Cam Queries And Rockers
How do you calculate the increased lift and duration when you install 1.65 rockers on a cam with specs for a 1.5 rocker arm? My cam is a Crane PN 28SR00002/Grind No. SR-244/362-2S-12.
How would increasing the rocker arms affect the centerline, if at all? Do the increased-ratio rocker arms make the motor act as if the cam was advanced? If so, by how much?
My cam card indicates the power band is between 3,400 to 7,200 rpm. However, with my 10-inch, 3,400-rpm stall converter, I would like to see the power band moved to the 3,000-6,800 range, especially since I do not plan to run the motor any higher than 6,500 rpm. I have been told that by advancing my cam 4 or 6 degrees, I can, in effect, move the power band of the cam down several hundred rpm. Will advancing the cam accomplish this? Will there be any other changes in the characteristics of the power curve?
Vehicle Specifications
Car: '68 Tempest
Weight: 4,000 pounds
Engine: '70 455 (+0.030) 4-bolt mains
Pistons: Arias aluminum
Compression Ratio: 11:1
Rods: Howards aluminum
Rockers: Roller, 1.65:1
Cam Lift with 1.5 Rockers: 0.543/0.561-in
Advertised Duration: 294/302-deg
Duration at 0.050: 244/252-deg
Heads: Edelbrock with mild porting and polishing, 72cc chambers
Valves: 2.11/1.66-in
Carburetor: Holley 1,000-cfm, mechanical secondaries
Intake: Performer RPM
Transmission: Turbo 400
Converter: TCI 10-in, 3,400-rpm stall
Rearend Ratio: 3.73:1 (12-bolt Posi)
Tires: 26x8x15 M/T slicks
Ignition: MSD 6A
Timing: 36-deg total, 14-deg initial
-Tom Laskowski
Cleveland, OH
Jim Taylor responds:
The way to calculate gross lift with different rocker-arm ratios is very simple. You list your cam as PN SR-244/362-2S-12. SR is for solid roller, 244 is the degrees of duration at 0.050-inch lifter rise, and 362 is the cam's lobe lift if measured alone with a dial indicator. From the base circle of the lobe (no lift area) to the top of the lobe would measure 0.362 inch.
The rocker multiplies that 0.362 inch by its ratio in mechanical operation. A 1.50:1-ratio rocker arm increases the 0.362-inch lobe lift one and a half times (0.362 inch x 1.5 = 0.543 inch of gross lift). The 1.65-ratio rocker produces about 10 percent more lift than a 1.5 (0.362 inch x 1.65 = 0.597 inch of gross lift).
This cam has 252 degrees of exhaust duration. These added degrees are designed into the exhaust lobe, which may include a higher lift lobe. Find the lobe lift on the exhaust lobe, then multiply again by the rocker ratio to be used to find the gross lift of the exhaust event.
Gross lift is read at zero lash, but your roller cam needs lash, the clearance at which the valves are set. A cam like yours probably requires about 0.020-inch lash, reducing gross lift of 0.543-inch to 0.523 net lift.
Advancing a cam typically moves the power band lower, giving the engine more low-end grunt, but where is yours now? It's typical to install a cam as per the recommendations on the cam card, such as "110-degree centerline" or "intake opens at 16-degrees before-top-dead-center." This is your starting point, your baseline. If you find in dyno testing or time-speed testing (drag racing) a change in cam position may be beneficial, then advance or retard the cam.
Your 244-degree roller is a small cam, and with a 112-degree lobe separation in a 455 engine, it may have a dead smooth idle. This cam will naturally make about 16 hg of vacuum at 1,000 rpm and have huge low-end torque. Don't worry about revving to 6,500 rpm. This cam's power curve will go flat at about 5,400 rpm in a 455 engine.
The Edelbrock heads and Performer RPM are good choices, but the 1,000-cfm carb is too big. An 850 will be just right. Higher ratio rocker arms will not "make the motor act as if the cam is advanced." If you take out some lash-that is, reduce the 0.020-inch clearance to 0.016-inch-you, in fact, make the valve event happen sooner, "advancing the cam" a very small amount. Valve adjustments are a very important part of tuning. Get a good baseline then make some changes to see what your engine likes. Good luck.