How to Choose Your Cam.. Duration, Overlap and more.

Choosing performance cams can often seem like a difficult task for many, considering the many variables that can apply to any one camshaft profile or application. I’ve seen too many cars with the wrong cam profile struggle on the dyno or even worse make their owners struggle with just driving.

Before choosing a cam however, don’t forget to remember what you are choosing the cam for; drag racing? road racing? any daily driver usage?

 

Let’s begin by tackling each term and what it means.

What is valve lift?

How far the cam opens the valves, when increasing lift you increase the length the valve opens, which will produce positive gains in airflow. Be careful when selecting your cam, as too much lift may cause valve float as you will see many manufacturers list recommendations for upgraded springs, retainers or guides.

What is duration?

The amount of time that a valve is open, regardless of intake or exhaust and measured in degrees of crankshaft rotation for that period. What does this mean to you? It tells you what the cam’s potential is within a specific rpm range. Shorter duration cams provide low end torque, while longer duration cams allow for more top end flow.

For all the SOHC ninjas 🙂

What is overlap?

The amount of time that both exhaust and intake valves are open in any one cylinder at the same time. Usually this is close to Top Dead Center as the piston stroke begins down the bore and the exhaust valves must stay open until the piston pushed the gases out.

 

What is timing?

Enthusiasts most commonly mistake this term “timing” to ignition timing when it comes to valvetrain events. Trust me, this happens way too often to make any sense.

What is camshaft timing? It’s what you can adjust when you have an adjustible cam gear, that will allow you to alter the timing events.

When you advance the camshaft timing, you are making the intake valve open sooner which will give you more low end torque. As always, when advancing or retarding the camshaft timing you must be conscious of the safety window in piston-valve clearance as to avoid a “catastrophic” event.

Retarding the camshaft timing will delay the intake closing and keeps the intake valve open for longer. This is one of the oldest tricks in the book for turbo Eclipses, Talons and Lasers in the mid-90s by selecting just one exhaust cam and retarding it to -5.

EPSON DSC picture

By retarding the one exhaust cam you could sacrifice idle quality for some terrific top end by holding the intake valve open to move the overlap higher in the powerband.

This is a shot of my old 1999 Mitsubishi Eclipse Spyder, before AWD conversion, retarded negative -5 on the exhaust cam and putting down 395 to the wheels @ 22 psi on 91 octane, extremely reliable.

Of course in today’s age of variable valve timing in everything, many of these older tricks are not needed to generate huge amounts of power without sacrificing idle and start response. In the mid 90s however, you weren’t doing that unless you had a turbo VTEC motor, which had it’s own set of problems in that era.

Increasing the duration is increasing the amount of time the valve is open, therefore helping the engine effectively fill the cylinders and produce power. Many camshaft profiles maximize flow by opening the exhaust cam extremely early in the cycle, increasing exhaust output and increasing the exhaust valve fully open when the exhaust stroke begins. During the power stroke, the burning fuel has used about 80 percent of its available force on the piston by the time the crank has turned 90 degrees.

The bottom half of the power stroke actually provides very little in terms of engine power, and it can be better used to help exhaust the combustion chamber so that there is more efficient cylinder filling on the intake stroke.

Meanwhile a more aggressive profile with higher lift velocities will shorten the duration which will help power, but narrow the powerband.

What is LSA?

Lobe separation angle is the number of degrees between the centerlines of the intake and exhaust lobes on any one cam. The lower the LSA, the more overlap you create while increasing the separation decreases overlap.

Remember to avoid the most common mistake, and that’s going with the most aggressive cam available only because “It sounds so badass” An overly aggressive long duration cam may sound cool at idle, but will give you a very top heavy and small window of power in the rpm range.

Add to that equation the idle, emissions and starting issues, and you can quickly see how a mistake in cam selection can ruin your enjoyment with your car. Which isn’t the point… right?

In the end, choosing a cam that’s right for you is a big part of your performance equation. If you have any more questions, feel free to shoot me an email or post below! As always, if in doubt ask your nearby tuner in your area!

Happy shopping!

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