The following collection of technical information should provide a basic understanding of everything from learning cam profile terms to more advanced topics such as verifying your valvetrain geometry.
RAMPS The parts of a camshaft lobe that actually initiate the lifting and descending movement of the lifter are called "ramps". Ramps include the lash ramp, the opening ramp, and the closing ramp. Camshaft lobe ramps are ground to have different rates of lifter movement in terms of velocity and degrees of duration, as measured in degrees of crankshaft rotation.
The "lash ramp" of a camshaft lobe is a mid-point location between the opening ramp and closing ramp.
The "opening ramp" of a camshaft lobe is the point where the lifter just begins to lift until the point that it reaches the nose of the lobe.
The "closing ramp" is the point of the camshaft lobe from the nose back down to the lash ramp.
NOSE The "nose" of a camshaft lobe is the top or the highest maximum lift point for the valve. It is where valves are kept open for as long as possible before making the transition to the closing ramp.
BASE CIRCLE The "base circle", also known as the "heel", is the lowest point of the camshaft lobe and is the place where the valve is in the closed position. The "base circle" is the point where all valve lash settings are made.
SYMMETRICAL is a term that refers to the "profiles" of the opening and closing ramps of a camshaft lobe. All "early technology" camshafts were ground on a symmetrical design, meaning both sides are exactly the same. That is to say the profile of the closing ramp is a "mirror image" of the opening ramp.
ASYMMETRICAL refers to a camshaft lobe profile where the opening and closing ramps are not exactly the same. The reason some camshafts are this way is to try to achieve an opening ramp profile that has a high velocity and a closing ramp profile that has a slower velocity. In this way the valve can be set down more "gently" than the rate at which it was first opened.
A DUAL PATTERN camshaft has an intake lobe profile design that differs from that of the exhaust lobe profile design. For example, camshaft "A" has intake lobes of 260 degrees duration while the exhaust lobes are 270 degrees duration. Camshaft "B", has intake and exhaust lobes that are both at 260 degrees. Camshaft "A" is referred to as a dual pattern, while camshaft "B" is referred to as a single pattern.
With the advent of emissions laws and the widespread use of computer systems, more modern single and dual profile pattern designs have been developed. A dual pattern camshaft is ground to "bias" the duration of either the intake or exhaust lobe. For example, if an engine is restricted on the exhaust side, compared to the intake side, the camshaft designer would try to compensate by grinding in more lift and/or duration on the exhaust lobe.
If you are installing a new flat tappet camshaft in your engine, you must install new lifters. The lifter/lobe contact area is under a tremendous amount of pressure, and the lifter and lobe must wear in together. The following illustration demonstrates why:
Lift refers to maximum valve lift. This is how much the valve is "lifted" off its seat at the cam lobe’s highest point.
How is it measured?
Cam Lobe Lift Valve Lift is the amount (usually in inches) that the valve is lifted off of its seat. It is usually measured with a dial indicator at the tip of the valve. Lobe Lift is the amount (usually in inches) that the cam lobe increases in radius above the cam base circle.
Tip: To quickly find maximum lobe lift, measure the base circle of the cam and subtract it from the thickness across the cam lobe’s highest point (see the diagram below).
Tip: Maximum valve lift can be calculated by multiplying the maximum lobe lift times the rocker ratio. For example, a 0.310" lobe lift cam yields 0.496" of valve lift when using a 1.6 ratio rocker arm.
Formula: valve lift = lobe lift x rocker ratio
What does it do?
The intake and exhaust valves need to open to let air/fuel in and exhaust out of the cylinders. Generally, opening the valves quicker and further will increase engine output. Increasing valve lift, without increasing duration, can yield more power without much change to the nature of the power curve. However, an increase in valve lift almost always is accompanied by an increase in duration. This is because ramps are limited in their shape which is directly related to the type of lifters being used, such as flat or roller.
Duration is the angle in crankshaft degrees that the valve stays off its seat during the lifting cycle of the cam lobe.
How is it measured?
Advertised duration is the angle in crankshaft degrees that the cam follower is lifted more than a predetermined amount (the SAE standard is 0.006") off of its seat. Duration @.050" is a measurement of the movement the cam follower, in crankshaft degrees, from the point where it’s first lifted .050" off the base circle on the opening ramp side of the camshaft lobe, to the point where it ends up being .050" from the base circle on the closing ramp side of the camshaft lobe. This is the industry standard, and is a good value to use to compare cams from different manufacturers. Both are usually measured with a dial indicator and a degree wheel.
What does it do?
Increasing duration keeps the valve open longer, and can increase high-rpm power. Doing so increases the RPM range that the engine produces power. Increasing duration without a change in lobe separation angle will result in increased valve overlap.
