Mr. Mopar's Lingo Page

Base Circle: The round part of the camshaft lobe, or heel, where no valve lift occurs, valve lash adjustment is set, or the "zero lift" plunger depth of a hydraulic lifter is set.

Cam Follower: Lifter, tappet, or in some overhead camshaft applications, the rocker arm. This is the device which is used to transfer the rotary motion of the camshaft into the reciprocating motion of the valve. Hydraulic, solid, roller, hydraulic roller, and mushroom are types of followers. There are several versions of arm type cam followers which operate with a fulcrum which can be solid or hydraulic, a contact surface with the cam which can be solid or roller, or in some engines a hydraulic lash adjuster between the tips of the arm and the valve stem tips.

Cam Master: The pattern of a cam's profile made as a permanent template to be used for cam grinding machines to follow or copy to produce that exact profile on the camshaft.
These masters are much larger than the camshaft lobe being generated providing great profile accuracy.

Cam Profile: The actual shape of the cam lobe from the heel or base circle to the opening ramp, flank (or side of the lobe), nose (or top of the lobe where maximum lift occurs), closing flank and closing ramp.

Clearance Ramp: The portion of the cam lobe profile, adjacent to the base circle, which slowly starts the motion of the cam follower to lift the valve. On the closing side of the lobe, the closing ramp again slows the motion of the follower, and hence the valve, so that the valve does not slam against the valve seat, which could cause valve bounce--resulting in damage to the valve seat face and valve seat.

Coil Bind: (a BAD thing!) When a valve spring has been compressed (during valve opening) to the point where the coils of the spring come into contact with each other. If the valve lift produced by the camshaft is too much for the spring, many small parts are made quickly--see Catastrophic Failure

Duration: The time, measured in degrees of crankshaft rotation, that the cam follower is off the base circle, causing the valve to be off the seat. There is an industry accepted standard of using .050 lift to reference duration. Most agree that .050 lift is the point at which effective flow through the valve opening occurs. Because of the various opening and closing ramps of cam profiles, the so-called "advertised" specs are virtually impossible to compare from one cam profile to another.

Long duration cams are used where low rpm, smoothness, and quiet operation (such as "stock" land yacht engines) is desired. Short duration cams also produce high low rpm torque.

Generally, the higher the cam's duration, the higher the "power band" of the engine. For instance, a stock engine in an Imperial might have a cam duration of 190 degrees (at .050 lift) which would give the engine tremendous torque from idle speed to perhaps 2500-rpm and peak power at 3800-rpm. Even in a "heavy", a 440 with this cam would fry the tires, but the power range is short and it would lay down quickly. For example, if you took the same engine and vehicle combo and upped the duration to 210, the power band would come up to maybe 1200 to 4200; 230 degrees and you'd be up to 1800 to 4800; 245 and 2500 to 5400; 254 degrees would get you to peak torque around 4000-rpm and max power at 6000.

You would, at this point, have had to change the intake, carb, valve springs, exhaust, and torque converter--you'd have almost no vacuum to operate the brakes--you'd have a hot...rod...Imperial?!?!

Dynamometer: A device used to measure the torque produced by an engine. Horsepower can be calculated using the torque produced at the RPM at which it (the torque) was measured.

Gross Lift: Theoretical maximum valve lift calculated by multiplying the cam lift by the rocker arm ratio.

Hydraulic Lifter: A cam follower which has an internal chamber where engine oil pressure is channeled under a floating pushrod socket which allows the valve train (ie., lifter pushrod, rocker arm) to operate at "zero lash" or no clearance. The engine oil pressure is not high enough to push the plunger up to open the valve, but the hole in the lifter which channels the oil in is too small for the oil to be pushed out as the cam turns to open the valve. Within the limits of travel of the pushrod socket or plunger the hydraulic lifter is self-adjusting.

Hydraulic Roller: A cam follower which incorporates the benefits of a quiet zero lash, zero maintenance valve train with the more agressive cam profiles which can be had with the roller follower.

Installed Centerline: This is the number of degrees of crankshaft rotation after TDC (Top Dead Center) where the centerline of the intake lobe (max lift) occurs. This is NOT a camshaft specification, but rather an installation parameter. Moving the installed centerline (advancing or retarding the cam relative to the crankshaft) can have a major affect on performance. This also affects the valve-to-piston clearance--see Catastrophic Failure

Lift: The distance the valve is raised from its seat in the cylinder head. Generally, increases in lift increase the torque produced by the engine. Lift is something which can be increased with very little detrimental effect to the other parameters of engine operation.

