Understanding engines: how a camshaft works (part 1)
While we love our choo-choo spooly bois and whine-box screamy screws, when it comes time to taste the big-capacity pushrod life of a V8 we really need to start learning about camshafts and how they make your engine awesome. This will be a multi-part series as we look at the process of upgrading a cam in a modern, street-driven pushrod V8 engine, starting with the basics of what a camshaft is, and how it works.
When General Motors released the LS V8 on the world in 1997 it ushered in a new era of smoothness and enhancements not found in traditional old pushrod designs and there was one area above all that proved to be the secret for unlocking the new-age pushrod small-block V8’s potential: the camshaft.
First, let’s look at how this “dinosaur technology” works. In a traditional pushrod V8 the cam actually sits in the engine block, directly above the crankshaft and under the valley.
As the camshaft rotates, turned by the timing chain that connects it to the crankshaft (see Image A, below), the lobes of the cam push lifters (see Image B, below) and pushrods (see Image C, below) that open and close intake and exhaust valves. The pushrods do this as they’re attached to rocker arms, which are acting against valve springs that are trying to keep the valves closed.
Image A: the top sprocket is bolted to the end of the cam while the chain runs down to a sprocket on the cranksaft. As the crank spins, it turns the cam.
Image B: hydraulic lifters which are moved up & down by lobes on the camshaft
Image C: pushrods which sit in the end of the lifters, and use the lifting force to push the rockers (the silver arms at the top) and open intake or exhaust valves
Factory cams need to be smooth and subtle to pass emissions laws and be nice to drive for a wide variety of people buying the new car. The more aggressive, chopzilla cams like those fitted to race cars have aggressive lobe shapes to give higher lift and duration and change how far and how long the valves are opened for. Yes, you can go too extreme in your cam and open the valve too far, which buries that valve in a piston and causes extreme sadness as you just turned your engine into a paperweight.
There hundreds of options when it comes to LS cam upgrades for street cars, and while everyone wants the angriest sounding hog-ass choppy-boi in their V8, the wrong cam specs can make a car slower because the power comes in too narrow an RPM range, comes in too high in the rev-range, won’t idle, or dozens of other issues.
Most cam specs shown are lift, duration and lobe separation angle (LSA). Lift refers to how far the intake and exhaust valve is opened (lifted) off its seat and increasing this can increase power, while duration refers to how long the cam holds the valves open, shown in degrees of crankshaft rotation (though the cam rotates at half the speed of the crank).
Holding valves open longer allows more air and fuel into and out of the combustion chamber, but will also shift power higher in the RPM range as duration increases, sacrificing low-end torque.
Cam manufacturers will generally display duration specifications both “as advertised” and “at 0.050-inch lift”, which measures from the point the cam moves the lifter up 0.050in until 0.050in until the lifter is all the way back down. Cam manufacturers often differ in the range they show for advertised duration so the “at 0.050in lift” numbers are the best to go by as they are a universal measurement.
When you see 228/232, that’s a cam with 228-degrees duration on the intake lobes and 232-degrees on the exhaust. Usually, the exhaust lobes have more duration and lift than the intakes but there are also “reverse pattern” cams that have more intake duration to fill the chamber, which is especially handy on boosted applications.
Finally, LSA refers to the spread of centreline between intake and exhaust lobes of the cam, showing the peak opening points of the valves. LSA is another term for overlap, which is when both intake and exhaust valves are open on a cylinder at the same time, lowering cylinder pressure.
The lower the LSA number the longer both intake and exhaust valves are open at the same time on that cylinder, and this leads to side-effects like choppy idle and a narrow power band. Stock cars generally have quite high LSA numbers, up around 120-degrees, while many aftermarket cams start at 115-degrees and can go as low as 106-degrees.