Action-Packed Big-Block Test In these days of all-aluminum, injected small-blocks, is it hard to get excited about something as simple as a carbureted big-block Chevy? Not if you're a loyal reader of Super Chevy it isn't, as the mere mention of the phrase "big-block" should get those old juices flowing.
Why such a fuss over old technology? While a cast-iron, carbureted motor that weighs more than a metric ton is hardly a technological marvel, there is no denying the results of replacement/displacement principle. When it comes to making power, bigger really can be better. The greater the displacement, the easier it is to reach a given power goal. Obviously you are not going to make big power with displacement alone, but with something as simple as a set of heads and cam, serious power is just an afternoon away.
Perhaps the best thing about owing a big-block Chevy is in addition to the serious horsepower potential, big motors also offer big torque. No amount of technology is going to artificially create thunderous torque at lower engine speeds unless that technology includes some sort of boost pressure. Having a Rat that makes 500 hp is one thing, but that 500 hp feels more like 600 when you combine it with over 500 lb-ft of torque low in the rev range. It is all that wondrous torque that pins you back in your seat and puts a smile on your face.
Having extolled the virtues of big-block ownership, let's take a look at making a good thing even better. In this case, our good thing started out life as a simple 454 offered by Demon Engines. This Demon 454 was created to cater to cost-conscious BBC enthusiasts and as such featured nothing more exotic than a cast crank, forged rods and a set of forged pistons.
The forged aluminum pistons featured 18cc domes to help improve the static compression ratio over your typical flat-top or even more performance-sucking dished factory BBC slugs. When combined with the typical 119cc combustion chamber, the 18cc domes produced a static compression ratio of roughly 9.5:1 depending on exact deck clearance and head gasket volume. Think of the Demon 454 short-block as a sensible upgrade to a high-mileage factory motor.
The Demon 454 short-block also featured a four-bolt (Gen IV or V) block. Some may question the cast crank, but we have exceeded 1,000 hp using factory cast cranks so you are unlikely to ever damage one under power. Crank failures are usually the result of oil starvation or detonation, all of which can be traced back to neglect by the owner. Demon also offers the 454 short-blocks with 4340 forged cranks, but feels the money for the crank upgrade is better spent on displacement or airflow.
The cost-conscious 454 from Demon was sporting a set of reconditioned peanut-port heads. The factory iron heads were given the once-over, including a fresh valve job, surface and new springs and retainers courtesy of Comp Cams. This particular Demon 454 (it offers a number of different BBC combinations, ranging from this near stock rebuild to 1,500-plus-hp supercharged stroker assemblies) was configured for towing and/or mild street performance. Use of the stock peanut-port heads naturally restricted the peak power output, but the combination was designed to offer plenty of low-speed torque at an affordable price point.
Applications for the 400hp Demon 454 included performance street, marine and even truck and/or RV usage to replace the typical tired (and sluggish) high-mileage, big-block. Continuing the torque theme was a mild hydraulic flat tappet cam and dual-plane intake manifold. The mild XE256H Xtreme Energy cam used in the 400hp 454 offered a 0.480/0.485 lift split, a torque-producing and idle-friendly 212/218 duration split and a 110-degree lobe separation angle.
The dual-plane intake manifold from Pro Comp offered a near-ideal combination of torque and power production for use with the factory (peanut) oval-port heads. The intake was fed by a 750hp Holley carb, while exhaust chores were handled by a set of 2.125-inch Hooker Super Comp headers. In addition to the intake manifold, Pro Comp also supplied the billet distributor and plug wires, while the spark plugs came from EG. Equipped as described, the Demon 454 produced 401 hp at 4,700 rpm and 508 lb-ft of torque at 3,400 rpm. Hardly a high-rpm screamer, the combination offered plenty of torque right in the most usable portion of the rev range.
Torquey big-blocks are always popular, especially when they are affordably priced, but what if we are looking for something a little more action-packed? Not a full-on race motor, but something that can show taillights to the modern competition without breaking the bank? In reality, our 454 was just begging for the opportunity to strut its stuff. With over 500 lb-ft, torque production from the mild 454 was already impressive. Now all we had to do was improve the upper portion of the rev range by allowing the motor to breathe.
With an excellent short-block at our disposal, we took a long, hard look at the power producers, namely the heads, cam and intake. The dual-plane intake was already perfect for our application, but unfortunately the Pro Comp manifold was designed for the oval-port heads and would not fit the rectangular-port heads we had in mind for our big block. The Pro Comp dual-plane intake was replaced by an Edelbrock Performer RPM Air Gap. Similar in design to the Pro Comp piece, the dual-plane intake offered a balanced combination of mid-range torque and peak horsepower production. For street use, a dual-plane is almost always preferred over the peakier, single-plane manifold.
