In a previous installment, through our engine buildup and dyno sessions, we tried to defend the honor of those legendary open-chamber, rectangular-port big-block Chevy heads in tests atop a very streetable 454. The dyno hinted that just maybe those hallowed heads aren't the hot ticket for the street. However, since we race cars, not dynos, we decided to drop our Rat between the framerails of a '75 Camaro and duplicate our four-way test on the dragstrip, translating horsepower and torque into e.t. and mph. In doing so, we're giving those storied heads one last chance to redeem themselves. Along the way, we'll cover just what's needed to Rat-infest a '70-'81 Camaro and how to get 560-plus horsepower to the ground safely and reliably.
Rat-ify a Camaro
Since big-blocks were optional in '70-'72 Camaros, bolting one in a '75 was a snap. The original 350 motor mounts, transmission mount, four-speed crossmember, and clutch linkage were reused. The driver-side motor mount had to be notched to clear the header according to instructions included with the Hooker Competition headers. Dig up last month's article for part numbers on the deep-groove pulleys and alternator mounting brackets. For cooling, we used a standard '69-'87 cast-iron long-style water pump combined with the original '75 Camaro flex fan. Big-block second-generation Camaros use a specific fan shroud; an excellent reproduction is available from Year One. To clear the supercharger drive, the fan had to be spaced forward, causing interference at the very top of the shroud. The shroud was sectioned and a thin-gauge aluminum cap was fabricated to clear the fan. Pop rivets and 3M high-bond foam tape permanently secured the cap to the shroud.
Year One also provided a trick, OE-appearing big-block radiator. Although the radiator features a thick, four-row cooling core, the end tank thickness and core width match the original 350 unit, resulting in a hassle-free, drop-in installation. Molded big-block radiator hoses added to the OE look. We topped off the engine with a low-profile open-element air cleaner featuring a 14x4-inch K&N element. The total height of the engine with supercharger and air filter dictated radical hood surgery or a new hood. We opted for a Glasstek bolt-on fiberglass cowl-induction hood. The 6-inch rise cleared the induction system with just under an inch to spare and definitely gave the Camaro the look of a serious player. The combination of the Glasstek hood, Weld Racing Draglite front wheels, and Mickey Thompson ET Fronts cut nose weight by 72 pounds! Even though the Glasstek hood uses the stock latch, we used a couple of Mr. Gasket hood pins as a safety measure.
The stock throttle cable fell short of reaching the Holley carb atop the blower. Lokar provided a slick cut-to-length stainless throttle-cable kit, along with bracket and return springs. The kit greatly simplifies connecting the gas pedal to the carb on almost any GM car originally equipped with a throttle cable.
To support 500-plus horsepower, we used a Holley blue pump just in front of the gas tank, with the bottom of the pump flush with the bottom of the tank. A Holley regulator was mounted to the framerail inside the engine compartment. An Auto Meter fuel-pressure gauge was tapped into the fuel line near the carb and mounted at the rear of the hood, inside the cowl-induction hood intake. For safety's sake, this gauge must be mounted outside the vehicle. To minimize the chance of fuel starvation, the sintered-bronze fuel filters were removed from the 850 Holley float bowls and replaced with a high-capacity GM inline filter just after the regulator.
Next, we installed a Torque-Tech full-length 3-inch aluminized exhaust system with Dynomax Ultra-Flow mufflers. Absolutely no welding was required, although the street hookups had to be cut to length, as well as heated and slightly bent to align the head pipes with the headers. The finished job has a very sanitary appearance with more ground clearance than expected. The sound level? What's pleasing to one is an annoyance to another, but we personally found the exhaust tone at idle and cruise both pleasing and authoritative.
Suspension The car was already equipped with a pair of no-name traction bars, less affectionately known as slapper bars. Earlier experiments with a stick-shift 327 revealed that the rear of the car would rise so much at launch that the stock rear shocks would hit full extension, literally yanking the slicks off the track! With over 100 more horsepower on tap and a heavy big-block up front, things would only get worse, so changes were in order. To lower the rear of the car, we removed the shortest leaf from each rear spring, replacing them with short aluminum plates of the same thickness to maintain the spring stock height. We modified the front spring-mounting brackets to relocate the front spring eyes 3/4 inch higher in their pockets, effectively lowering the car a like amount. We also used a set of Koni SPA-1 bolt-on rear shocks from Summit Racing, which feature increased adjustable extension damping. Up front, we installed a pair of Lakewood 90/10 drag shocks from Mr. Gasket.
Putting a stick-shift trans behind a 560hp and 650-lb-ft engine used to be a recipe for driveline breakage. Now, solid driveline parts are available to reliably handle this kind of output. We used a McLeod Soft-Lok clutch featuring an adjustable pressure plate, aluminum flywheel, and a sintered-iron clutch disc. It's an extremely versatile unit, equally at home behind a normally aspirated 327 or a supercharged 454. The static pressure is easily adjusted according to the engine's output, just above the threshold of slippage. We also used a simple bolt-on counterweight to compensate for the external balance requirements of the 454. The clutch is streetable, although for street use we would recommend a sprung-hub disc over our solid-hub unit to minimize chatter. The G-Force G-101 four-speed transmission and Long shifter can handle whatever abuse we dish out. A Spicer 1350-series driveshaft provides a reliable connection to the Strange Engineering Dana 60 rear axle.
