Your Harley-Davidson flathead 45, flathead 80, or knucklehead engine will not develop its full power without improving the cam. Increasing the stroke length, intake valve size, porting & relieving, etc. all help increase power, but they do not extend that power into the upper RPM range that really gets results. Obviously, the choice of which cam to use depends on how your engine was already modified, and what you’re going to use it for. A hotter cam does not always mean more power, or even more peak power.
   The original 45 & U-Series cams have approximately the same mild duration on both the intake and exhaust cams. However, for performance use, Harley-Davidson side-valve (flathead) motors need more intake cam duration than exhaust cam duration (the factory race motors used about 30-35° more). This is partially due to the fact that the intake ports are fairly small, badly shaped, and include many inefficient curves, while the exhaust valves are relatively large with much better ports.
   When planning a street motor, or a motor for a heavy bike, a mild intake cam can be used with the stock exhaust cams. As the intake cams get hotter (more duration), the same bias should be retained - use an exhaust cam with less duration (milder than the intake cam). Never install hot exhaust cams with stock intake cams.
   A high performance motor must use hotter intake cams to increase volumetric effiency (torque), and move the power higher up on the RPM scale. These should be used with somewhat milder exhaust cams as mentioned, supra, but this will not work with a restrictive exhaust system. A high backpressure muffler will completely confuse the carburetor and make accurate jetting nearly impossible. These cams only work well with completely open exhaust, or low restriction muffler such as Screaming Eagle, Cycle Shack, SuperTrapp, etc.
   Straight pipes with internal baffles are an extremely bad idea for any motor.

   No motor “needs” backpressure, there is no “minimum backpressure” required to make a motor work. If the motor runs better with more backpressure, the carburetor adjustment was incorrect. What is required is a minimum exhaust gas speed for the motor to operate cleanly, which is why larger diameter exhaust pipes are absolutely not needed. The tuned exhaust pulse produced by open pipes will interfere with carburetor operation at certain RPM. This can be reduced inserting a reflective object to “break up” the pulse. This can be done by drilling a ¼” hole 1” from the open end and inserting a 1” long bolt into the pipe secured with washers & nut, and reduce the effect without loss of power. A better shape is the old school “Snuff-or-Not” product. This is no longer available, but one can be made from small hardware parts. Note the spring and adjustment knob for selecting position: with or against the gas flow. Click the image for a larger view.

   45 strokers using 4-7/16” or larger flywheels should use bigger cams than stock stroke engines.    No sidevalve cams require piston-to-valve clearance. Some will require valve clearance in the head (as noted). All hotter cams work better with heavier valve springs. All hotter cams require minor carburetor adjustment for best results. Linkerts and other carburetors with limited adjustment range may be difficult to tune with hot cams, especially with open exhaust. Mikunis, Amals and other carburetors with multiple circuits are easier to re-jet but will still require substitution of minor components especially to the idle/pilot system.    The re-grinding process re-shapes the original cam lobes. No welding has been needed in the past, because the roller tappet does not require a specially-hardened surface (as flat tappets do), and the new lobe shape can be made from the original with only minor metal removal (mostly from the base circle). This is why these profiles are limited to only slightly more lift than the original lobe.
   Regrinding can also repair minor lobe surface damage (same prices), but not chipping, broken or missing gear teeth, worn or damaged journals, etc.
   The regrinding process locates the new lobe center exactly at the existing point of maximum lift. The extra duration (in degrees) is divided equally between both ends of the cam event.

   For example, an intake cam with 30° extra duration will open the valve 15° sooner, which extends the overlap duration, and will require idle circuit enrichment and slightly reduce low speed power. If a hotter exhaust cam is also used, overlap is extended even more. Higher idle speed may be needed.
   An intake cam with 30° extra duration will close the intake valve 15° later, which reduces cranking compression and low speed power. This can and should be partially offset by higher compression ratio.
   Alternate lobe centers are not possible. For those who wish to get the best results from their cam installation, minor adjustments to the installed position are possible by modifying the motor itself.
   For engines with stock or near stock compression ratio, and stock stroke 45 engines using large port cylinders such as WLDR, WR, or K, the intake valve should close no later than 60° ABDC.
   For engines with significant compression ratio increase and high engine RPM, and strokers, the intake valve should close no later than 65° ABDC, as shown in the Table, right. Click here for more details: .
   This means that the intake event timing for several different durations can be predicted:

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