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Original Model T Ford Camshaft Design

 

Figure 1. Post-1912 Model T camshaft lobe and lifter, 45 degrees before maximum lift

 

The specifications for the original post-1912 Model T Ford camshaft are listed in Table 1. This cam is of the three arc or harmonic design. One arc forms the base circle, a small radius arc forms the nose and a large radius arc forms the flank. Designs of this type were common during the Model T era. Unlike later designs, there is no special ramp area modification for this cam. The shapes of the intake and exhaust lobes are identical and symmetric.

 

Table 1

Original Model T Camshaft Specifications1

Intake Opening

12.67 deg. ATC

Intake Closing

50.82 deg. ABC

Intake Centerline

121.75 deg. ATC

Intake Duration

218.15 deg. (crankshaft)

Exhaust Opening

37.87 deg. BBC

Exhaust Closing

0 deg. TC

Exhaust Centerline

108.94 deg. BTC

Exhaust Duration

217.87 deg. (crankshaft)

Angle Between Lobes

115.35 deg. (camshaft)

Gross Lift

0.2502 in

Base Circle Radius

0.4060 in

Nose Radius

0.0313 in

Flank Radius

1.2601 in

Lifter Radius

0.5 in

Bearing Outside Radius

0.6875 in

 

Fig.1 is a drawing of a single lobe constructed from the data in Table 1. Equations are available to calculate the lift curves from this information.2 The lift is:

 

Opening Flank:    L = 0.8541 [ 1 cos(68.57 + f ) ]    for -68.6 < f < -40.3

Nose:                   L = 0.2502 0.6250 ( 1 cos f )    for -40.3 < f < 40.3

Closing Flank:     L = 0.8541 [ 1 cos(68.57 - f ) ]     for 40.3 < f < 68.6

 

where f is the angle of the camshaft relative to the lobe centerline. As the cam rotates counterclockwise (as viewed from the front of the engine) with no lifter clearance or valve lash, it rides on the base circle until an angle of -68.6 degrees, i.e. 68.6 degrees before the lobe centerline. From this point it rides on the opening flank until -40.3 degrees is reached. It rides on the nose from -40.3 to +40.3 degrees, then on the closing flank and finally on the base circle at +68.6 degrees. Fig. 1 depicts the cam at -45 degrees, i.e. immediately before it leaves the opening flank and begins riding on the nose.

 

Fig. 2 compares the lift curves calculated from the equations to those measured on a NOS (new old stock) cam. The measurements, taken with a simple degree wheel and dial indicator, agree well with the calculated curves. The measured centerlines for the lobes were 121.1 deg ATC for the intake lobe and 110.4 deg BTC for the exhaust lobe, which agree well with the specifications listed in Table 1.

 

Figure 2. Lift curves for stock cam, measured (symbols) and calculated (lines)

 

Figure 3. Opening lift curve with measured data showing symmertry

 

Fig 3 is an expanded view of half of a lift curve. Since the intake and exhaust have identical symmetrical lobes, the curve in Fig. 3 is representative of the complete curves in Fig. 2. All of the measured data (both opening and closing) from Fig. 2 are plotted on Fig. 3 to illustrate this point.

 

Model T camshafts are installed with a valve lash or clearance of 0.010 to 0.032 inches. The duration of valve opening for any given lash can easily be determined from Fig. 3. For example, a lift of 0.020 occurs at 56.1 degrees, so a valve lash of 0.020 would produce a seat-to-seat duration of 2x56.1 camshaft degrees or 224.4 crankshaft degrees. Table 2 shows the duration for various values of lift. By comparing the values in Table 2, it is apparent that valve lash has a significant influence on the seat-to-seat duration. To obtain a duration of 218 degrees as specified in Table 1, the valve lash should be 0.0256 in. for a perfectly ground stock cam. This lash is quite large, even by standards of the Model T era. As a result, the actual valve lift is only 0.225 in.