Obviously, the stock peanut-port heads had to go. With a combination of small intake and exhaust ports as well as small-block-sized intake and exhaust valves, the stock iron heads could hardly be considered performance oriented. That they flowed just 235 cfm on the intake and 172 cfm on the exhaust meant they were better suited for a small-block application than a healthy big-block. Sealing the replacement deal was the fact hat they were constructed of heavy cast-iron. Just the mere thought of lifting them during the head swap strained a muscle or two. For any motor, but particularly for a Rat, aluminum heads should be considered a must.
For our stump puller, we turned to Racing Head Service (RHS). Though it offers both aluminum and cast-iron performance heads for the big-block (as well as a wide range of small-block heads), we stuck with the lightweight aluminum castings. This narrowed our selection down to the 320cc Pro Action and 360cc Pro Elite heads. Since our relatively mild 454 was not in need of CNC-ported cylinder heads that offered 400 cfm, we chose a set of 320cc as-cast, Pro Action aluminum heads.
The RHS Pro Action heads offered patented Clean-Cast technology, which is designed to optimize port-to-port transitions and airflow efficiency. The 320cc heads offered impressive airflow numbers, especially compared to the stock peanut-port heads. Where the stock heads topped out at 235 cfm, the RHS Pro Action heads offered 375 cfm (at 0.700 lift). Closer to lift values we planned to run, the RHS heads still outflowed the stock heads by as much as 100 cfm.
The impressive intake and exhaust flow can be partially attributed to the increase in valve size from the 2.06/1.72 combination on the stock heads to 2.25/1.88 combo on the RHS heads. The stainless steel valves were combined with a multi-angle valve job, hardened seats and premium valve guides to make for serious out-of-the-box performance. Just for good measures, RHS tossed in a refined water jacket to improve thermal conductivity and a thick deck surface for superior sealing.
Compared to the stock peanut-port castings, the RHS 320cc BBC heads were a serious step up the performance ladder. To take full advantage of the airflow offered by the RHS aluminum heads, we swapped out the mild, torque oriented XE256H cam and replaced it with a more performance oriented XE284H grind. The XE284H hydraulic flat-tappet offered 0.574/0.578 lift split, a 240/246 duration split (at 0.050) and the same 110-degree lobe separation angle.
The new heads, cam and intake were run with the same carb, headers and distributor. The only other change was to install a set of 1.7 ratio roller rockers in place of the stock (stamped steel) rockers employed on the stock iron heads. Naturally the pushrods were upgraded as well, from the stock 5/16-inch pushrods used with the stock heads (and guide plates) to 3/8-inch Pro magnum pushrods from Comp cams. After some minor tuning to the carb and a quick timing sweep to determine the best combination, we were rewarded with peak numbers of 527 hp at 5,800 rpm and 533 lb-ft of torque at 4,700 rpm. Torque production was up significantly with the new combination, as the 454 exceeded 500 lb-ft from 3,200-5,500 rpm.
As expected, there was a slight loss in power below 3,200 rpm, but the extra 125 hp and 27 lb-ft more than made up for the losses below 3,200 rpm. Despite the wilder cam timing, the 454 still managed to produced 420 lb-ft of torque at 2,500 rpm, enough to bark even the biggest baloneys. With over 500 lb-ft from 3,200 rpm to 5,500 rpm, this 454 offers one meaty power band.
No matter how you slice it, adding a set of RHS heads and Comp Xtreme Energy cam to your big block certainly qualifies as "action-packed."
Graphs: Demon 454 Head/Cam Test-Stock/Comp Cam vs RHS/Comp Cam Any time you add over 100 hp to your motor, you know you've made the right move. Replacing the stock peanut-port iron Chevy heads and mild Comp XE256H cam with a set of aluminum RHS (320cc) heads and XE284H cam resulted in a jump in peak power from 400 hp and 508 lb-ft of torque to 527 hp and 533 lb-ft of torque. Adding the RHS 320 aluminum heads and larger Xtreme Energy cam improved the power output by 127 hp and 25 lb-ft of torque. Had we continued to rev the motor equipped with the stock heads to 6,000 rpm, the difference would have easily exceeded 150 hp. Note that the gains in peak power were not without a minor trade off in low-speed power, as the stock-headed motor offered more torque up to 3,300 rpm.