Camshafts are the command center of the engine. As a primary contributor to VE they govern the precise valve action that controls the intake and exhaust system and thus exert enormous influence on an engine’s power potential. Choosing a camshaft for any given racing application can be a daunting experience for even the most experienced engine builder. While the primary mechanical processes are well understood, the actual physics of engine airflow and camshaft timing are considerably more challenging, particularly as they relate to the requirements dictated by each individual racing application.
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Some engines today are built strictly for dyno work just to see how much peak power they can make. That seems like a lot of fun, but most of us are racers seeking an optimum combination to reinforce our racing requirements. The question of defining the power curve requirement becomes paramount. What is the required optimum operating range for the intended application? Is the powerband requirement broad or narrow and how does this relate to the overall engine speed, gearing, tire diameter, and shift frequency for the application?
The builder/cam designer must determine how much duration, lift, lobe separation angle (LSA), and other factors are required to optimize the engine’s performance in the required operating range. Consideration must also be given to valvetrain stability and durability and their long-term contribution to the precision operation of all the relative components.
Like many other functions within a competition engine, mathematical specifics govern the operating principles and dictate the physical and mechanical limits of operation as they relate to the application in terms of engine displacement, stroke and rod length, compression ratio, flow path dimensions, vehicle weight, gearing, rear axle ratio, tire diameter, and a host of other factors each of which contributes to or affects the overall VE equation. Arriving at a suitable compromise among all these factors is often a formidable task for race engine builders and cam designers alike.
Many races have been won by cars with less power than their competitors. Usable power complements the specific requirements of a racing application is what’s being sought. Peak numbers are irrelevant if the powerband is not properly shaped and positioned to support the final application.
Understanding Cam Specs
In terms of opening and closing valve events, the intake opening (IO) and exhaust opening (EO) represent the intake and exhaust opening points in crankshaft degrees. Intake closing (IC) and exhaust closing (EC) are the intake and exhaust closing events. Cam cards show these points based on the manufacturer’s chosen reference points: typically .006 inch for advertised duration and .050 inch for a universal checking reference based on an agreed amount of lobe lift where reasonable flow is initiated.
The following formula is used to calculate intake and exhaust duration. It applies to any lift as long as your cam card specifies opening and closing figures for a particular lift value. (Note that all camshaft specifications are designated in crankshaft degrees except the LSA, which is measured in camshaft degrees.)
Duration at Specified Lift = opening point + 180 degrees + closing point
For example, a Comp Cams XE274H-10 hydraulic cam lists the following opening and closing points for a checking lift of .006 inch:
IO = 31° BTDC IC = 63° ABDC EO = 77° BBDC EC = 29° ATDC
Intake Duration = 31 + 180 + 63 = 274° at .006-inch lift Exhaust Duration = 77 + 180 + 29 = 286° at .006 inch lift
From these numbers you can calculate the intake centerline of our example using the following formula:
Intake Centerline = (duration ÷ 2) – IO Intake Centerline = (274 ÷ 2) – 31 = 106°
Sometimes with a very mild or stock cam the IO occurs after TDC (ATDC). In this case just add the IO figure to half of the duration.
On the exhaust side the formula is similar, but instead of subtracting the IO point, you subtract the EC point.
Exhaust Centerline = (calculated duration ÷ 2) – EC Exhaust Centerline = (286 ÷ 2) – 29 = 114°
Once you know these numbers, it’s easy to find LSA, which is the difference between the two centerlines. You add the centerlines together and divide by 2. Continuing our example, the LSA is calcualted using this formula:
LSA = (intake centerline + exhaust centerline) ÷ 2 LSA = (110° + 114°) ÷ 2 = 110°
If a cam is ground “straight up” both centerlines are the same.
More commonly, cam companies grind their street cams 4 degrees advanced to help boost low-speed torque on longer duration cams. Our Comp XE274 is an example of this: the IC is 106 degrees, and the LSA is 110 degrees (4 degrees advanced). Note that 110 degrees is exactly halfway between 106 degrees and 114 degrees (the EC).
