Cam No.
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Duration
Advert. .050
|
hp/rpm
|
Torque/rpm
|
Overlap
|
LSA
|
Intake Centerline
|
|
9777254
|
269/277
|
—
|
265/4,600
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397/2,400
|
47
|
113.0
|
112.5
|
9779066
|
273/282
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200/210
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325/4,800
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445/2,900
|
55
|
111.25
|
106.5
|
9779067
|
273/289
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200/213
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350/5,000
|
445/3,000
|
54
|
113.5
|
113.5
|
9779068
|
288/302
|
212/225
|
360/5,100
|
445/3,600
|
63
|
116.0
|
113.0
|
9785744
|
301/313
|
224/236
|
360/5,400
|
445/3,800
|
76
|
115.5
|
112.5
|
9794041
|
308/320
|
231/240
|
366/5,500
|
445/3,900
|
87
|
113.5
|
112.0
|
Notes: The 254 cam has .374" lift on intake, .407" on exhaust. The 041 cam has .407" lift on both intake and exhaust (.313 lobe height). The other camshafts listed have .407" lift on both intake and exhaust. These lifts are measured with a 1.5:1 rocker arm ratio. Intake centerlines indicate degrees of factory cam advance, usually 1 to 3 degrees. It should also be noted that during the horsepower wars of the first muscle car era, advertised horsepower was not always accurate, due both to competition from other manufacturers and from various racing association rules. This is apparent by looking at the horsepower ratings for the last three cams listed.
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Cam Terms and Definitions
Duration
The time in crankshaft degrees that the valve is open. Pontiac measured their advertised duration at different points on liver rise and fall. Most members of the cam industry now use a lifter rise of .050 inch as a standard reference to measure the total duration. The advertised duration of both Pontiac and aftermarket cams is somewhat meaningless because neither are/were based on a common standard.
Cam Lift
The actual lift of the cam in inches at the maximum cam lobe height. Valve lift is derived by multiplying the cam lift by the rocker arm ratio.
Valve Timing
The point(s) in crankshaft degrees that the valve(s) actually open and close.
Lobe Separation Angle (LSA)
The angular displacement between the centerline of the intake lobe and the centerline of the companion exhaust lobe. LSA is expressed in camshaft decrees.
Overlap
The period of time in crankshaft decrees that both the intake and exhaust valves are open. Again, the overlap at .050 duration is significantly different than that at the advertised duration.
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About Rhoads Lifters
Variable or leakdown lifters are designed to lessen the cam duration and lift at low engine speeds by allowing more oil leakage within the lifter body while the lifter is under valve spring pressure (on the cam lobe). As the leakage is a function of time, the lifters leak the most at low rpm, because there is a relatively long time during the lift cycles. This allows the Rhoads lifter, with its machined leakdown groove in the body, to shorten the lobe duration by 10 to 15 degrees and reduce the lift at low rpm.
As the engine speed increases and the cycle time becomes shorter, less leakage occurs. At 3,000 to 4,000 rpm, the actual leakage and the effect on cam timing become inconsequential. The dramatic increase in vacuum and low-end torque provided by these lifters more than offsets the slight loss of upper-end horsepower in a heavy, automatic-equipped vehicle, such as my wagon. The overall result is the ability to use a fairly radical camshaft to maximize mid-range and top end power while developing strong lowed torque.
Additionally, a variable lifter will operate to a much higher rpm before it begins to hold the valve open due to lifter pump-up. The weight/viscosity of the engine oil used will have a significant effect on leakdown lifters. The thicker the oil, the less pronounced the leakdown eject becomes.
It is neither necessary nor advisable to use Rhoads lifters with a streetable cam such as the 068, because the cylinder pressure may rise too high at low rpm, causing detonation or pre-ignition. There are benefits and disadvantages in all types of variable fibers, but the Rhoads units provide the best overall performance in my wagon.
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The effect of 1.65:1 Rocker Arms on Duration
We all know that 1.65:1 rocker arms provide 10 percent more lift than 1.5:1 arms, but how do they affect the cam durations. Technically, the 1.65 rockers do not change the duration of the cam lobes. However, the 1.65 rockers cause the valve to reach the .075-inch (.05-inch x 1.5 rocker arm ratio) lift point 1.5 to 2 cam degrees earlier than with the 1.5 rockers. Therefore, the use of 1.65:1 rockers will make a cam of about 230° (measured at .050 inch lift) act about 3° to 4° greater in duration, depending on the lobe design. This duration difference must be considered when attempting to duplicate the performance of a cam using 1.65 rockers with a different cam and 1.5 rockers.
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