Back To Basics: Understanding A Cam Card With Erson Cams

Cam specs can be a confusing set of numbers and terms that might often seem like a foreign language. When you purchase a new camshaft from a company like Erson, in the cam box will be a card that lists all the specifics for this particular camshaft. If you don’t know what these numbers mean, the cam card is next to useless.

Yet this card is your passport to make sure the cam is both what you actually ordered (or thought you ordered) and also a great resource for future reference. The cam card also offers critical information you will need when it comes time to degree the camshaft in the engine to ensure the cam is installed correctly.

For the benefit of those who do not, as yet, speak camshaft (or a refresher for those who do), we’ll run through each of the pieces of data on the card and explain what they are and how they relate to camshaft operation. This will take some time to explain, so sit back, grab your favorite caffeinated beverage to keep you awake and we’ll get started!

All cam lobes convert rotational movement into linear movement by way of an eccentric. The base circle is the round portion of the lobe (A). With rotation clockwise, the opening flank (B) leads to the nose of the cam (C) that generates the maximum lobe lift. Further rotation moves into the closing flank (D) before returning to the base circle.

The Cam Card Might Not Actually Be A Card

Cam cards from various companies will look different, but will all deliver the important data you need. Think of your cam card like a menu. The food you want to eat is right there at your fingertips. All you have to do is pick out what you want. For this exercise, we’ll use both a small-block Chevy and LS engine for the specific data, but all four-stroke engines employ the same data regardless of the name on the valve cover.

For this initial foray, let’s use a simple, hydraulic flat-tappet camshaft from Erson (Card A). We’re going to start with some basics on cam design so we don’t lose anybody in the description. A cam lobe is designed around a few basic parameters such as duration and lift. Let’s start with the simplest which is valve lift. A cam lobe converts rotational movement into linear — up and down — movement to open and close the valves.

As the lifter follows the eccentric portion of the lobe from the base circle toward the highest point, this generates upward movement of the lifter that can be measured in fractions of an inch. On Cam Card A for this P/N: E11044 cam, find the heading “Checking Figures @ 0.050” Tappet Rise” and find Lobe Lift Intake.

Cam Card A

Bread and Butter of Specs — Lift and Duration

The spec is listed as 0.320 which is 0.320-inch of vertical lift of the tappet. If we multiply that number by the rocker ratio (in this case 1.5:1) we will get the theoretical valve lift. In this case, that’s 0.480-inch of lift at the valve, which is not actually listed on this cam card. We call this theoretical lift because, with pushrod deflection and a rocker ratio that may not always be 1.5:1 at all lift points, the lift may not always come out to be exactly as the math predicts. But for the sake of this discussion, we can use that 0.480-inch valve lift as a close approximation of true valve lift.

The other half of that spec for a given cam lobe is something called duration. This is defined as the number of crankshaft degrees in which the lobe generates lift. Since establishing exactly when actual lift begins is difficult to determine consistently, most cam manufacturers use a lobe lift spec of 0.004 or 0.006 inch as their opening and closing points to establish what is termed advertised duration.

We’ve arbitrarily marked this lobe with two sets of marks. If we assume the cam is rotating in a clockwise direction, the black marks represent advertised duration that starts earlier and ends later. The red marks represent duration at 0.050 inch of tappet lift. You can see that the 0.050 numbers will always be shorter in duration than the advertised numbers.

On this same card, the upper section describes “advertised” duration even though it is not specifically called out as such. You will find the duration numbers listed in the upper section as Intake 292 and Exhaust 292 degrees. As a further definition, you may see cams described as single- or dual-pattern.

Because this particular cam uses the same duration for both intake and exhaust, it would be a single-pattern cam. If the cam used 292 degrees for the intake and 308 degrees for the exhaust, then that would be a dual-pattern cam.

Cam Card B

Valve Event Timing — Opening and Closing

Notice that the lower section of the card lists Intake Opening, Intake Closing, Exhaust Opening, and Exhaust Closing. We’ll deal with the intake side to show how these numbers relate to duration. Remember that a camshaft operates at half-speed compared to the crankshaft, but all these numbers are listed as crankshaft degree positions for opening and closing points.

All the numbers on this cam card are listed using 0.050-inch of tappet lift. That means that when reading the numbers on a degree wheel on the front of the engine, all these numbers will be in reference to the tappet at 0.050-inch off the base circle both on the opening and closing sides of the lobe.

Let’s start with Intake Opening. This card does not qualify the term as degrees Before Top Dead Center (BTDC), but all lobes begin lift before the piston arrives at TDC. In this case, the intake opens when the lifter gets to the 0.050-inch mark at 8 degrees BTDC. Next, the intake closes at 42 degrees After Bottom Dead Center (ABDC).

If we were to look at the degree wheel while we were measuring these two points, you would notice that the crankshaft turned an additional 180 degrees between TDC and BDC. So to calculate duration at 0.050-inch tappet lift, we would then add 8 (BTDC) + 180 degrees (between TDC and BDC) + 42 degrees (ABDC). This adds up to the 230 degrees listed as Intake Duration at 0.050-inch tappet lift.

Cam Card C

Looking at the numbers in the upper part of the card, you can see that the advertised intake duration is a much larger number at 292 degrees because the opening and closing points occur both earlier and later at 0.006-inch tappet lift instead of 0.050-inch. Advertised intake duration will look like this: 39 + 180 +73 = 292 degrees. Now that we have a grasp on duration, we can move on to more sophisticated parts of the cam specs.

Sometimes with 0.050 intake opening and exhaust closing numbers for a short duration cam, you may see an opening and/or closing listed as a negative number. In the case of an intake lobe, this is because the cam has such a short duration that the intake valve does not open until After Top Dead Center (ATDC). If you look at Card C, you can see the Intake Opening point is listed as -7. This means the lobe doesn’t achieve 0.050-inch tappet lift until 7 degrees After Top Dead Center.

