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As you know, in most cars it is not easy to turn the intake or exhaust camshaft by hand. You probably need to use both hands, grab the cam sprocket and apply a lot of force to rotate them even with the timing belt removed. This is due to valve springs resisting against compression. However, when you rotate the camshaft a few degrees, the springs should start to decompress which makes it much easier to continue turning the camshaft but is this what happens next? I think the camshaft will be hard to rotate through the full 360 degrees of its rotation because even if some springs are decompressing, the other springs of other valves have started to compress. Correct me if I'm wrong.

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Generally, your assertion that as one spring compresses another spring decompresses is correct. There are some caveats to this, though. This mainly has to do with the camshaft setup (ie: OHV v. SOHC v. DOHC) and the number of cylinders. Here is why each makes a difference ...

In a four stroke engine, there are intake and exhaust lobes on the cam which actuate their corresponding valves. The more cam lobes you have on any given cam, the more acculative tension you'll have on those lobes due to how the valvetrain works. If this is a single cam as in an OHV configuration, the tension is fairly equalized on the cam as there will be some lobes compressing valve springs while other springs are decompressing. The more cylinders, the more equalized it will be. Due to this, you most likely will not get the snapping effect you were asking about here.

If this is a V engine in a SOHC configuration (ie: Ford mod motor), the cam lobes are spread across the two camshafts. As long as the two cams are tied together via the timing chain, you should again, not see the snapping you're asking about. If the cams are not tied together, you have a lot higher chance of it happening. I've personally seen the snapping you're talking about on smaller, SOHC engines (ie: inline 4 cyl). There aren't enough cam lobes for there to have equal compression/decompression forces throughout the spin of the camshaft, so it will snap (or pop or spontaneously turn) due to the forces.

On a DOHC configuration, this becomes even greater, because the lobes are now spread out over four cams (in a V configuration) or two cams in a four cylinder. Again, if the cams are tied together via the timing chain (or belt), you'll see a lot less of it, but the fewer the cylinders, the higher the probability.

Something you didn't ask about, but I wanted to point out. There is a basic conservation of energy going on with the camshaft and spring compression/decompression. Yes it takes energy to open the valve as it comes up on the lobe. The energy is then returned back into the system when the valve is closing. There are some energy losses due to friction as the cam rotates in the cam bores as well as the lifters against the cam, but all-in-all, the system gives back to itself for the most part.

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  • @Paulster2 - Thanks a lot for your informative answer. Based on your answer, I edited my question and added something to it which I think is related to the subject. I'll be happy to know your opinion about it too. Jan 21 at 9:01
  • @NarimanAsgharian - Please don't edit your question to include another question, especially considered the original question had been answered and you selected the answer as answering it. If you have a new question which is related, create a new question and link this question into it for reference. Jan 21 at 16:46
  • I did what you said and created another topic for my question though it is directly related to the things discussed here. Jan 22 at 13:55
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    @NarimanAsgharian - TY ... that's the way SE works, so we should do it that way :o) Jan 22 at 15:26
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First of all you should NOT do this. Rotating the cams while the crank is stationary can damage the valves and/or the pistons.

But assuming the head is removed from the engine and there is clearance below for the valves to move, you should be able to rotate the cams.

That being said, the needed force is not constant but varies depending on what valves/springs are being compressed or decompressed at that time. You will feel that it's "jerky" to some degree.

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  • I don't do this with the head on the car. The thing I want to know is the pattern of rotation. Imagine you start turning the inlet camshaft at a certain point which is hard. What happens next if you continue rotating? What happens when you pass the compression of springs? Do the springs suddenly rotate the camshaft when they start to decompress? This is what I want to know. Jan 20 at 18:59
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    It really depends on a number of factors like the number of cylinders, the profile of the cams, the strength of the springs, the friction of the cams to lifters, etc.
    – jwh20
    Jan 20 at 21:07

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