 

Table 2

Stock Cam Duration and Velocity vs. Lift

Lift

inches

Duration

Cam Degrees

Duration

Crank Degrees

Velocity

in/cam deg

0.010

119.6

239.2

0.0023

0.015

115.6

231.2

0.0028

0.020

112.2

224.6

0.0032

0.025

109.3

218.7

0.0035

0.0256

109.0

218.0

0.0036

0.030

106.7

213.4

0.0038

0.035

104.2

208.4

0.0042

0.040

101.9

203.8

0.0045

0.050

97.7

195.5

0.0050

0.065

92.1

184.2

0.0057

 

The lift curve controls the flow of gases in to and out of the combustion chamber. Other important aspects of the design are the valve train velocity and acceleration rates produced by the cam. The velocity is the rate of change of the lift with respect to a change in the camshaft angle and is normally expressed in inches per degree of camshaft rotation. These values must be multiplied by half the engine speed to obtain an actual velocity, i.e. rate of change of distance with time. A typical Model T engine rarely exceeds 2000 RPM, which would correspond to 50 mph with a stock drive train. However, with an accessory overhead valve conversion, e.g. Frontenac, Roof or Rajo, engine speeds in excess of 4000 RPM are possible.

 

For the stock Model T camshaft, the opening velocity in units of inches per camshaft degree is:

 

     Flank:    V = (0.01745)(0.8540) sin(68.57 + f )

     Nose:    V = -(0.01745)(0.6250) sin f

 

The factor 0.01745 is p /180. Equations for the acceleration rates are also available.2 Fig. 4 shows the velocity and acceleration rate together with the lift curve.

 

Figure 4. Calculated lift, velocity and acceleration for stock cam

 

The ramp velocity is important. On opening, the ramp velocity is the velocity at the point of first contact between the cam and lifter, so it determines the impact of the first contact. On closing, it governs the impact as the valve comes into contact with the seat. A large ramp velocity can contribute to noisy operation and high wear. The velocities for various lifts are listed in Table 2. Later methods of cam design incorporated special modifications of the ramp area. Ramp velocities of 0.0005 to 0.0010 were recommended for trucks and low speed engines and 0.0015 to 0.0050 for large diesel and aircraft engines.2 By comparing these numbers to those in Table 2, the Model T ramp velocity appears to be relatively high.

 

The rate of acceleration times the valve train mass determines the force caused by valve train motion. During valve opening this force acts together with the force of the valve spring to determine the total force acting on the camshaft. During valve closing this force must be overcome by the valve spring to insure that the lifter remains in contact with the cam lobe. High acceleration rates require a stiffer valve spring and contribute to greater wear. Due to its simplicity, the mass of the model T valve train is low. However, for an accessory overhead valve head, the mass would be much larger due to the addition of push rods and rocker arms. No calculations of valve spring and other valve train requirements have been performed in this study. However, detailed measurement on a reground cam showed maximum acceleration rates of about 0.0006 to 0.0007 in/(cam deg)2. This reground cam works fine even though the maximum acceleration rate is more than twice that of a stock T cam. Values in this range are used as target maximum acceleration rates in this study.

 

Another important aspect of the cam design is the location of the point where the lifter contacts the cam lobe. The radial distance of this point from the lifter centerline determines the required lifter diameter. This distance can be calculated by dividing the velocity by 0.01745. From Fig. 4, the maximum velocity is 0.00705 in/deg at -40.3 degrees giving a maximum radius of contact on the lifter of 0.404 inch, which is well within the 0.5 inch radius of a stock Model T lifter. For reference purposes, Fig. 1 is a scaled drawing of the cam and lifter at -45 degrees. At this angle, Fig. 4 shows the velocity is 0.00595, so the contact point in Fig. 1 is at 0.341 inches.

 

References

  1. Model T Ford Service Bulletin Essentials, Dan R. Post, ed., Lincoln Publishing, Lockport, NY, 1997

  2. M.C. Turkish, Valve Gear Design, First Edition, Eaton Manufacturing Co., Wilcox Rich Division, Detroit Michigan, 1946