspirated — Optimizing A SBC Camshaft For Nitrous

What was once possibly the most popular engine in the enthusiast arena, today is considered as “old tech” by some. While it’s true that the spotlight has moved away from the Gen-I small-block Chevrolet engine, just because other platforms might be considered the latest and greatest, doesn’t make the original sub-par overnight. In fact, the exact opposite is true, as the platform enjoys extensive support and continuous advancement thanks to its younger, hipper sibling, the LS.

One such example comes from our own Brian Petty, who you might recognize as the host of several Horsepower Wars series, such as the $10K Drag Shootout, LS vs. Coyote Part 2, and Pony Wars. A man of varied automotive tastes, one platform that has held his attention for a significant period, is the early small-block Chevy.

Petty has a 408 cubic-inch small-block which he has campaigned on the dragstrip for several seasons, which he wants to revamp to take a significant amount of nitrous. Already a stout naturally-aspirated combination, Petty turned to COMP Cams in order to upgrade the camshaft and associated valvetrain components to optimize the combination to ingest copious amounts of giggle-gas.

This unassuming 408-cube small-block Chevy makes north of 860 horsepower with only a 150-shot of nitrous. If Petty were to get a little more greedy, a bigger number is just a jet change away.

Nitrous Vs. Naturally Aspirated

There are a lot of people who might assume that a nitrous camshaft would like everything that a naturally aspirated engine would, and they would be somewhat correct. At the end of the day, from a high-level view, a naturally aspirated and nitrous-assisted system do have a lot of similarities, especially when compared to turbo- or supercharging. However, when you dive into the nitty-gritty, there are some differences that need to be accounted for in a nitrous-specific camshaft.

The big difference between the two is on the exhaust side. Even though the actual volume of the intake charge is similar with nitrous, it contains a lot of extra oxygen, which produces a greater volume of exhaust gasses when completely combusted. The exhaust side of the combination can be “opened up” with a carefully selected balance of additional lift and duration to fully expel those additional gasses.

On the intake side, there are some considerations that can be given as well, since the intake charge is denser and heavier than just simple air. This can require a little extra motivation to get moving into the cylinder. However, as Chris Mays, Edelbrock Group’s Sales Manager, pointed out during the selection process, depending on your final goals, it may just be the exhaust that needs adjustment.

“The cam that was in the engine previously was P/N: 12-824-14 [.630 lift, 276/284 degrees at .050-in lift, 107-degree lobe-separation angle]. It’s a great camshaft, just not suited for nitrous,” Mays explains. “If we wanted to stick to something similar, but suited for the nitrous, I’d spec the same exact intake lobe, but an eight-degree larger exhaust lobe along with a seven-degree wider lobe separation angle for the nitrous.”

Spinning the small-block north of 7,000 rpm requires some good valvetrain parts. The lightweight tool-steel retainers are 33-percent lighter than traditional steel retainers (within a few grams of titanium) while retaining the strength and durability steel retainers are known for.

Mays also mentions that the torque band might not be as wide with that camshaft, naturally aspirated. But, the performance on the nitrous oxide would be significantly improved over just spraying on top of the original naturally aspirated camshaft that was originally in the engine.

However, Petty isn’t looking for peak naturally aspirated power in this project, but rather to maximize the combination for nitrous oxide. “I’m sure the HXL/HXX lobes will make more power. Since we are wide open in camshaft size, we can make a big jump with lobe design,” Mays says. Before we get much deeper into the new camshaft specs, let’s dive into the engine combination.

408 Gen-I Small-Block Chevy

As a base for the engine, Petty started with a Dart Little M iron block. The stout block was designed to not only be capable of holding big power, but to do it utilizing as many standard small-block Chevy components as possible. With priority main oiling, extra-thick-wall siamesed bores, and billet-steel four-bolt main caps, the block will easily withstand the extra cylinder pressures of nitrous. Eckman Machine performed the final machining on the block, as well as assembly.