Lobe separation is the angle in camshaft degrees between the maximum lift points of the intake and exhaust valves. It is the result of the placement of the intake and exhaust lobes on the camshaft.
How is it measured?
Cam Lobe Separation Lobe separation can be measured using a dial indicator and a degree wheel, but is usually calculated by dividing the sum of the intake centerline and the exhaust centerline by two.
What does it do?
Lobe separation affects valve overlap, which affects the nature of the power curve, idle quality, idle vacuum, etc.
Overlap is the angle in crankshaft degrees that both the intake and exhaust valves are open. This occurs at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift duration and/or decreasing lobe separation increases overlap.
How is it measured?
Overlap can be calculated by adding the exhaust closing and the intake opening points. For example, a cam with an exhaust closing at 4 degrees ATDC and an intake opening of 8 degrees BTDC has 12 degrees of overlap. But keep in mind that since these timing figures are at 0.050" of valve lift, this therefore is overlap at 0.050". A better way to think about overlap is the area that both lift curves overlap, rather than just the crankshaft angle that both valves are open. Therefore, one can see that decreasing the lobe separation only a few degrees can have a huge effect on overlap area.
What does it do?
At high engine speeds, overlap allows the rush of exhaust gasses out the exhaust valve to help pull the fresh air/fuel mixture into the cylinder through the intake valve. Increased engine speed enhances the effect. Increasing overlap increases top-end power and reduces low-speed power and idle quality.
The intake centerline is the point of highest lift on the intake lobe. It is expressed in crankshaft degrees after top dead center (ATDC). Likewise the exhaust centerline is the point of highest lift on the exhaust lobe. It is expressed in crankshaft degrees before top dead center (BTDC). The cam centerline is the point halfway between the intake and exhaust centerlines.
Advancing or retarding the camshaft moves the engine’s torque band around the RPM scale by moving the valve events further ahead or behind the movement of the piston. Typically, a racer will experiment with advancing or retarding a cam from "straight up" and see what works best for their application. Lunati camshafts are ground to provide maximum performance and are designed to be installed to the specifications listed on the cam card.
How is it measured?
A cam with a 107 degrees intake lobe centerline will actually be centered at 103 degrees ATDC when installed 4 degrees advanced.
Most Lunati camshafts have a certain amount of advance ground in. "Ground-in advance" can also be found by subtracting the intake lobe centerline from the lobe separation.
What does it do?
Advance improves low-end power and response. For a general summary of the affects of camshaft timing, refer to the following tables:
The hydraulic flat tappet is self-adjusting, due to the valve controlled plunger within the tappet body. It operates to pre-load the push-rod by using the oil system pressure to maintain this pre-load in the closed valve position. Hydraulic tappets are quieter than mechanical tappet lifters since there is no lash or free-play. However, it is generally agreed that they fall short of offering optimum performance above 6,000 - 6,500 RPM. Many cheaper designs fall even shorter than this. This poor performance at high RPM is due mainly to the inability of the lifter to "bleed down" the excessive oil pressure , and thus does not allow the valves to seat.
The mechanical (solid) tappet is essentially a solid "link" between the cam lobe, and the push-rod. In most cases it is a simple heat-treated cylinder with a radiused contact face. It allows more RPM potential than that of the hydraulic tappet since there are no worries about the inability of the lifter to "bleed down." Solid lifters do, however require lash or clearance to allow for part expansion as the engine heats up.
The mechanical (solid) roller tappet allows for the most aggressive lobe designs. Roller tappets allow faster, "steeper" opening and closing ramps. This allows the cam to produce more lift for a given duration. They are not limited to a particular lifter diameter to obtain higher cam lifts. They also contain a roller that reduces friction between cam and followers. Roller cams require the use of higher valve spring forces making high engine speeds (over 10,000 RPM’s) possible.
The hydraulic roller tappet camshaft can provide the best of both worlds. Diesel engines and some motorcycle engines have used this design for many years. They provide most of the virtues of a solid mechanical roller tappet while providing the benefits of quiet operation and ease of valve lash setting.
This type of design still has the limitations of an oil bleed-off control type follower. If your application requires high RPM potential you should use a solid roller tappet design.
The following is a method of verifying proper valve train geometry. After you have estimated the required pushrod length using a Lunati Pushrod Length Checker, use this method to verify that the valve train geometry is correct (using the rockers you are using in your engine.)
1. The first step is to install a solid lifter and an adjustable pushrod. Mark the tip of the valve with a marker.
2. Install your rocker arm and set it up with zero lash. Rotate the crankshaft clockwise several times. Remove the rocker arm. The contact pattern of the rocker tip will be where the marker has been wiped away from the valve tip. The pattern should be centered on the valve tip, and as narrow as possible. If it is not, experiment with varying the pushrod length to yield the best pattern.