This practice moves the IC event 4 degrees ahead, which diminishes top end power in favor of more low-speed grunt for street engines. Another point to note is the use of parentheses around some timing points (on a cam card). This notation indicates that the cam actually closes the valve ATDC instead of before, even though the card indicates BTDC. This is only found on short-duration cams, but it is important to note if you’re making calculations with a small cam.
Calculating Valve Lift
Net valve lift is a function of camshaft lobe lift and rocker arm ratio. Lobe lift (sometimes called cam rise) is the height of the eccentric portion of the cam lobe above the base circle. The rocker arm transfers the motion of the valve lifter riding on the cam lobe to the valve and increases the lobe lift by the amount of the rocker ratio, which is typically 1.5 to 1.7:1. It provides a convenient means of increasing valve lift. This is very evident in a pushrod engine where the valvetrain is compact and easily packaged compared to the complication and excessive size required for single- and double-overhead cam arrangements.
Net valve lift varies according to the type of lifter. To accommodate thermal expansion, clearance is built into the system in the form of clearance ramps and valve lash for mechanical (solid) lifter cams. The valve lash clearance must be subtracted from the total valve lift to obtain the net valve lift for this type of camshaft.
Mechanical Lifter Cam Net Lift = (lobe lift x rocker ratio) – valve lash
For example, an engine with a lobe lift of .300 inch and a 1.5:1 rocker ratio with a .022-inch valve lash: Net Lift = (.300 x 1.5) – .022 = .428 inch
Hydraulic camshafts that are often used in lower-tier circle track applications automatically adjust for thermal expansion via lifter preload against an internal hydraulic plunger. No clearance is necessary and these lifters are typically adjusted with a specified amount of preload or a preferred degree of turn down from zero lash usually one-quarter to one-half turn down. In this case the net valve lift is based on the lobe lift and the rocker ratio alone.
Hydraulic Lifter Cam Net Lift = lobe lift x rocker ratio Net Lift = .300 x 1.5 = .450 inch
Mechanical cams are typically smaller than their hydraulic counterparts due to loss of lift attributable to valve lash. But mechanical cams, unlike hydraulic cams, can be tuned slightly by altering valve lash. Tightening the lash adds lift and starts the valve event sooner, effectively mimicking a larger cam.
To accommodate various tuning changes, lash changes are often limited to either the intake valves or the exhaust valves and sometimes only on the end cylinders to accommodate variations in runner length. A racer might tighten the lash on the exhaust side to increase the exhaust event if he feels that the engine is exhaust limited. Or he might tighten the lash on the outer four corner cylinders to compensate for the longer intake runners on those cylinders. That’s equivalent to running slightly more camshaft on those cylinders.
In many cases, dual torque peaks and a broader torque curve can be generated by running different-size (c/s area) primary pipes on alternating cylinders in the firing order. This is a worthy fine-tuning measure, and in some cases you can combine this step with valve-lash adjustments on selected cylinders to further tune the torque output at different speeds. In theory this is predictable, but in practice it almost always requires dyno verification to quantify gains.
Valve lash changes should be limited to a maximum of .004 inch, and consideration should be given to the known valve-to-piston clearance before going too far on the exhaust side. These tuning measures can net small gains, but the correct combination can effectively broaden a torque curve with surprisingly good results. This may be just enough to give you some added leverage on the competition without making major engine modifications.
Locating TDC accurately is absolutely essential to proper camshaft installation. Exact TDC is the timing basis for all camshaft timing events. The method for locating it varies according to the engine’s state of assembly. Whatever that is, a temporary piston stop is used to stop the piston at some arbitrary distance before and after TDC. For fully assembled engines that are not already equipped with an accurately set TDC indicator, a threaded piston stop can be installed in the spark plug hole of the number-1cylinder.
On most V-8s, the number-1 cylinder is almost always the farthest one forward in the V configuration. Paired rod-and-piston assemblies on each crank throw dictate that one is always offset farther forward than its counterpart. Study the front of the block to see which of the front cylinders is farther forward. That is the number-1. If the degree process is being performed during engine assembly, it is best to do it with only the number-1 piston-and-rod assembly installed on the crankshaft. Rotating the engine to degree the cam is much easier this way. In this case a flat-bar piston stop is bolted to the block deck surface above the number-1 piston. This type of piston stop has a center bolt that can be adjusted to stop the piston at any desired point below TDC.