Residing in the mains is an Eagle Specialty Products forged 4340 steel crankshaft with a 3.750-inch stroke. Attached to that is a set of Eagle forged 4340 steel H-beam connecting rods, while Clevite H-series bearings keep both parts rotating smoothly. A set of 4.165-inch diameter SRP Pro 2618 dome pistons from JE Pistons sit in the bore, making the total displacement a little over 408 cubic inches. Total Seal Gapless rings seal the pistons in the bore.

A Moroso steel oil pan with a deep sump, drag race-specific baffling, and windage tray, along with a Moroso billet oil pump keep the lifeblood of the engine where it needs to be under the heavy G-forces on launch and shutdown. An ATI Super Damper keeps torsional vibrations in check while a Cloyes timing set keeps the valvetrain in time with the crankshaft.

The base of the combination is a forged 4340 steel rotating assembly from Eagle. You can see the billet four-bolt mains of the Dart Little M block as well. The stout bottom end should handle anything Petty throws at it.

Up top are a set of Dart Pro 1 230cc cylinder heads. Using the conventional 23-degree small-block Chevy valve angle, these heads are considered maximum-performance cylinder heads aimed at high-RPM, big-displacement small-block combinations. The intake and exhaust ports come CNC machined from the factory, but Petty’s set features some additional light polishing work to even further clean them up.

The heads feature 2.080-inch intake valves and 1.600-inch exhaust valves. These fit inside a 64cc combustion chamber, which makes for approximately 13.0:1 compression. The hardened exhaust seats and bronze valve guides are designed to withstand all the abuse thrown at the combination, regardless of power adder type. Translating the motion of the camshaft to the valves is a set of T&D 1.60:1 aluminum roller rockers.

Fueling the combination is a Quick Fuel 950cfm carburetor feeding into a Dart single-plane intake manifold designed specifically for the 23-degree heads. An MSD distributor, coil, and wires handle the engine’s spark, while a Magnafuel pump supplies the VP110 race gas for the combination. A set of Hooker 1-7/8-inch-primary headers get the spent gasses out of the engine as efficiently as possible.

Remember how we said this was an unassuming combo? From the outside, you’d never know that it was full of high-quality, solid components designed to make serious power.

Upgraded Valvetrain Components

Since the crux of this article is the new camshaft designed for nitrous use, we should start there. As mentioned earlier in the article, the previous camshaft had a lobe lift of .420 inches, intake, and exhaust, which equated to .672-inch of lift at the valve. This was coupled with a split duration of 276 degrees on the intake and 284 degrees on the exhaust side.

For the new cam, Petty’s engine builder, Buz Nelson of Eckman Machine, conferred with Mays about what he’d like to see in the new camshaft. “I was thinking something like 266 degrees on the intake and 280 degrees on the exhaust,” says Nelson. “I looked at some lobes in COMP’s catalog and found 2229, which is an intake lobe with .448 inch of lobe lift and 266 degrees at .050-inch, and the 12206 exhaust lobe with .445 inch of lobe lift and 280 degrees at .050 inch.”

Similar to Mays’ earlier recommendation of adding seven degrees of lobe separation to the previous cam, this cam comes in with a 113.5-degree lobe-separation angle and 2.5 degrees of advance built-in. The larger-than-stock 1.6 ratio rocker arms bring the total valve lift up to an impressive .718 inch on the intake and .712 inch on the exhaust. It also slightly increases the duration at .050-inch numbers as well (since those were calculated with a standard 1.5:1 ratio).

The custom solid-roller camshaft for this combo was cut from an 8620 tool steel cam blank. Besides being a strong, wear-resistant camshaft material, tool-steel blanks allow a greater range of grind profiles to be cut.

