Camshafts are a subject that all car guys love to talk about, and those who listen tend to roll their eyes at the staggering number of terms and numbers. There’s lift and duration, overlap and lobe centers, lobe separation angles and so on, and all of them have numbers attached. Most enthusiasts have a basic understanding of the cam, lifter, pushrod, rocker arm, and valve relationship, but keeping all the numbers straight is often intimidating. This chapter looks at the basic calculations relating to camshafts and valvetrain components and how you can use them to equip and tune your particular combination for top performance.
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Imagine a line passing from the center of the cam directly through the highest point on the cam lobe. This is the geometric centerline of that particular lobe. To avoid confusion when comparing cams, remember that the lobe center angle is measured between the centerlines of the intake and corresponding exhaust lobes, while lobe centerline is the angle measured between the center of the lobe and TDC. Lobe center angle is fixed and cannot be changed after the cam is ground. The lobe centerline can be altered by advancing or retarding the cam. When you do this you are effectively moving the intake lobe centerline closer to or farther from TDC.
In terms of engine performance, the lobe center angle is significant. A larger angle yields less valve overlap (the period when both valves are open at the same time). This permits the cylinder to begin building pressure sooner and that boosts low-speed torque. Decreasing the angle crates greater overlap and moves the torque curve higher in the RPMrange, effectively narrowing the engine’s powerband. For most street applications always select a cam that builds as much torque as possible. Generally you want valve events that produce a wider lobe center angle, decreasing valve overlap. One of the advantages of the new high-velocity street roller profiles is that you maintain good idle quality by using wider 112- to 115-degree lobe centerlines, but you also have a high-RPM boost with more aggressive lobe profiles (increasing effective duration). The result is a broad torque curve ideal for street use.
Supercharged or turbocharged applications should avoid cams with excessive overlap because the pressurized intake system already provides effective cylinder filling and forced exhaust scavenging. In these applications, long overlap can be detrimental because some of the intake charge can be blown right through the engine without being burned. For the average street and strip enthusiast, all of these factors are taken care of by the cam manufacturer. Their vast experience lets them provide you with a cam that they know will work for you application.
Understanding Cam Specs
For the purpose of this discussion I will speak in terms of opening and closing valve events. 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 publish these points based on the manufacturer’s chosen reference points: typically 0.006 inch for advertised duration and 0.050 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.
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 0.006 inch:
IO = 31-degrees BTDC IC = 63-degrees ABDC
EO = 77-degrees BBDC EC = 29-degrees ATDC
Intake Duration = 31 + 180 + 63 = 274 degrees at 0.006-inch lift
Exhaust Duration = 77 + 180 + 29 = 286 degrees at 0.006-inch lift
From this you can calculate the intake and exhaust centerlines. To find the intake lobe centerline, divide the intake duration by two and subtract the indicated intake opening point as shown in our example with the Comp XE274 example.
Intake Centerline = (duration ÷ 2) – IO
Intake Centerline = (274 ÷ 2) – 31 = 106 degrees
Sometimes you find a very mild or stock cam where the IO occurs after TDC (ATDC). In this case just add the IO figure to one half of the duration.
On the exhaust side the formula is similar, but instead of subtracting the intake opening point, subtract the exhaust closing point.
Exhaust Centerline = (calculated duration ÷ 2) – EC Exhaust Centerline = (286 ÷ 2) – 29 = 114 degrees Once you know this it’s easy to calculate the lobe separation angle (LSA) which is the difference between the two centerlines. Simply add the calculated centerlines together and divide by 2.
Lobe Separation Angle = (intake centerline + exhaust centerline) ÷ 2
LSA = (106 degrees + 114 degrees) ÷ 2 = 110 degrees
If a cam is ground “straight up,” both centerlines are the same and the LSA is one half of their sum. More commonly you find that cam companies grind their street cams 4 degrees advanced to help boost low-speed torque on longer-duration cams. You can see this in the Comp XE274 example where the intake centerline is 106 degrees, but the LSA is 110 or 4 degrees advanced. Note that 110 degrees is exactly halfway between 106 and 114 degrees. This practice moves the IC event 4 degrees ahead, which tends to diminish top end power in favor of more low-speed grunt for street engines. One other point to note is the use of parenthesis around some timing points. This notation indicates that the cam actually closes the valve after TDC instead of before, even though the card indicates BTDC. You only find this 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 without a space or packaging penalty. 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 differs according to the type of lifter. To accommodate thermal expansion, clearance is build 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 cam.
Mechanical Lifter Cam Net Lift = (lobe lift x rocker ratio) – valve lash
Example: For a Lobe Lift of 0.300 inch and a 1.5:1 rocker ratio with a 0.022-inch valve lash:
Net Lift = (0.300 x 1.5) – 0.022 = 0.428 inch
A hydraulic camshaft automatically adjusts 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 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.
Net Lift = lobe lift x rocker ratio
Net Lift = 0.300 x 1.5 = 0.450 inch
Mechanical (solid) 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 somewhat 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, this is 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 a bigger cam on those cylinders.
You may recall from Chapter 8 that sometimes you can effect dual torque peaks and a broader torque curve by running different-size (c/s area) primary pipes on alternating cylinders in the firing order. This is a fine tuning measure, but in some cases you can combine this 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 often requires dyno verification to quantify gains.
Valve lash changes should be limited to a maximum of 0.004 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 having to make 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-1 cylinder. Note that on most V-8 engines, 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 will be 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 one 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 the balancer if it is already installed.
- Before installing the piston stop, rotate the engine until the piston top visually appears to be at TDC. You should be able to see this through the spark plug hole on an assembled engine. It doesn’t have to be exact—just close.
- Install a temporary wire pointer and adjust it so 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.
in degrees before top dead center (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 after top dead center (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 we have to 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 will depend on the depth of your piston stop in the cylinder bore, but it is all relative. TDC is halfway between the recorded readings. Loosen the degree wheel and rotate the degree wheel only until the pointer reads 32-degrees. Lock down the degree wheel and make sure not to touch or move the pointer from this point forward. 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 your 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 your engine. Set up your dial indicator and check 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 0.050-inch lifter travel. Note the reading on the degree wheel. It should match the intake opening point (IO) indicated on the cam card for 0.050-inch lift. Continue rotating the engine through full lifter travel and down the other side of the lobe until you reach 0.050- 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 0.050-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 intake opening event doesn’t match the cam card, you will have to 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 0.050-inch lift), but your measurements show that it is opening 34-degrees BTDC and closing 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. Note that they can also be used to reposition the cam if you deliberately choose to advance the cam to promote lowend torque or retard the cam for a little more top end power.
If your degree results are plus or minus 1 degree of your published specs, consider leaving the engine as assembled because it is entirely possible that the small degree wheel you are probably 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. You may well find that your wheel is not 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, then zero the indicator. Now rotate backward about 0.100 to 0.150-inch to compensate for timing chain slack. Then rotate clockwise until you reach 0.050 inch. This is 0.050 inch before max lift.
Note the reading on the degree wheel. Then continue over the nose of the cam until you reach 0.050 inch again. This is the 0.050 inch after max lift. Note the degree wheel reading again. Now add the two readings together and divide by 2 to find the center line. It should match the cam card.
For example, if your numbers are 80 and 132:
(80 + 132) ÷ 2 = 106-degree 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, go back and 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 degrees + 23 degrees = 52 degrees overlap
Written by John Baechtel and Posted with Permission of CarTechBooks