This means we must subtract 7 degrees from the 180-degree travel between Top and Bottom Dead Center. In this case then (-7) + 180 + 21 (intake closing) = 194 degrees which is the rated duration of this cam at 0.050-inch tappet lift. The same situation occurs at exhaust closing.

All four-stroke engines operate the camshaft at half engine speed as can be seen by this timing set layout.

No Separation Anxiety Here

Right at the very top of all the cam cards is a category labeled lobe separation angle (LSA). This spec calls out the angle between the intake and exhaust lobe center lines. The intake center line on Cam Card A is not specified, but for the sake of discussion, let’s say that the intake centerline for this cam is 108 degrees After Top Dead Center (ATDC) and the exhaust lobe center line is 108 degrees Before Top Dead Center (BTDC). If we add those two numbers together and divide by 2, this will be the LSA.

In this particular application, the LSA and the intake center line are the same. Many camshafts intended for street operation are machined with a few degrees of advance built into the cam. If this is the case, it is easy to spot because the intake centerline number will be a smaller number compared to the LSA.

As an example, let’s say our cam card lists the intake center line as 106 degrees while the Lobe Separation Angle (LSA) is listed at 108 degrees. This means the intake has been machined with two degrees of built-in advance. This also means that the exhaust lobe center line must be 110 degrees in order for the LSA to be 108 degrees (106 + 110 = 216 / 2 = 108).

This is important because with this information you can instantly learn from the card whether the camshaft has been machined with a built-in advance. In the case of the Erson E110044 camshaft (Card A), we can assume that since the intake center line spec was not included, that the cam has not been ground with built-in advance.

A camshaft by itself doesn’t tell you very much as camshafts all look alike. It requires either a cam card or installing the cam in an engine and measuring the opening and closing points for both the intake and exhaust in order to establish the cam’s actual specs. If you have an unknown aftermarket camshaft, the part number is often etched on the end of the cam but stock cams are often not labeled.

An example of built-in advance can be found on Cam Card B. Check out the left column, eight lines down where the description says Lobe Separation called out at 110 degrees. Note right below this is “Adv. or Ret. Deg.” with a spec of 5 degrees. This is telling us that this cam was ground with a 110-degree LSA but the intake lobe was intentionally advanced 5 degrees. This would put the intake center line at 105 degrees ATDC and the exhaust center line at 115 degrees (105 + 115 = 220 / 2 = 110).

Moving back to Cam Card A, you‘ll note right below the Lobe Separation Angle is something called Overlap. Overlap is the number of crankshaft degrees where both the exhaust and intake valves are open at the same time. This is something cam designers back in the 1930s discovered.

They realized that if they opened the intake valve early, this overlap improved intake breathing at higher engine speeds, where there is less time to get the air moving into the cylinder from the intake tract. This process is now much better understood, and can now be utilized to attain greater than 100-percent volumetric efficiency from naturally aspirated combinations.

This graph of a typical intake and exhaust lobe clearly illustrates overlap and lobe separation angle. The graph shows the exhaust lobe first through its lift curve followed by the intake lobe. Note that before the exhaust fully closes, the intake begins to open. That small triangle area represents the amount of overlap in crankshaft degrees. You can learn quite a bit about camshafts from this simple graph.

Adding lots of overlap reduces low-speed throttle response but also creates that now-classic choppy idle that is so sought after by street enthusiasts. As with almost all things in life, too much overlap is not always a good thing and will kill off-idle power especially when mistakenly combined with a low static compression ratio. The combination of excessive overlap and low static compression ratio is a recipe for a very soggy and weak street engine.

One interesting aspect of lobe separation angle is that you may often see a long duration camshaft with a wider LSA that often will produce more overlap than a short duration street cam with a narrow LSA. Increasing the duration on both the intake and exhaust will naturally increase the amount of overlap if the LSA remains the same. It’s something to keep in mind when evaluating camshafts.

Checking cam position in the engine is referred to as degreeing the cam. We use a large diameter degree wheel for accuracy and check the actual position of the cam against the numbers on the cam card. This is an iron 5.3L LS engine that our friend Bill Irwin is checking.

Degreeing The Cam — Points To Check

The best way to ensure that the camshaft is installed properly in your engine is to go through the motions of checking cam position by the degree method. Various cam companies use different methods for this process. Erson prefers to check all four positions of Intake Opening, Intake Closing, Exhaust Opening, and Exhaust Closing for one cylinder (usually Number One).

Erson then says to evaluate all four points and see if they occur within a 1 to 2-degree spread. If so, then the cam is installed correctly. If the numbers do not fall within this 1-2 degree spread, the cam may need to be repositioned by advancing or retarding. Of the four points, the intake closing is the most important.

This overview has purposefully omitted several related camshaft talking points so we could maintain this story at a reasonable length. The goal was to create a great entry point for those interested in learning about cam timing and its effect on engine performance. It’s also important to remember that all engines operate as a system and that each one will respond a little differently to the same cam choice. That’s what makes performance engine building such a challenge.

Just lining up the dots between the cam and crank gear is no guarantee that the cam will be installed properly. It may be close, but you won’t know for sure unless you degree the cam. As you can see on this crank gear, it offers multiple positions of advance or retard in two-degree increments and that is installed at this point at the zero or straight up position.

This overview has purposefully omitted several related camshaft talking points so we could maintain this story at a reasonable length. The goal was to create a great entry point for those interested in learning about cam timing and its effect on engine performance. It’s also important to remember that all engines operate as a system and that each one will respond a little differently to the same cam choice. That’s what makes performance engine building such a challenge.

Article Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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