Modified Mustangs & Fords
Ford Camshaft Timing - Degreed To Go
Check Your Cam Timing to Know It's Right
You lean on the throttle and your classic Ford runs like it's chained to a tree. Maybe it's overheating or the exhaust headers glow red-hot under a light load. Performance and reliability begin with proper cam timing during an engine build or cam swap. It's crucial to performance and engine life as well.
We take so much for granted when we plan and build engines. We install new parts right out of the box under the assumption they are ready for installation. Did you know, however, that each and every new part should be thoroughly inspected before installation? Trust the manufacturer-but verify quality control. Assuming you have a good part can become time consuming and expensive if you don't inspect it before installation.
Take, for example, camshaft/valvetrain parts and installation. If you believe degreeing a cam is unnecessary and a waste of time, consider this: The single greatest cause of engine failure across the board is camshaft and valvetrain malfunction. Improper valve timing and irregular cam-lobe indexing can cause engine failure in a nanosecond. Failure typically occurs at high rpm when an engine is most vulnerable, when you're on the throttle and need power most.
How to Check Cam Timing
Why should you check a camshaft for proper timing? Degreeing a camshaft is what separates engine builders from engine assemblers. Engine assemblers gather parts and screw them together. Engine builders are architects and engineers who know what to expect at the end result. They want absolute assurance that all parts will work together in perfect harmony. This mandates fierce discipline and close attention to detail in every phase of engine planning and execution.
When you buy parts, there's no guarantee they'll be safe. Even the most reputable manufacturers in the industry experience failure issues from time to time, in everything from Main Street to NASCAR and NHRA competition. As long as human beings make the parts-or program the computers that make the parts-there will be failure. Inspection, your last line of defense, is what saves you the expense and inconvenience of engine failure.
Degreeing a camshaft helps to confirm valve-timing events as they relate to piston travel and crankshaft rotation. Although most cam manufacturers have a good track record when it comes to cam-card accuracy, there's always a chance you might wind up with a mispackaged camshaft or a defective cam out of index that doesn't match specifications. It's always better to discover a defective or mispackaged camshaft during installation than to wonder why performance isn't as expected once an engine is buttoned up. Camshaft flaws don't have to be huge to adversely affect performance. Lobes that are off one or two degrees will alter performance significantly and can lead to serious engine damage.
You have to think of your engine not as a V-8 or a six-cylinder, but as eight or six individual single-cylinder engines sharing a common crankshaft. Each bore has endless variables-compression, airflow, chamber size, piston-to-cylinder-wall clearances, ring dynamics, piston weight, deck height, cylinder finish-that make it different from the rest. Each cylinder is as unique as a human fingerprint.
A camshaft consists of egg-shaped lobes and journals on a common shaft. These elements are positioned on a core machined to specifications. Each cam lobe has a base circle and lobe. A pushrod V-8 has 16 of them, sixes have 12. Lobes open valves and springs close them. Each lobe consists of two clearance ramps, a nose, and two flanks. When valves are closed, lifters (also known as tappets) sit on a base circle 180 degrees opposite the nose. Flanks control the speed at which valves open. Clearance ramps take up valve lash. Lifters ride cam lobes to convert rotary camshaft motion (roundy-round) into linear (back and forth) motion.
Valve timing is related in the number of degrees of crankshaft rotation (not camshaft rotation) as before top dead center (BTDC), after top dead center (ATDC), or top dead center (TDC). When the piston is at its highest point in travel, it's at TDC. At its lowest, it's at bottom dead center (BDC). Cam specs also include before bottom dead center (BBDC) and after bottom dead center (ABDC). Expect to see these terms on your manufacturer's cam card.
The amount of time expressed in crankshaft rotation is the number of degrees between valve unseat (opening) and valve seat (closed) and is called duration. Cam manufacturers have two ways of expressing this specification-duration and advertised duration. Advertised duration is almost always different than actual duration. Cam-shaft manufacturers tend to show both duration and duration at 0.050 inch in these specifications. It has been said that Harvey Crane of Crane Cams started a camshaft revolution by establishing the industry-standard duration of 0.050 inch.
Duration at 0.050 inch refers to the number of degrees of crankshaft rotation from 0.050 inch of lifter rise until the lifter reaches 0.050 inch of rise before the valve closes completely. True duration is from valve unseat to valve seat. Cam cards typically have both specifications. Some manufacturers base their numbers on duration at 0.006 inch or duration at 0.002 inch, so be fully prepared for any possibility. It's a shell game based on the desire to look better-on paper-than the competition.