3. Rotate the crankshaft clockwise several times. Remove the rocker arm. The contact pattern of the rocker tip will be where the marker has been wiped away from the valve tip. The pattern should be centered on the valve tip, and as narrow as possible. If it is not, experiment with varying the pushrod length to yield the best pattern.
4a. Pushrod Too Long: Notice how the pattern is wide, and shifted to the exhaust side of the valve tip
4b. Pushrod Too Short: Notice how the pattern is wide, and shifted to the intake side of the valve tip.
4c. Pushrod Length Correct: Notice how the pattern is narrow and is centered on the valve tip.
Valve spring pocket clearance is the gap between the inside diameter of the valve spring pocket (or cup, if used) and the outside diameter of the valve spring.
The valve spring retainer should fit the valve spring being used. A slightly snug fit is acceptable, however a fit that is too tight can overstress the top coil, and cause it to fail. A fit that is too loose can lead to spring "dancing."
The installed height of the valve spring is the distance between the valve pocket (or cup, or shims) and the outer edge of the spring retainer (which is the height of the valve spring) when the valve is closed. To check installed height, follow the following procedure:
The distance between the innermost step on the valve spring retainer and the valve guide must be 0.090" larger than the maximum valve lift of the camshaft. Measure the distance between the top of the valve seal to the bottom of the valve spring retainer. After adding 0.090" to your measurement, it should still be larger than the maximum valve lift of the camshaft. If not, machining of the valve guide in necessary for adequate clearance.
Coil clearance is the distance between the valve spring coils when the valve is it maximum lift (fully open). A minimum of 0.060" must exist between the coils at maximum lift. Coil bind is when the valve spring is compressed fully-to the point that all of the coils are "stacked up" on top of each other. For high RPM applications, .100" is recommended . Coil bind is a catastrophic condition that will result in valve train failure. Disassemble each spring (if multiple springs are employed at each valve). Check all the springs (both inner, and outer springs) If there is not 0.060" - 0.100" minimum of clearance between the coils, the solutions are: the valve retainer, the valve locks, the valve, or the spring must be changed; the spring pocket must be machined. Keep in mind that these modifications will change the valve spring installed height
When installing the rocker arms, check to see that the inside of the rocker arms clear the spring retainers. Many rocker arms have a "relief" to accommodate large valve spring retainers.
Each set of Lunati valve springs are hand-selected to keep load variations below +/- 10% of the next. However, it is important to "run in" your new valve springs at low RPM using the following procedure:
The below topics will help you answer the question above:
Before selecting a piston, the desired compression height must be known. As shown, compression height is the distance between the centerline of the pin bore and the top of the piston. To determine the compression, three things about the engine must first be known: block height, connecting rod length and crankshaft stroke length.
Deck height is measured from the crankshaft centerline to the deck (cylinder head mounting surface) of the block.
Connecting rod length is measured between the centers of the "big end" (journal end - rotating) and the "little end" (piston pin end - reciprocating).
Stroke length is twice the distance from the centerline of the crankshaft main bearing journals to the centerline of the connecting rod journals. It is also the distance the piston moves up and down in the cylinder.
Immediately below is a list of the various charts/calculations on this page. Clicking on an item in the list below will take you to the respective section of this page.
Engine displacement = bore X bore X stroke X 0.7854 X number of cylinders
Example: Cylinder bore diameter = 4.000", Stroke length = 3.480", Number of cylinders = 8
Engine displacement = bore X bore X 0.7854 X number of cylinders
Engine displacement = 4.000 X 4.000 X 3.480 X 0.7854 X 8
Engine displacement= 349.8586 cubic inches (round up to 350 cubic inches)
Stroke Length = engine displacement / (bore X bore X 0.7854 X number of cylinders)
Example: Engine Displacement = 350 cubic inches, Cylinder bore diameter = 4.000", Number of cylinders = 8
Stroke Length = engine displacement / (bore X bore X 0.7854 X number of cylinders)
Stroke Length = 349.8486 / (4.000 X 4.000 X 0.7854 X 8)
Stroke Length = 3.480"
Cylinder bore diameter = square root of [engine displacement/(stroke X 0.7854 X number of cylinders)]
Example 1: Engine Displacement = 350 cubic inches, Stroke Length = 3.480", Number of cylinders = 8
Cylinder bore diameter = square root of [engine displacement/(stroke X 0.7854 X number of cylinders)]
Cylinder bore diameter = square root of [349.8486 / (3.480 X 0.7854 X 8)]
Cylinder bore diameter = 4.000"
Example 2: NASCAR® has a 358 cubic inch maximum engine size rule. If we use a 3.480" crank, what is biggest bore allowed? Engine Displacement = 358 cubic inches, Stroke Length = 3.480", Number of cylinders = 8
Cylinder bore diameter = square root of [engine displacement/(stroke X 0.7854 X number of cylinders)]
Cylinder bore diameter = square root of [358 / (3.480 X 0.7854 X 8)]
Cylinder bore diameter = 4.046"
For 4032 material only
Piston dome cc's to gram conversion: 1cc (volume) = 2.8 grams (weight)
This is a good way to remove excess dome without having to re-cc piston: Mill a small amount and re-weight piston until total reduction is reached.