With the car having 10-second quarter-mile potential, track rules dictate a helmet, rollbar, safety harness, driveshaft loop, and, in the case of our stick-shift car, a scattershield. An eight-point, NHRA class-legal Art Morrison rollbar (installed by Terry Myers of Hephzibah, Georgia) provided the necessary rollover protection. Crow Enterprizes supplied the 3-inch- wide, five-point safety harness. We couldn't find a universal driveshaft loop that cleared the exhaust system, so we fabricated our own out of 1/4x2-inch steel.
It can be tough keeping an eye on the tach when you're busy bangin' gears, so we opted for an Auto Meter shift light, which we parked right above the tach. With it, we could hit our shift points without taking our eyes off the track.
To the Track
After much thrashing, we were finally ready to test the four engine combinations at the track--or so we thought. The first glitch we encountered was the loud popping and banging coming from the exhaust system when the car was staged against the rev limiter. Apparently, the rev limiter we were using had such a long "stutter" interval that it allowed the exhaust system to load up with unburned fuel, which would then ignite. It sounded like the mufflers were going to blow clean off the car! MSD recommended its Digital 6-Plus Ignition Control, which features a much shorter "stutter" interval. The MSD unit instantly solved the problem.
The supercharged 454 quickly found the limits of our nearly stock suspension. Even with the Koni rear shocks set on full hard, the 9x29.5 Mickey Thompson slicks would lose traction just a few feet off the starting line, the result of the shocks hitting full extension. As a crutch, we fabbed some shock extensions, allowing about 2-1/2 inches more chassis separation before the shocks hit full extension. That helped, but it was evident that more extension damping was needed. How could we do it? Mark Ball of Koni recommended revalving the shocks to further increase extension damping. The modified shocks made the most dramatic change we'd seen in the car (other than putting the blower on!), dropping the e.t.'s a full quarter-second with no other changes! We consider them "36hp shocks."
As pumped as we were about finally getting the car to launch, our excitement was short-lived when our rocker arm breakage problem returned (see Part 1). We thought the shimmed valvesprings cured the problem. At Editor Smith's suggestion, we deburred the top of the oil hole on each rocker. This eliminated a sharp edge, which was an apparent stress riser. This simple, no-buck mod has kept us breakage-free.
The Moment of Truth
We had an opportunity to test the unblown combinations back-to-back. The instant we bolted on the oval-port heads, we noticed a distinct improvement in throttle response, especially when cracking the throttle. The increased sharpness was akin to that found when going from carburetion to fuel injection. At the track, we noticed the oval-port heads would cause the car to pull hard at launch, while the engine nearly bogged off the line with the big heads. Despite the noticeable improvement in torque, there was not a giant difference in e.t. or speed between the oval and rec ports. The Camaro responded with a series of high-11-second passes with both the oval- and rectangular-port configurations.
We decided to jump right into the supercharged combination to see what the little blower was worth. Our first pass with the rectangle-port heads generated our first 10-second e.t! What a great, eye-opening ride! Greedy for more, could we trim a little more e.t. off with our supercharged/oval-port combo? Although we were traction-limited to a 3,000-rpm leave, we wanted to see if we could get the Camaro to pick up down-track. The rev limiter had been set at 5,800 rpm, causing the engine to stutter before reaching the finish line. We bumped the rev limiter to 5,900 and put in a fresh set of plugs. We were rewarded with a 10.853/ 123.66, with a 1.548 60-foot time. For our last pass, we shifted sans clutch. By leaning on the shifter prior to the gear change and allowing the engine to hit the rev limiter, the trans would unload and immediately drop into the next gear.
Admittedly, this is not the way to maximize transmission life, but our clutchless pass yielded a best-ever 10.847, and we sounded like Warren Johnson doing it (well, maybe just a little). Don't try this with your factory gearbox, though, or you'll wind up with a handful of synchronizer dust.
We "kept the faith," defending the honor of the big, rectangular-port cylinder heads to the end. However, the dyno pulls and track results indicated that the oval-port heads simply made more power. Keep this in mind the next time you're tempted to put those big heads on a street-driven car.
Another discovery that came as no great surprise is that the combination of a blower, a big-block, and a four-speed makes for one E-ticket ride! Despite its animalistic tendencies, the supercharged 454 would idle quietly and smoothly at 800 rpm without overheating, yet can rip off 10-second e.t.'s through the mufflers.
This 454 would be easy to duplicate given its stock cylinder heads. The iron castings benefited from absolutely no bowl work, porting, or port matching to create these power numbers. A total of 51 dyno pulls and 38 passes down the track demonstrated that the equipment we settled on was rock-solid reliable, with the exception of the stamped-steel rocker arms. Would we have had any rocker-arm failures had we deburred them up front? We'll never know.