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Camshaft Duration

Real vs. Advertised

As you probably know, the two key attributes of every camshaft are camshaft lift and camshaft duration.

Camshaft Lift determines how much a valve is open, and is expressed as the maximum distance of the away from the seat reached by the valve during engine operation.

On the other hand camshaft duration determines how long the valve is off the seat, that is how long the valve remains open, and this is expressed in degrees of crankshaft rotation.

There’s nothing deceiving when it comes to lift values. The valve opens this many mm and that’s your valve lift, and that’s pretty much it. It’s always just one simple number and it is impossible to misrepresent anything when it comes to camshaft lift.

On the other hand camshaft duration values can be pretty tricky or even deceiving and to understand why this is the case we first have to understand how is camshaft duration measured. The starting point for the measuring of the duration is when the valve gets off the seat, and the end point for measuring duration is when the valve returns to the seat.

Now camshaft duration is actually expressed in degrees of crankshaft rotation. Our example intake camshaft has a duration of 240 degrees, so what does this mean? It simply means that the intake valves of any particular cylinder in this engine are open for 240 degrees out of the 360 degrees of the crankshaft’s full single rotation.

Simple right? Well not really, because this raises an important new question: at what exact amount of valve lift do we actually start measuring duration. Is it when the valve is 1 mm of it’s seat? Or maybe 0.5mm off the seat? Or maybe we should measure in inches and talk about 0.05 inches or 0.006 inches from the valve seat? Does it even matter?

Well, it definitely matters, because the starting valve lift has great impact on the final camshaft duration value. It may seem that the difference between 0.1 mm, 0.5 mm and 1 mm of valve lift is negligible for duration, but it’s really not and has a very significant impact.

For example this particular camshaft, which is an OEM Toyota 4AGE 16v bigport camshaft, has 240 degrees of duration when you start measuring duration at 0.1mm or 0.003 inches of valve lift, on the other hand it only has 204 degrees of rotation when you start measuring at 1.2mm lift off the seat or 0.05 inches. That’s a difference of 34 degrees, which is very significant when it comes to camshaft duration.

Often duration numbers are thrown around the internet and forums are stated simply in degrees of duration, without the valve lift point at which duration was measured. And this can make it very difficult or even impossible in some cases to compare different camshafts when looking for an upgrade for your engine. If all you have is the advertised duration it’s almost like having nothing, because you have no idea where that duration value comes from, and what might seem like comparing two camshafts, one with higher and one with lower duration might in fact be completely misleading, the long duration cam, could very well be the one with shorter duration.

So what’s the key takeaway here? Well it’s very simple, you need to know at which lift point duration is measured so you can actually compare different camshafts, otherwise you’re a bit in the dark as advertised duration can be misleading so the next time some dude on the forums or a Facebook group tries to sell you an aggressive long duration camshaft make sure to ask him about that lift point.

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