Example: A piston has 12.5cc effective dome volume. The desired effective dome volume is 10.5cc. To remove 2.0cc, cut 5.6 grams (2 X 2.8) from the piston dome.
Compression ratio = (swept volume + total chamber volume) / total chamber volume
It is important that we understand two terms and their relationship to compression ratio: Swept Volume and Total Chamber Volume. Swept Volume is the area the piston travels through bottom dead center to top dead center. Total Chamber Volume is all the area above the piston at top dead center. This would include the area above the piston in the cylinder block, the area of the compressed head gasket, the combustion chamber, the valve pocket, and the dome of the piston. The compression ratio is the relationship of the swept volume to the total chamber volume.
To start, we need to know the Swept Volume of one cylinder. The size of one cylinder figured in cubic centimeters.
Swept volume (cc) = cylinder bore diameter (inches) X cylinder bore diameter (inches) X stroke (inches) X 12.8704
Example:
Cylinder head cc = 72.18 cc
Piston = flat top with two valve pockets that measure a total of 4 cc
Head gasket = 4.000" round and 0.038" thick when compressed
Deck clearance = The piston at top dead center is 0.010" below the surface of the deck
Gasket cc = bore X bore X compressed thickness X 12.8704
Gasket cc = 4.000 X 4.000 X 0.038 X 12.8704
Gasket cc = 7.83 cc
Deck clearance volume = bore X bore X deck clearance X 12.8704
Deck clearance volume = 4.000 X 4.000 X 0.010 12.8704
Deck clearance volume = 2.059 cc
Total chamber volume = 72.18 + 7.83 + 4 + 2.059
Total chamber volume = 86.07 cc
We are finally ready to calculate the compression ratio!
Example:
Swept volume = 716.62 cc
Total chamber volume = 86.07 cc
Compression ratio = (swept volume + total chamber volume) / total chamber volume
Compression ratio = (716.16 + 86.07) / 86.07
Compression ratio = 9.33:1
Cylinder head chamber volume = swept volume / (desired compression ratio - 1)
Example:
Swept volume = 716.62 cc
Desired compression ratio = 11:1
Cylinder head chamber volume = swept volume / (desired compression ratio - 1)
Cylinder head chamber volume = 716.62 / (11:1 - 1)
Cylinder head chamber volume = 71.66 cc
Cylinder head deck material removal = (current chamber volume - desired chamber volume) X deck material per cc
By experience, we have learned that a small block Chevy cylinder head will need 0.006" deck removed for each cc we want to reduce. An open chamber big block will take 0.005" per cc. These numbers will put us in the ballpark. Always check by "cc-ing" the cylinder head chamber volume for accuracy.
Example:
Current chamber volume = 86.07 cc
Current chamber volume = 71.66 cc
Deck material removal per cc = 0.006"
Deck material to remove = (current chamber volume - desired chamber volume) X deck material per cc
Deck material to remove = (86.07 - 71.66) X 0.006
Deck material to remove = 0.086"
Connecting rods probably receive the highest stresses of any bottom end engine component. The forces the rod receives when the piston direction reverses from top dead center can exceed 12,000 lb. in a 500 C.I. Pro Stock engine. This is why Lunati rods are forged from the finest premium quality 4340 alloy steel for strength and utilize Lunati/ARP rod bolts for superior clamping forces on the rod journal.
Closer tolerances and pride in workmanship, along with strict quality control are what makes Lunati Connecting Rods the only logical choice for serious racers. We at Lunati feel that no other manufacturer will hold the close machining tolerances in manufacturing that we do.
Apply the molybdenum base lubricant under the head of the bolt and on the threads.
With the bolts installed hand tight, install the guage and zero the dial indicator. The stretch guage must fit into the dimples on each end of the bolt.
Alternating from one side of the rod to the other, tighten the bolts.
DO NOT STAMP THE ROD NUMBERS ON THE RODS! ETCH THE NUMBERS. STAMPING WILL DISTORT THE ROUNDNESS!
Connecting rod length is measured between the centers of the big end (journal end) and the little end (piston pin end). Below is a table with stock connecting rod lengths for various engine families.