Camshaft Basics and Terminology
THE CAMSHAFT IS THE "HEART" OF THE GASOLINE ENGINE.The Model T engine will not perform to its highest potential unless the cam is precision ground to provide performance at the speeds that you require. Everything else in your engine may be performing tip-top, the carburetor and ignition systems for example. We have only 20 horsepower to work with and we need all of it!
Camshaft/Crankshaft Basics The old time race drivers called the cam the "Jiggler stick" and it was just as mysterious to most of them as it is to us today! Most of us however, are familiar with the strokes of the four cycle Model T engine, intake, compression, power and exhaust.
Each stroke is one half of a revolution or 180° of crank movement. All four strokes (4 x 180° = 720° ) represent two complete revolutions of the crank.
The cam is connected by a 2-1 ratio of gears, so it only turns once to the cranks two turns (Two crank degrees equal one cam degree). When viewed from the front of the car, the crankshaft turns clockwise and the camshaft turns counterclockwise. The purpose of the cam is to operate the valves in the correct sequence in relation to piston movement.
Click on this link for some cool Camshaft Animations.
Camshaft Terminology There are several terms and abbreviations which are used when discussing camshafts. The following abbreviations have to do with the location of the piston in the cycle. For the complete specifications for a Model T cam, look at the Cam Design page.
TC Top Center (piston at the highest point)
BC Bottom Center (piston at lowest point)
BTC Before Top Center (piston rising)
ATC After Top Center (piston lowering)
BBC Before Bottom Center (piston lowering)
ABC After Bottom Center (piston risinng)
Some of the other terms used are illustrated in the drawing and are explained below.
Valve Opening and Closing Angles the angles (usually measured in crankshaft degrees) when the valves first leave and then return to their seats.
Duration - This is the number of degrees the valves are "off their seats" in the four stroke cycle. Duration is usually expressed in crankshaft degrees. Duration may also refer to the number of crankshaft degrees that the lift is greater than a specified value, e.g. duration at 0.050 lift.
Lobe Lift (gross lift) The difference between camshaft lobe height and lobe width. For an installed cam, lobe lift may be measured from the lifter.
Valve Lift (actual lift) This equals lobe lift less the tappet gap measurement or it is the lift measured from the top of the valve.
Valve Clearance (Tappet Gap) or Valve Lash The maximum space between the end of the valve stem and the lifter.
Overlap This is the number of degrees of duration that the intake and exhaust valves are open at the same time. Overlap should not be present in extremely low RPM engines. In fact, the stock Model T Ford cam has 13° of negative overlap. Overlap is present in most high RPM engines and most of the reground cams for Model Ts.
Full Lift or Lobe Centerline The centered point of the highest lift of a cam lobe, expressed in crankshaft degrees.
Lobe Center Angle or Lobe Separation Angle The angle formed between the full lift of the intake lobe and the full lift of the exhaust lobe for a cylinder, expressed in camshaft degrees.
Lobe Terminology - Some of the terminology, which describes a single lobe is illustrated in the drawing below.