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We wish to design three cams for the Model T:
In order to produce the higher performance 280 cam and Laurel-Roof cam, the lift must be increased beyond the stock lift, while not exceeding an intake duration of 225 degrees. The Model T has several physical constraints, which makes these designs difficult. First, in order to increase the lift without increasing the duration, the base circle needs to be larger. To gain an understanding of the interrelationship between lift, duration and base circle size, consider a cam with completely flat sides. This shape has the maximum lift for a given base circle size and duration, so it is useful to consider it as a limiting case. The lift for a flat sided cam with base circle radius Rb, nose radius Rn, valve lash d , and duration of 4fo is: Lmax = [(Rb - Rn)(1 – cosfo ) + d ]/cosfo This equation gives an absolute physical limit on the lift. In reality a cam with flat flanks would be too radical in terms of velocity and acceleration. A cam with a suitable shape will need to have a lift, which is at least 0.030 to 0.040 less than this limiting case. For this reason, we have subtracted 0.030 from the equation and plotted the results in Fig. 5 for a nose radius of 1/32, a valve lash of 0.010 and four values of duration. For example, for a stock duration of 218 and a stock base circle of 0.406, the maximum lift obtainable is about .260. If the duration is increased to 225, the lift could not exceed 0.288. Keep in mind that these are approximate physical limits, a cam with suitable shape may need to have somewhat less lift.
Figure 5. Physical limits on camshaft lift In order to produce a cam by regrinding a worn cam, the size of the original cam imposes additional constraints on the design, because there is only so much metal in the worn cam. A typical worn Model T cam may have a lift of only 0.225. The maximum lift obtainable on a regrind is then: Rb + Lmax < 0.406 + 0.225 = 0.631 This relationship is also plotted on Fig. 5. If we combine this constraint together with the previous one, we find that this severely limits the lift, which can be obtained unless the duration is increased considerably beyond stock specifications. For example, with stock duration, the maximum lift would be about 0.245. If a lift of 0.300 is required, the duration would have to be about 243 degrees. When these constraints are taken into consideration, it is not surprising that the currently available reground Model T cams, especially those with high lift, have durations in excess of 235 degrees. Based on the results from The Cam Project, it would be better to produce a cam with less lift than the available regrinds, so that the intake duration could be maintained at a value closer to stock. From the discussion above, it is apparent that a regrind cannot produce the higher performance cams, which are desired. If we consider a new cam, there are still several constraints imposed on the design. First, the improved stock and 280 lift cams must slide through holes in the block which have a radius of 0.6875 in. The distance from the center of the cam to the tip of the exhaust lobes must be less than this value unless the engine is disassembled and notches are cut in the block. This constraint applies to the exhaust lobes of the first and third cylinders, because the camshaft must be centered before these lobes clear the holes for the front and middle bearings. The intake lobes can be considerably larger than this value, since the cam does not have to be centered as the intake lobes pass through the holes. In terms of the cam design parameters, the base circle radius, Rb, plus the maximum lift, Lmax, must be less than the radius of the holes in the block: Rb + Lmax < 0.6875 This limit is like the one imposed by a regrind, but it is substantially less restrictive. This limit is also plotted on Fig. 5. Using Fig. 5, we find that a new cam with stock duration could have a lift of about 0.265, while one with a duration of 225 could have a lift of about 0.285. The higher lift of the Laurel-Roof cam, will require that the holes in the block be notched to allow the cam to be installed. The requirement that the 280 cam run on stock lifters imposes still another constraint on the design of the cam. As described previously, the radius of the contact point on the lifter is related to the velocity. Applying a small safety factor, a design lifter radius of 0.485, means that the velocity must be restricted to: Vmax < 0.485(0.01745) = 0.00847 in/deg This value is only 20 percent larger than the maximum velocity of a stock T cam. This constraint indirectly restricts the lift and duration of the cam, because the average velocity is given by: Vavg = Lmax /fo where 4fo is the duration in crankshaft degrees. The maximum velocity will tend to increase when the average velocity increases. Due to the restriction imposed by the lifter radius, we have selected a design method called the Triple Curve Design,1 because this method takes into account the lifter restriction and it is relatively easy to apply. We have modified the method to use a constant acceleration ramp instead of a constant velocity ramp. This method is less sophisticated than modern design methods, but it is more advanced than the three-arc design method used on the original Model T cam. Like the three-arc method, the triple curve design produces continuous velocity profiles, but step changes in the acceleration rates. These designs could be refined using a method which produces smooth acceleration rates. All three cams are designed for a valve lash of 0.010 inches. The designs use a smaller acceleration for the ramp area and a larger one for the flank. The smaller ramp acceleration is used for the first 0.012 inches of lift. The ramp velocities are all less than the velocity for a stock cam at 0.020 lift. Due to their higher lift and smaller ramp velocity, all of the cams have maximum acceleration rates, which are larger than the maximum acceleration for a stock cam.
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