Camshafts: What You Need to Know, Part 2

The Comp Cams Thumpr (image from compperformancegroupstores.com). This particular camshaft is advertised with a 275/295 with a 109 LSA.

When selecting camshafts, there are a lot of numbers to understand. If you have never seen a breakdown of these numbers, it may be difficult at first to fully comprehend what each one means.

All camshaft numbers have the same meaning. Whether you’re building an LS based race engine or upgrading the cams in your 5.0 Mustang, the numbers always represent the same thing. Every aftermarket camshaft will come with a “cam card,” a sheet providing the cam’s specifications.

Because every engine is different in design, some engines only utilize one cam, while others utilize up to four. GM/Chevy overhead valve (OHV) engines use only one cam, which operates both the intake and exhaust valves. On a single overhead valve engine, the same principles apply. One cam operates the valves. In a double overhead valve engine (DOHC), one camshaft is used for the intake valves, while another camshaft operates the exhaust valves, per cylinder head. A DOHC V8 engine therefore has four camshafts.

 

Decoding a Camshaft

Camshafts: What You Need to Know, Part 2
This illustration from Speedway Motorsports shows the breakdown of a cam lobe. As the cam turns, the angle of the nose opens, lifts and then closes the valve.

Let’s take a look at the 2000 LS1 camshaft used in the Camaro and Firebird.

198/209 @0.05”
0.500”/0.500” int/exh lift
119.5 LSA

The first two numbers are degrees in crankshaft rotation that the intake and exhaust valves are open. This is known as “duration.” Valves are considered open at 0.05” of lift. This is a standard number set for all camshafts, so there is no confusion between different camshafts. It gives a baseline for camshaft durations for every cam.

198 degrees intake refers to the intake valve being opened for 198 degrees out of the 360 degrees of the rotating crankshaft. Don’t get confused; this is only per cylinder, per crankshaft revolution. 209 is the number of degrees the exhaust valve is open while the crankshaft is turning.

Camshafts: What You Need to Know, Part 2
The author’s edited image shows duration. Occurring between the opening of the valve and closing of the valve – measured in degrees.

 

Lift

Lift refers to the maximum lift of the valves during operation of the camshaft. When you upgrade to an aftermarket cam, you’re most likely going to see a change in lift. Because lift is changing, you’ll need new valvetrain components. Research your individual engine, because parts vary. Do not skimp on these parts either, because if you break something, you could drop a valve into the combustion chamber and destroy your engine.

 

LSA

LSA refers to lobe separation angle. This is the distance between the lobes on the camshaft for the intake and exhaust valves, measured in degrees, per cylinder. Every cylinder will have the same LSA. The wider the LSA (larger number), the more vacuum the engine will have at idle, resulting in a smoother idle. This is not the number you’re seeking for a rough and lumpy idle with a hot cam. Too small of an LSA (narrow) will result in a terrible idle, and very low engine vacuum. Most aftermarket camshafts are in the 95 to 120-degree range for LSA.

LSA must be strongly considered and matched according to your needs. Running a cam with a 101 LSA in a street car will most likely result in stalling out at multiple stoplights and is not recommended for daily driving, whereas if you trailer your car to the track, the lower LSA will give the benefit of a larger RPM band, and also more torque. LSAs will trade torque for horsepower. As the LSA increases, so does horsepower, but torque declines.

Camshafts: What You Need to Know, Part 2
Lunati’s illustration of lobe separation, or LSA, on a camshaft.

 

Overlap

Overlap occurs when in the exhaust valve closes and the intake valve opens at the same time. How long this occurs is measured in degrees. Although mostly unadvertised, overlap can be easily calculated for both positive values (overlap) and negative values (no overlap). Let’s compare the stock LS1 cam to a Comp Cams Thumpr.

Camshafts: What You Need to Know, Part 2
The Comp Cams Thumpr (image from compperformancegroupstores.com). This particular camshaft is advertised with a 275/295 with a 109 LSA.

Camshafts: What You Need to Know, Part 2

Overlap = (Intake + Exhaust)/4, then subtract the cam’s LSA, and multiply the answer by 2.

Camshafts: What You Need to Know, Part 2

Camshafts: What You Need to Know, Part 2

LS1 Cam = -35.5°
Thumpr = 67°

Consider these timing events, and that the Thumpr cam increases overlap over the LS1 by a total of 102.5°!

 

Side Note

Overlap is what gives the cam its lumpy idle, along with LSA. A lower LSA isn’t the only thing necessary for a desired lumpy idle. Mild cams with carb tuning can result in lumpy idles. In computer-controlled vehicles, duration matters the most. Aftermarket cams have been proven to produce the iconic idle of a cammed engine, even with higher LSAs.

 

Summary

If you think you are ready to select a camshaft, consider your purpose and desired RPM use, along with stall speed and any power adders. Once you’ve taken these factors into account, research the necessary valvetrain components and engine specifications needed to run the camshaft. Finally, choose your duration and LSA, and consider the overlap of the exact camshaft you’re selecting.

If you become confused or cannot make a decision regarding a camshaft, it is always recommended to contact the camshaft manufacturer directly, or contact a reputable shop or engine builder to help determine which camshaft suits your application and needs.

4 Comments on Camshafts: What You Need to Know, Part 2

  1. Some good basic info here, but you’re incorrect in that ALL manufacturers spec their cams at .050″ lift.
    Many cams are spec’d at 1mm lift, which is .040″, and are also sometimes listed at the normal lash dimension, which is effectively 0″ at the valve.

  2. “Valves are considered open at 0.05” of lift. This is a standard number set for all camshafts, so there is no confusion between different camshafts”

    This is absolutely true – for US camshafts. However, it is not true of European camshafts. Europeans consider the valve to be open as soon as the cam is off the base circle of the camshaft, not at .05 inch lift. As you might expect, this causes problems if you are comparing specs between a US made camshaft and a European made camshaft.

    • Many European makers, such as Kent Cams, also give specs at 1mm.
      It sure would be nice if everyone would use the .050″ standard, as it’s almost impossible to compare cams otherwise.

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