In addition to the above specs, the camshaft also features a 4/7 firing order swap. The whole thing was built out of an 8620 tool-steel cam core with standard SBC cam journal diameters. Besides being extremely durable, the tool steel core allows for much more versatility in the selection of the cam’s specs, due in large part to the depth of the heat treatment. (For more information on cam core materials, check out our article on the subject here.)

In order to translate the camshaft’s rotational motion to vertical motion reliably, COMP supplied a set of its premium Sportsman-series solid-roller lifters. These tie-bar-style mechanical lifters are made from 8620 tool steel and utilize roller bearings for a minimum of friction. The skirted wheel design offers additional strength as well as a greater bearing surface for increased stability. The lifters are the standard 0.842-inch diameter and feature a .160-inch offset.

The COMP Sportsman solid-roller lifters borrow some of their advanced features from their more expensive siblings while coming in at a more affordable price point. Features like 8620 tool-steel bodies, skirted, roller-bearing wheels, and stainless-steel tie bars come standard.

To control the valves at the projected 7,300rpm redline, COMP suggested a set of Elite Race Dual Valvesprings. With a maximum lift of .805 inches, these dual springs have a combined rate of 502 pounds per inch and offer a seat pressure of 195 pounds and 563 pounds of open pressure. These springs are designed specifically for roller-cam sportsman racing applications and are designed to balance performance with longevity.

To hold the springs in place, COMP also recommended a set of 10-degree lightweight tool steel retainers. They offer the benefits of steel — high-strength and significant wear resistance — while coming in 33-percent lighter than a traditional steel retainer and only several grams heavier than an equivalent titanium retainer. Complementing the retainers are a set of 10-degree Super Locks, machined out of an extremely durable alloy to withstand both wear and fatigue.

Dyno Time — Spray and Pray

With the valvetrain upgrade complete, it was time to bolt the engine up to the dyno. Before plumbing up the nitrous, The engine was run naturally aspirated as a baseline, and to see how it compared to the previous combination, even though N/A power wasn’t the goal. With the dyno set for a 2,500-rpm sweep, the pull was started at 4,900 rpm and the handle pushed all the way forward. Once the engine wound down, there was a pleasant surprise on the monitor, peak horsepower of 645.8 horsepower at 6,800 rpm, with peak torque of 546.3 lb-ft coming in at 5,800 rpm. That is a gain of roughly 10 horsepower and 14 lb-ft of torque over the previous N/A-specific camshaft.

Here you can see that even naturally-aspirated, this combination is no slouch. While not optimized to run on motor alone, the custom camshaft picked up more than 10 peak horsepower over the previous, N/A-specific camshaft.

With the all-motor number established, it was time to place the Nitrous Express Stage 6 nitrous plate under the carb and wire up the solenoids. With a 70 nitrous jet and a 62 fuel jet plumbed in, for an expected 150 horsepower boost, the bottle was cracked, the solenoids armed and the engine started. A more conservative sweep range of 5,000-7,000 rpm was chosen for the nitrous test, and after the engine finished singing the song of its people the numbers appeared on the screen.

And here are the final numbers. 864 horsepower and 766 lb-ft of torque on a simple 150-shot. Picking up 70 horsepower and 70 lb-ft over the book value for the jetting used shows the efficiency of the system with the new camshaft optimized for nitrous use. Unfortunately for us, Petty and his tuner decided not to be greedy on the dyno, because we would have loved to see the numbers if it were jetted up some.

With the relatively mild jetting, the engine achieved a peak horsepower number of 864 at 6,400 rpm and 766.2 lb-ft of torque at 6,300 rpm. That’s an increase of almost 220 horsepower and 220 lb-ft of torque on a jet size that should have been worth 150 horsepower according to the NX manual. Working backward, that’s almost 50-percent more power added than expected.

Diving into those numbers, it starts to become apparent that optimizing the camshaft for a nitrous setup can yield some really impressive results. So while nitrous oxide can add power to any combination, to do it right and unlock its full potential, you need to optimize the system.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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