Begin by installing the degree wheel on the crank snout or on the balancer if it is already installed. Before installing the piston stop, rotate the engine until the piston top appears to be at TDC. You should be able to see this through the spark plug hole on an assembled engine. It does not have to be exact just close. Install a temporary wire pointer and adjust it so that the tip is close to the graduated marks on the degree wheel. Adjust the degree wheel so the pointer indicates TDC (0 degrees) and snug it lightly. Rotate the engine counterclockwise approximately one half turn and install the piston stop.
Tighten it securely so it won’t move when the piston contacts it. Slowly rotate the engine clockwise until the piston contacts the piston stop. Note the pointer reading on the degree wheel. It will be in degrees BTDC. Record that number and then rotate the engine in the opposite direction (counterclockwise) until it completes a revolution and contacts the piston again. Record the reading on the degree wheel and note that it indicates degrees ATDC.
If your calibrated eyeball is very accurate, the recorded numbers indicate the same number of degrees on either side of TDC and the pointer reads zero with the piston stop removed and the piston brought to the top. In practice, most of us aren’t that accurate, so locate TDC based on a common reference point on either side of TDC. That’s the piston stop. The reason you can’t accurately locate TDC visually is because the piston experiences a brief period of dwell (stationary) at the top of its stroke as the rod angle transitions from one side to the other. The piston is stopped at this point and you have to split the dwell point exactly to find true TDC.
Since the piston stop does not move, it represents a fixed reference point before and after TDC. True TDC is found by splitting the difference between the degree-wheel readings.
For example, let’s say your recorded numbers are 34 degrees BTDC and 30 degrees ATDC. The exact number depends on the depth of the piston stop in the cylinder bore, but it is all relative. TDC is halfway between the recorded readings. Loosen the degree wheel and rotate it only until the pointer reads 32 degrees. Lock down the degree wheel and make sure not to touch or move the pointer from now onward. Check your work by rotating the engine back and forth to the piston stop in both directions. The pointer reading should be the same in both directions (32 degrees in our example). If it is not the same, repeat the steps until the pointer indicates the exact same number of degrees before and after TDC. Once it does, remove the piston stop and degree the cam with confidence that you are locating the timing events based on exact TDC.
Degreeing the Cam
There are two methods for degreeing a camshaft. One compares the opening and closing points of the intake valve to see if they match the manufacturer’s specs on the cam card. The other method locates the intake lobe centerline relative to TDC. Both methods are successful, but the intake centerline method does not verify the intake opening and closing points according to the cam card. Both methods are described below, but the intake opening and closing method is recommended for initial setup. Then you can check your work with the intake centerline method. In either case you need an accurate means of reading lifter travel.
I prefer the cam checking tool available from Jegs, Summit, and many other suppliers, but successful results can be obtained using a solid lifter or a modified hydraulic lifter with the internal plunger reversed to give the dial indicator plunger a flat surface to bear against. You can also locate the plunger against the edge of the lifter. Make sure that the contact is stable and that the direction of the indicator travel is parallel to lifter travel. Then adjust the dial indicator to ensure that it has enough available range to read total intake lifter travel for the number-1 cylinder.
Intake Opening Method
Install the cam with the timing marks correctly aligned for the engine. Set up your dial indicator and checking lifter, or the cam checking tool in the number-1 intake lifter hole as described above. Zero the dial indicator and rotate the engine in the normal direction of rotation for several revolutions to verify that the dial indicator reads full lifter travel and returns to zero each time. You can take this opportunity to verify that lifter travel matches the indicated lobe lift on the cam card. If the lifter does not return to zero on the base circle, determine the cause and correct before continuing.
Once you’re satisfied, begin with the lifter on the base circle and slowly rotate the engine clockwise until the indicator shows .050-inch lifter travel. Note the reading on the degree wheel. It should match the intake opening (IO) point indicated on the cam card for .050-inch lift.
Continue rotating the engine through full lifter travel and down the other side of the lobe until you reach .050-inch lift before the intake closing point. Since you know the lobe lift and the recommended closing point from the cam card, you should be able to anticipate the closing point as you rotate the engine. If you miss it, simply back up about 60 degrees to compensate for timing chain slack and approach the .050- inch closing point again. Compare it to the cam card and then continue rotating to verify that the lifter returns to zero again.
