First Choice

How to Choose the Right Cam

Bob Mehlhoff Feb 6, 2004 0 Comment(s)
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One of the most defining aspects of a real performance car is a performance cam, a stick that raises lift, adds duration, and gives the engine a distinctive idle that sounds really cool. But sometimes choosing the right camshaft to deliver that extra power while maintaining good driveability can be challenging. Sure, it's easy to just pick the cam listed at the bottom of the cam manufacturer's page to ensure a radical idle, but you want more than just a signifying sound.

Like all other performance modifications, the best results are obtained when parts are matched to complement each other. Lower gears, freer-flowing exhaust, cylinder-head characteristics, and transmission type must all be considered before deciding which camshaft to choose. To make this decision properly, you must determine what you want from your car. Do you want it to be a double-duty street/strip car, a tow vehicle, or a very dedicated performance ride that sees limited street use? Compression ratio and engine size will also affect which camshaft is best. For example, the same performance camshaft installed in a 400ci small-block will sound significantly tamer and have a lower powerband than the same cam installed in a smaller 283. This is because (all else being equal) the smaller engine has less volume (less pumping capability) and less engine vacuum.

Before we travel too far ahead, let's lay down some guidelines first. For our camshaft discussion, we'll use five separate stages of engine and performance categories. Stage I is a daily driver--a 305 engine that we just intend to make perform better. Stage II uses a relatively stock 350 engine that has a performance exhaust system, aftermarket intake manifold, and a four-barrel carburetor. For Stage III, we'll use a 383 with Stage II components and add headers, 3.55 gears, and a 2,500-stall converter. Stage IV is a stout 406 small-block with better-flowing cylinder heads, 3.70-4.10 gears, aftermarket ignition, a 3,000-stall converter, and a shift kit. Finally, Stage V focuses on an all-out 502 Rat equipped with rectangular-port cylinder heads, lots of compression, and a single-plane intake with an 850-cfm carburetor.

Next, we'll consider camshaft duration, which is critical to the amount of power the engine will produce and at what rpm range it will do so. Since a camshaft with more duration bleeds off more cylinder pressure, it's important to consider displacement and compression ratio. As an industry standard, most camshaft companies measure duration at 0.050-inch valve lift. As a general guideline, we've provided the following chart to explain the range of camshaft durations and the anticipated powerbands.

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This stout 406ci small-block uses a Comp Cams Extreme Energy (XE274) hydraulic flat-tappet camshaft. With 230/236 degrees of duration (at 0.050) and 0.490-inch lift, it produces gobs of power on the street and strip. Its latest outing produced a mid-12-second pass with 3.73:1 gears and a high-stall converter.

Hydraulic, Solid, or Roller Lifter

You'll have to choose between hydraulic, mechanical, or roller lifters. Hydraulic lifters are typical. They provide quiet operation and almost never need adjustment. However, for very-high-rpm operation, hydraulics will often bleed down and cause valve float. Mechanical lifters allow higher engine rpm (above 7,000) but require frequent valve adjustments and have a noisy clatter at idle. Today, most of the antipump-up hydraulic lifters work well to almost 7,000 rpm.

For the more serious street contender, enter the world of roller camshafts. The name roller describes the small wheels installed on the bottoms of the lifters that reduce friction and allow a very aggressive camshaft profile that can directly enhance power levels. Roller camshafts are available in both hydraulic and mechanical designs. Plan on spending considerably more for a roller camshaft, especially if your engine was originally designed for flat-tappet lifters.

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It's a Spring Thing

The most frequently encountered problem when adding a new camshaft is using valvesprings that were not designed to work with it. Stock springs will often be too weak, bound-up, or just plain worn out. These conditions ensure limited rpm use or poor performance. Stock springs work fine for stock camshafts with low lift; with high-lift cams, always use the manufacturer's recommended valvesprings.

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

Sources

Crane Cams
Daytona Beach, FL 32117
866-388-5120
http://www.cranecams.com
Comp Cams
Memphis, TN 38118
800-999-0853
http://www.compcams.com
Erson Technical Service
Carson City, NV 89701
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