There is a physical limitation to the amount of lift which can be designed for a specific duration with flat surface followers. Consequently, increases in rocker arm ratio are often used to get "more lift" from a given cam profile.

Lobe Centers: This is the number of degrees of crankshaft rotation between the centerline of the intake and exhaust lobes for a given cylinder, generally in the 106º to 114º range. This relationship greatly affects the performance of the engine. Careful consideration should be given to this specification on any camshaft application.

Mushroom Lifter: A cam follower which has a cam lobe contact surface diameter larger than the "lifter bore" or the small cylinders in the cylinder block which guide the cam followers as they travel to the camshaft rotary motion into reciprocating. Necessarily, they are installed from the bottom of the lifter bore before the cam is installed.
Many early engines and most "flat head" engines had mushroom lifters. Because of the assembly and maintenance problems, they are not in common use today, however, they do exist and allow "more cam", meaning a more agressive profile, than would a smaller diameter lifter face. This is why the "off-brand" boys are always trying to sneak larger diameter lifters into their engines. Mopar engines have the largest stock diameter in the industry.

Overlap: This is the number of degrees of crankshaft rotation during which both the intake and exhaust valves are open. (This is best evaluated using the degrees measured at .050 lift.) This is the camshaft characteristic which creates the idle quality--the more overlap, the rougher the idle.

Rocker Arm Ratio: The rocker arm is used to change the direction of travel of the lifter and pushrod which follow the cam lobe. The cam pushes up but the valve has to go down. The rocker on a shaft (or stud, now that we have cheapened the engine) "rocks" to push the valve down. If we make the "arm" on the valve side of the shaft longer, we increase the amount of valve lift that the same motion on the pushrod side gives us. The most common rocker arm ratio is 1.5:1, meaning that .300" lifter rise would move the valve .450". Changing to 1.6 would make the valve move .480", 1.7 ratio gets you up to .570"--see Coil Bind

Roller Lifter: A cam follower which has a cylindrical wheel which turns on needle bearings around an axle. This wheel rolls on the surface of the cam lobe, the obvious advantage being an elimination of friction. The physical limitation of the "edge" of a flat lifter is also eliminated, allowing much more rapid opening and closing events on the cam lobe.

Solid Lifter: Also called a flat tappet, this is a cam follower which makes a solid mechanical connection between the cam lobe and the pushrod and requires "clearance" to be allowed between the rocker arm tip and the valve stem tip. While not perfectly flat, the surface of the lifter that interfaces with the cam lobe is called flat, hence "flat tappet"--"tappet" is the old terminology still used by hard-nosed engine heads.

Spring Bounce: When the spring cannot control the motion of the the valve due to low tension (might be the wrong spring!) or improper or radical cam lobe profile.
This is what is often referred to as "valve float"--see Catastrophic Failure

Torque Converter: A fluid coupling between the engine and transmission in an automatic. The outer case or shell of the converter is attached to the engine's crankshaft and the inside is connected to the input shaft of the transmission. The shell has turbine blades which push the transmission fluid (which is being forced into the converter by a pump in the transmission) against the vanes of the impeller splined to the input shaft (see pictures).

As engine rpm's increase, the "fluid drive" starts to turn the impeller and hence, the car. The stall speed which the engine has to be to trn the impeller and hence, the car. The stall speed of a converter is the speed which the engine has to have to turn the impeller. A "loose" or high stall converter requires a higher engine rpm to move the car. This stall speed can be tailored to the requirements of the engine as dictated by teh camshaft (among other things).

Valve Spring Surge: A condition caused by improper design or application of the spring in question, allowing the valve train (the pieces between the valves and the cam--springs, rocker arms, etc.) to separate and the valve to move uncontrolled--see Catastrophic Failure.
Proper design of the springs involves calculation of the natural frequency of the spring based on the type of material, wire diameter, number of coils, as well as operating parameters such as intended lift and RPM.

Catastrophic Failure: The process of making a big mess in a big hurry, yielding lots of unusable parts and little pieces thereof, most generally because the wrong parts were used or the parts were used wrong.

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