Your readings should show the intake opening and closing points and the total lifter travel or lobe lift. If the IO event doesn’t match the cam card, you advance or retard the cam to bring it into spec.
For example, if your cam is supposed to open the intake valve at 36 degrees BTDC and close at 70 degrees ATDC (at .050-inch lift), but your measurements show that it is opening at 34 degrees BTDC and closing at 72 degrees ATDC, the cam is retarded. The valve event is occurring later than the recommended spec. If it were to open at 38 degrees BTDC and close at 68 degrees ATDC, it would be 2 degrees advanced because the valve event is occurring 2 degrees earlier than specified.
In either case it is easy to correct using offset cam bushings or a crank gear with multiple keyways. Both allow you to adjust the position of the cam and then recheck it for compliance with the cam card specs. They can also be used to reposition the cam if you deliberately choose to advance the cam to promote low-end torque or retard the cam for a little more top end power.
If degree results are plus-or-minus 1 degree of the published specs, consider leaving the engine as assembled because it is entirely possible that the degree wheel you are using is not that accurate. Larger-diameter degree wheels space the degree marks farther apart and therefore have a greater chance of improved accuracy.
You can check the accuracy or your wheel by placing it on a large sheet of paper and marking the four 90-degree positions of the wheel. Then move the wheel to various positions and check to see that each 90-degree mark is an equal number of degrees from 90. Your wheel might not be completely accurate. This is why fussing over less than 2 degrees (unless for example, the cam is retarded 2 degrees and you want 2 degrees advanced) may not be worth the effort.
Intake Centerline Method
The intake centerline method finds the location of the intake lobe centerline relative to TDC. The recommended intake centerline is indicated on the cam card, and when correct it should yield the specified intake opening and closing points when you degree the cam.
Finding the centerline is easy. Rotate the engine clockwise until you find the maximum lobe lift, and then zero the indicator. Now rotate backward about .100 to .150 inch to compensate for timing chain slack. Then rotate clockwise until you reach .050 inch. This is .050 inch before maximum lift.
Note the reading on the degree wheel. Then continue over the nose of the cam until you reach .050 inch again. This is the .050 inch after maximum lift.
Note the degree wheel reading again. Now add the two readings together and divide by two to find the centerline. It should match the cam card. Let’s use an example of 80 and 132.
(80 + 132) ÷ 2 = 106° centerline
The cam card indicates the correct installed intake centerline. If it calls for 106 degrees and you come up with 108 degrees, the cam is early and you have to retard it 2 degrees to bring it into spec. If you get 104 degrees the cam is retarded and you have to advance it 2 degrees to correct it.
If you have degreed the cam with the intake centerline method, check to see if the intake opening and closing points match those indicated on the cam card. If incorrect, determine the direction of error and reposition the cam accordingly.
Calculating Valve Overlap
Overlap is the number of degrees where both valves are off their seats at the same time. It is a combination of the intake opening event and the exhaust closing event. Adding these two points together yields valve overlap.
Valve Overlap = IO + EC
For example, a cam with an intake opening point of 29 degrees BTDC and an exhaust closing point of 23 degrees ATDC has a valve overlap of 52 degrees.
29° + 23° = 52° overlap
Numerous terms and acronyms are used to describe camshaft operation. The following definitions are provided to explain camshaft lingo and make it easier for you to work the associated math.
ABDC (after bottom dead center): Position where the piston is accelerating away from BDC. Intake valve closing occurs in this area.
ATDC (after top dead center): Piston position where the piston has passed TDC and is accelerating away from it. The exhaust closing event usually occurs in this area.
BBDC (before bottom dead center): Piston position before reaching BDC. Normal area for exhaust opening event.
BDC (bottom dead center): Position of the piston at the exact bottom of its travel; exactly 180 degrees opposite of TDC.
BTDC (before top dead center): Piston position (typically number-1) before or approaching TDC. The piston is decelerating and the intake valve opening normally occurs in this area.
EC: Exhaust closing point in degrees ATDC.
EO: Exhaust opening point in degrees BBDC.
