Stage I--Mild 305 ci
Although it's only a 305, it has the same stroke dimension as a 350 (3.480 inches). This mild combo would include a stock intake manifold with a four-barrel carburetor, 8.0:1 compression, and small stock heads. To increase torque and horsepower, we'll keep the duration short. Mild performance camshafts offer as much as 200 degrees of duration. An improved dual-exhaust system combined with a stouter camshaft can wake up the little 305. These short-duration cams provide a smooth idle and 16-18 inches of vacuum. Axle ratios for most 305 engines should range from 3.08:1 to 3.42:1. These cams offer excellent low-end and midrange power and extended rpm ranges for improved performance. With these added modifications, a mild 305 can produce brisk power.
305 ci
Compression: 8.5:1
Cylinder Heads: Stock
Intake Manifold: Stock
Carburetion: Aftermarket 600 cfm or Quadrajet style
Ignition: Stock
Exhaust: Stock manifolds
Axle Ratio: 3.08 to 3.42:1
Stall Speed: Stock
Duration at 0.050: Up to 200 degrees
Idle: Stock
Stage II--Mild 350 ci
With a larger displacement, you'll gain even more power by adding a larger cam. Many mild 350s are built from Goodwrench engines or something of equal stature. With just about 8.5:1 compression, you'll want to keep the duration short, so we've chosen camshafts with durations from 201 to 214 degrees and approximately 0.430-inch lift. These camshafts offer a nearly stable idle combined with a gentle lope. You might want to add Vortech heads, an Edelbrock Performer intake manifold, and a performance carburetor and exhaust system. With these additions you should find added power from above idle to 5,000 rpm.
350 ci
Compression: 8.5:1
Cylinder Heads: Stock or Vortech
Intake Manifold: Aftermarket dual-plane
Carburetion: Aftermarket 650 cfm
Ignition: Stock HEI
Exhaust: Performance
Axle Ratio: 3.23 to 3.55:1
Stall Speed: Stock
Duration at 0.050: 201-214 degrees
Idle: Smooth
Stage III--Torquey 383
With more stroke (3.750 inches) and more torque, the 383 provides decent power levels and more fun. Because more displacement can utilize more camshaft, choose one with added duration and lift. A camshaft with 215 to 224 degrees of duration offers very strong midrange torque and throttle response, especially when combined with some compression. These cams typically have 0.450-0.460-inch lift. Better-flowing cylinder heads will add more sizzle to the package.
383 ci
Compression: 9.5:1 to 10.0:1
Cylinder Heads: Aftermarket with larger runners
Intake Manifold: Aftermarket dual-plane
Carburetion: Aftermarket 750 cfm
Ignition: Aftermarket
Exhaust: 13/4-inch headers with free-flowing system
Axle Ratio: 3.55 to 3.73:1
Stall Speed: 2,200-plus rpm
Duration at 0.050: 215-224 degrees
Idle: Fair
Stage IV--406ci SB
With a big bore (4.155 inches) and longer stroke (3.750 inches), duration from 225 to 240 degrees will really help power levels. Lift will be near 0.500 inch. The compression ratio should range between 10:1 to 11:1. Aftermarket cylinder heads with at least 200cc runners will help deliver tons of torque, and gears from 3.55 to 4.10:1 will help the 406 pull hard.
406 ci
Compression: 9.5:1 to 11:1
Cylinder Heads: Aftermarket with larger runners
Intake Manifold: Aftermarket dual-plane
Carburetion: Aftermarket 750 cfm
Ignition: Aftermarket
Exhaust: 13/4-inch headers with free-flowing system
Axle Ratio: 3.55 to 4.10:1
Stall Speed: 2,800-plus rpm
Duration at 0.050: 225-240 degrees
Idle: Rough
Stage V--Large by Huge 502 ci
Add mammoth stroke and big-port heads, and the Rat returns gobs of torque. Add to that an 800-cfm carburetor, 9.5:1 to 11:1 compression, and a camshaft to match and you have winning power. For this engine, we'd select a roller design with 0.540-0.560-inch lift. And with a 241-260 duration (at 0.050) camshaft, you'll have a rough idle, low manifold vacuum, and need 3.73 to 4.10:1 gears for ultimate performance.
502 ci
Compression: 9.5:1 to 11:1
Cylinder Heads: Rectangular port aftermarket
Intake Manifold: Aftermarket dual-plane
Carburetion: Aftermarket 800 cfm
Ignition: Aftermarket HEI
Exhaust: 17/8-inch headers with free-flowing system
Axle Ratio: 3.70 to 4.10:1
Stall Speed: 3,500 rpm
Duration at 0.050: 241-260 degrees
Idle: Radical