Give us cubic inches or give us death. In a world of me-too, late-model small-blocks, a big-block Chevy rings true. In the previous installment, we blueprinted the short-block of a 470ci big-block Chevy. Now it's time to finish up with an A-grade top end with products from Air Flow Research, Crower Cams, Trend Performance, Dart Machine, and Summit Racing.
When selecting a top end for your engine, the two components most critical to performance and operation are the heads and camshaft. While the intake manifold, fuel delivery method, and ignition also play vital roles, think of the cam-and-head package like the engine's DNA. While other variables can be externally manipulated, the DNA remains the same and will dictate when and how the engine makes power, what sort of fuel it will run best on, and what sort of driveability it will ultimately have.
This Rat is of the Pro Street variety and aims to balance overall engine output with relative affordability. Splurges were kept in check, but let loose on components such as aluminum heads and a hydraulic roller cam to maximize power and torque.
When choosing heads for a big-block Chevy, it's almost too easy to get greedy with runner volume well into the 400cc range, which are readily available. However, choosing a head with a runner volume matched to your displacement will yield far better throttle response and useable power and torque curves. For our build, we turned to Air Flow Research for a set of 305cc rectangular port heads, which are designed for engines up to 509 ci. The 305cc is no slouch either, flowing 362 cfm of air on the intake and 274 cc on the exhaust, all while shaving significant weight over any factory iron head. They feature stainless steel 2.25-inch intake and 1.88 exhaust valves, and we ordered our set equipped with hydraulic roller springs.
Moving air through the heads is a Crower camshaft, lifters, and 1.8:1 rockers, actuated by Trend Performance pushrods. The cam has 0.612-inch lift and 0.636-inch lift on tap for the intake and exhaust valves, respectively, and its duration specs out at 248-degrees at 0.050-inch on the intake and 253-degrees on the exhaust side. While not as aggressive as a competition-only cam, this grind will retain an acceptable street friendly demeanor while still helping the engine perform. It also won't require the lifter and lash maintenance of a dedicated solid roller cam.
For induction, since the engine will be tasked with power first and road manners second, and deliver maximum power upward of 6,000 rpm, we used a Dart Machine single-plane intake with a 4150-style carb flange. Westech's house 950-cfm Holley Ultra XP carb was borrowed to deliver the fuel. An ignition system from MSD and Summit Racing will light the fire.
With our ingredients list complete, we set to work assembling this beast, carefully checking every detail as we went.
Follow along and stay tuned for a dyno story coming soon. CHP
When any dome-top piston is used, such as this JE slug, it is necessary to verify dome-to-cylinder head clearance. This can be accomplished by using clay or Dykem layout fluid. With our piston slathered up, we saw no transfer to the cylinder head (without a gasket). This means we will have at least our gasket compressed thickness (0.040-inch) clearance when the head is installed.
The next step is to verify piston-to-valve clearance. We installed checking springs on the intake and exhaust valves, turned the engine to 10 degrees before TDC (top dead center) for the intake and subsequently 10 degrees after TDC for the exhaust and used a dial indicator to measure free drop. With our relatively conservative lift, there was miles of clearance.
Next, we checked radial valve clearance using a transfer punch inserted through the valveguides. The corresponding punch mark on the pistons was scribed using a compass set to half the valve diameter. This check ensures the valve faces fit inside the valve pockets.
With fitment of the AFR heads verified, it was time to install the Crower hydraulic roller cam. This cam has specs of 248/253-degrees duration at 0.050-inch, 0.612/0.636-inch lift, and is ground on a 110-degree lobe separation angle.
The Crower hydraulic lifters were soaked overnight in engine oil to lubricate and fill them. Next, they were installed in their bores with the link bar facing inward.
The Crower timing gearset was installed and includes several bushing inserts to advance or retard the timing, as well as an adjustable lower gear to do the same. We installed our cam with 3-degrees of retard to aid in top-end power and help bleed off a little bit of cranking compression.
This two-piece billet aluminum timing cover from Summit Racing is an excellent piece. It allows cam-thrust checking and camshaft adjustment/replacement without removing the oil pan. If you plan to mess with the cam, this one is a must.
With the cover in place, we used a dial bore gauge to verify our camshaft thrust. The Crower timing gear uses a Torrington bearing to prevent wear and a thrust button pushes against the timing cover to keep fore and aft movement in check. Our cam had a perfect 0.006-inch of endplay.
We used a Summit Racing oil pan kit, which included the correct pan, oil pump, and pickup. These were installed and sealed with a one-piece Summit Racing oil pan gasket.
We installed a set of JE Pro Seal MLS gaskets and set about installing the AFR 305cc cylinder heads.
ARP bolts were used to secure the AFR heads in place. The bolts were coated in ARP Teflon thread sealer to prevent pressurized coolant from leaking into the crankcase.
The AFR heads use 0.100-inch longer intake and exhaust valves, which requires a non-standard length pushrod. Using a pushrod length checker, we added 0.100 inch to the length of a stock pushrod as a starting point, installed the checking tool, a temporary solid lifter and rocker arm, and rolled the engine over a few times to see what the roller wheel pattern looked like.
Verifying the correct pushrod length is easy and requires only a felt tip marker and a glance at the valve stem. Here, you can see our pushrod length is spot on and the rocker wheel is wiping the ink off only the center of the valve stem during travel.
We placed an order to Trend Performance and they overnighted us a beautiful set of 8.850-inch intake and 8.000 exhaust pushrods with 5/16-inch ball ends and 3/8-inch diameters. A 0.135-inch wall thickness was chosen since the engine will see high rpm and valvespring pressure.
The AFR heads use adjustable guideplates that require setting. Install the guideplates under the rocker stud and set a pushrod and rocker on each. Rock the guideplate back and forth until both the intake and exhaust rockers sit dead center on the valve. Next, tighten the adjuster nut.
Once the guideplates are properly adjusted, use a torque wrench to tighten them to 65 ft-lb.
After the rockers are installed on each cylinder, verify the cam is on the base circle (valve is closed) then tighten the exhaust and intake rockers to zero lash. Spin the adjuster nut an extra half turn to preload them and use an Allen key to lock them in place.
With the valvetrain completed, it was time to install the intake manifold and last remaining parts. A set of Fel-Pro gaskets with ports the right size were set on the AFR head faces and sealed with a small amount of silicone around the water ports.
The Dart single-plane intake manifold was lowered into place atop the AFR heads.
With the majority of the engine complete, we took the opportunity to degree the camshaft. If the engine isn't equipped with a two-piece cover, this job should be tackled before the cover is sealed up.
With a dial indicator mounted against the Trend pushrod, we used a piston stop, inserted through the spark plug hole to find true top-dead center. The engine measured exactly 3 degrees retarded on both the intake and exhaust, which was exactly what we were after with our earlier adjustments.
With timing verified, we installed an ATI Performance harmonic balancer, which is both lightweight and SFI approved. At this time, we adjusted our timing pointer to show true TDC on the balancer.
We finished off the ignition system with an MSD Black series Pro-Billet distributor and a set of ceramic-booted spark plug wires from Summit Racing.
On the dyno, our big-block made some serious steam, cranking out 647 hp at 6,000 rpm. Torque was copious and flat, cracking 550 lb-ft at 3,500 rpm and peaking at 593 lb-ft at 5,300 rpm. The engine delivered everything we hoped for, and despite its small-for-a-big-block displacement of 469 cubic inches, both power and torque were excellent. Eventually, the engine is destined for a home between the fenders of a 1968 Camaro, but first it will see some additional dyno flogging.
Photography by Evan Perkins