IC: Intake closing point in degrees ABDC.
IO: The intake valve opening point, usually in degrees BTDC.
LDA (lobe displacement angle): The number of camshaft degrees separating the centerlines of the intake and exhaust lobes. Also referred to as lobe separation angle (LSA) or simply “lobe center.” LSA (lobe separation angle): Same as lobe displacement angle. Most commonly used reference.
TDC (top dead center): Position where the piston is at the exact top of its travel in the cylinder. It splits the dwell point where the rod changes its angle and represents the zero degree reference point for crankshaft degrees and cam timing events.
Rocker Ratio Tuning
You can fine tune a mechanical or solid lifter cam to some degree with small valve lash adjustments, but you can further tune any pushrod-style cam with a rocker arm change. For example, a Chevy 1.5:1 rocker arm can be replaced with a 1.6 :1 ratio rocker to achieve a 10-percent gain in rocker ratio. That doesn’t mean a 10-percent increase in lift, but the effective increase is substantial and often quite useful. This increases valve lift on a Chevy, for example, by about .030 inch. As a general rule, the higher the lobe lift, the greater the increase. To be certain of the gain, multiply your known lobe lift by the new rocker arm ratio.
Net Lift = lobe lift x new rocker ratio
For example, a lobe lift of .350 inch provides a net lift of .525 inch except with a mechanical cam. A 1.65:1—rocker arm bumps that value by more than .050 to .577 inch a substantial increase. Equally important are the other effects a ratio change brings to the table.
A higher ratio rocker arm accelerates valve opening and closing events at a faster rate, effectively increasing duration with the same opening and closing points. The rule of thumb is 1 degree of duration for every 1/2 point of rocker ratio increase. Hence, a switch from 1.5 to 1.6:1 can net a 2 degree increase in duration. Power gains acquired through ratio changes are split between the increase in lift and the increase in duration.
Rocker ratio increase may affect valve to piston clearance, valve-spring coil bind, and retainer-to-seal clearance; the faster valve action may induce valve float in some cases. Generally these problems don’t occur, but you have to check for them and take appropriate action if necessary.
Ratio changes are often made on either the intake or the exhaust only to help mask a deficiency elsewhere, and tuners sometimes run more rocker ratio on the end cylinders to compensate for differences in runner length. Whatever the case, rocker ratio is an effective tuning tool for racers who understand the advantages it can provide.
The difference between advertised duration and duration at .050-inch lift doesn’t have to be confusing. Cam manufacturers select an opening and closing lift point to specify the duration of their cams. This is typically a point of discernable lifter motion such as .006 inch for Comp Cams or .004 inch for Crane Cams. These lift points are the basis for the advertised duration that all cam manufacturers use because bigger numbers sound better from a marketing standpoint. Advertised duration is problematic because manufacturers all seem to use different lift points to specify duration, thus making cam comparisons difficult.
Years ago, at the urging of Harvey Crane, all manufacturers finally settled on a universal checking point of .050-inch to compare cam specs from different companies. This standard was adopted by the SAE and is currently the standardized checking point for all camshafts. Later, Harvey Crane introduced the concept of hydraulic intensity, which is the time in crankshaft degrees that it takes for the lifter to move from its advertised duration checking point to the universal .050-inch lift point. It’s determined by subtracting the duration at .050 inch from the advertised duration. The smaller the number, the greater the hydraulic intensity, which indicates a more aggressive cam. Naturally comparisons can only be made between cams with the same advertised checking point relative to the .050-inch standard. Cams with a more aggressive hydraulic intensity typically exhibit more valvetrain noise due to the steep lift curve, but they generally deliver more performance without sacrificing idle quality.
Camshaft Selection Criteria
In a nutshell, camshaft selection must be painstakingly matched to the application’s required engine speed range, the flow rates of the inlet and exhaust systems within the given RPM range and how well they complement piston motion, and cylinder pressure differential according to rod length.
Engine builders are often required to contemplate a specific engine speed or operating range, engine breathing characteristics within that speed range, and the mechanical requirements suitable to sustain durable high-speed operation based on the type of camshaft and valvetrain being used. This typically includes accommodations for potential mechanical conflicts such as inadequate piston-to-valve clearance, valvespring coil bind issues, and other critical interference issues typically associated with high-lift, long-duration cams.