In reality, duration is as unique as the cam grinder's specifications. Any way you slice or dice duration, it's the length of time in crankshaft degrees that the valves are off their seats.
Because we're dealing with four-cycle engines, the crankshaft turns 720 degrees for every power cycle-two complete 360-degree revolutions. Each valve opens once in those 720 degrees of rotation. Power and performance are determined by when we open each valve, how much, how aggressively, and for how long.
Duration gives us a beginning and end to valve action, but what about events in between? Study a cam lobe and you'll see the dynamics of valve action. Flat-tappet cam lobes look different than roller-tappet lobes, and roller smoothness enables us to open and close valves more aggressively. We can also open valves further without the shortcomings of a high-lift, long-duration, flat-tappet cam.
Flat tappets don't ride squarely on cam lobes. They ride to one side in order to achieve rotation and smoother operation. To make this happen, flat tappets are spherical-rising 0.002 inch toward their centers. They have to spin in their bores to prevent excessive friction and wear. Roller tappets ride dead center on cam lobes, thanks to roller technology and smoothness. This greatly reduces frictional power losses. What's more, roller cams virtually never wear out.
At one time, cam lobes took on all kinds of bizarre shapes and paths to achieve given results. This was known as triple-curve or harmonic shape and applied more to flathead designs. Overhead-valve technology, which became more common in the '40s and '50s, changed all the rules. Detroit had to approach cam design differently with the advent of overhead-valve engines.
To degree a cam, you need the following:
- Degree wheel
- Dial indicator
- TDC indicator
- 1/2-inch drive, 1 5/16-inch socket, and breaker bar
The degree wheel, which attaches to the crankshaft, tells you in degrees where the crankshaft is positioned relative to its 360-degree journey. Zero degrees is TDC. It's also 360 degrees when we make one full revolution of the crankshaft. All we have to do is establish TDC on the No. 1 cylinder. It doesn't have to be that cylinder, however; it can be any one you choose. Just make sure you're on the compression stroke at TDC for that particular bore when setting the degree wheel at zero.
Once you have TDC dialed in, don't disturb the degree-wheel position on the crank, as that will change TDC accuracy and throw off everything, forcing you to start over.
After establishing zero degrees at TDC, roll the crank one full turn (360 degrees), and make sure the degree wheel is properly positioned. Now you're ready for cam timing.
Locate the manufacturer's cam-specification card. The cam card will tell you exact valve opening and closing specifications (duration). To confirm accuracy, you need a dial indicator with a measuring range of at least 1 inch on each lifter. With both valves closed, set the dial indicator at zero. Begin with the intake valve; slowly turn the crank clockwise until the lifter rises 0.050 inch. Stop turning the crank and read the degree wheel. This begins duration at 0.050 inch. If our cam card says the intake valve opens at 31 degrees BTDC, the degree wheel should show 31 degrees BTDC at this time.
On a cylinder-by-cylinder basis, record your reading. Slowly rotate the crankshaft clockwise and watch the lifter rise to the top of the lobe and then record lift. Continue turning the crankshaft and stop when the lifter reaches 0.050 inch above zero (base circle). Our cam card says intake closes at 67 degrees ABDC; we should see 67 degrees on the degree wheel. Repeat this procedure on all eight intake lobes.
Once you've checked and recorded all intake lobes, look over your findings with the cam card. Don't be surprised to find readings that are off 2-4 degrees from cam-card figures. Sometimes it's the camshaft and sometimes it's your crankshaft keyway. The crankshaft keyway should be in perfect alignment with sprocket timing marks at 12 and 6 o'clock. You have options that will get your timing spot on. We'll get into those shortly.
Repeat the same procedure on each of the exhaust valve lifters and record your findings. See how they compare with the manufacturer's cam card. Again, slowly turn the crank with each lobe and carefully document your findings. Every lobe should yield the same findings.
Advancing valve timing raises cylinder pressure because both valves close earlier. This should improve low and midrange torque. Retarding cam timing improves high-rpm performance, sacrificing low-end torque. For street performance, it's a good idea-depending on the camshaft-to advance timing 2-4 degrees. This should improve overall performance. Pay close attention to piston-to-valve clearances whenever you're advancing or retarding cam timing. This is a lot of work, but so is building another engine because you didn't catch all the details.