The primary goal of the camshaft designer is to achieve and maintain optimum intake and exhaust flow velocity across the specified operating range. The intent is to maintain a minimum flow velocity of 240 to 260 feet per second in the ports, but high-speed engines frequently exceed those values by a factor of two or three. As lift and duration increase, the engine must run faster to make sufficient power.
Many factors contribute to the engine airflow equation. Superior breathing is a highly variable function of engine size and operating speed, inlet and exhaust flow path characteristics, and how closely they are matched along with intake manifold design and carburetor or throttle body size. Typically, short-stroke, small-displacement engines do not accept as much camshaft lift and duration as larger-displacement, longer-stroke engines that produce higher flow velocities with longer-duration, high-lift cams.
While the intake closing event has traditionally been viewed as the most important point in effective camshaft selection, more and more designers are now basing their design criteria on the valve overlap period and its relationship to engine speed and final application. The overlap period is critical to taking maximum advantage of exhaust pressure wave scavenging and its potentially optimizing effect on intake tuning. The second most important criteria is the LSA or lobe centerline angle (LCA, the angle between the centerlines of the intake and exhaust lobes). As compression ratio increases it calls for a wider LCA. The same goes for valve acceleration rate. Higher ratio rockers and quick action cam lobes also tend to require a wider LCA.
Firing Order Swaps
For some time now, many Chevy engine builders have routinely rearranged the firing order of their racing engines. The debate as to whether this accomplishes anything is ongoing, but it’s become common practice and the naysayers appear to be fading. The most common reordering is the 4/7 swap, where the firing positions of cylinders 4 and 7 are swapped in the standard V-8 firing order (18436572). This yields a firing order of 18736542 and was a well-kept secret in Pro/ Stock drag racing for years; many builders now favor it. Interestingly the power claims are generally not substantial (something on the order of 4 to 7 hp), although numbers as high as a 40-hp gain routinely circulate on Internet forums, particularly with regard to dual-plane-equipped circle track applications.
More common are claims of a broader, smoother torque curve with improved drivability and component durability. With the original firing order, cylinders 5 and 7 firing in sequence tend to saturate heat around those chambers, which could encourage detonation and dimensional issues with valves and seats.
Cooling issues are said to be resolved by separating cylinders 5 and 7 in the firing order, but the problem is really only transferred to cylinders 4 and 2, which are admittedly closer to the water pump in a domestic V-8.
Durability claims may hold some merit since the standard firing order has numbers 6, 5, and 7 cylinders all hammering the number-4 main bearing in sequence. The 4/7 swap relieves this, but transfers similar loading to the center main with cylinders 6, 5, and 4 all hitting in sequence. These issues are more prevalent on big-inch stroker applications where you’re slinging a good bit more mass in the crankcase.
In the case of GM’s Gen III firing order (18726543), loading is transferred from front to back and then cycled through the center cylinders (where the thrust bearing is now located) before hitting the end cylinders again. This firing order replicates the Ford Windsor and the later 5.0 HO firing order if you re-number the cylinders to match the Chevy. More importantly, this firing order cycles between cylinders with longer runners and cylinders with shorter runners, which must be tuned to different torque peaks, potentially broadening the overall curve.
Cam profile selection varies according to application and covers a broad spectrum. Many professional engine builders are well acquainted with the basic requirements of camshaft selection, but they still defer to the cam designer’s judgment in most applications and often the selection is a collaborative effort depending on the engine builder’s level of experience and recognition of the power curve requirements. The cam designer or profile builder can draw from a deep well of experience and engine builder feedback, which often translates to better camshaft selection for builders down the line.
A high-speed drag race profile, for example, requires very specific timing for successful operation, particularly on large-displacement engines running very high engine speeds, high compression ratios, and large-tube open headers. This type of engine generates a lot of exhaust volume at high RPM so the designer may emphasize early exhaust valve opening to minimize pumping losses during the exhaust stroke. Depending on the cylinder head, these engines may experience difficulty discharging the high volume of exhaust gas they produce, hence the emphasis on exhaust timing.
Written by John Baechtel and Posted with Permission of CarTechBooks