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This question is somehow related to my previous question here: Compression of valve springs and camshaft rotation

In that question, I asked about the pattern of camshaft rotation and how valve springs can affect it.

Now, another thing comes to mind and it is choosing the best possible timing positions for engines by manufacturers. According to my experience, it seems that engineers choose a position in which the forces of compression and decompression are equalized so that the camshaft(s) remain stationary even when you remove the timing pins. I think this is the reason camshafts don't tend to turn when they are positioned in TDC or timing position. Is this assumption true?

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The selection of choosing the best possible timing positions has nothing to do with keeping the camshaft(s) stationary. If the camshafts stay stationary, it's usually because there is a split between the intake and exhaust lobes opening times called the Lobe Separating Angle or LSA. Depending on how this is setup, the LSA is usually between about 110° of crankshaft rotation to around 122°. This depends on a lot of factors to include things like whether the engine is turbocharged or naturally aspirated. This usually means, depending upon the number of cylinders and thus cam lobes, there's one cam lobe opening a valve and one closing a valve, which offset each other.

The cams timing position is designed into the cam when engineered. The angle at which it is set is prescribed by engineers to account for whatever performance variables are required by the specific application. In performance applications, there is the ability to change the cam angle to affect how an engine behaves. Move it one way and it will improve low end torque (TQ), like you'd want with a pickup truck or towing application. This is to the detriment to high end horsepower (HP). Move the cam the other way and it improves high end HP at the cost of low end TQ. You'd want this for performance applications where you're running your engine at higher RPMs like racing applications. Usually, under normal conditions, the manufacturer's engineers try to find the sweet spot between the two areas to give the vehicle owner the happy medium. In recent years manufacturers have been able to do this using variable valve timing, which, depending on the vehicle's needs, can alter cam timing on the fly and give a little better low end TQ while also improving high end HP.

Bottom line here is, the cam position isn't set to provide an easier time for the mechanic. It's engineered how it is for what is needed for the specs of the vehicle. The fact most cams stay put with the belt/chain off them is just a happy coincidence. I say most, because some cams move if not held stationary. Some multi-cammed engines are nearly (if not completely) impossible to set timing without some sort of holding fixture. They just won't stay still.

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  • @Paulster2 - Thanks for your fast and first response! I'm completely aware of the things you mentioned in your 2nd paragragh. With this topic, I just wanted to know if " holding the cams stationary without inserting locking pins with the belt/chain removed " is one of the things engineers have in mind when they decide where to design the timing holes in the cylinder head (not the engine timing, cam profiles, valve lift, etc). In fact, you can put timing holes in a cylinder head at any point (throughout 360-degree rotation of camshafts) you want without affecting engine timing. Jan 22 at 16:01
  • @NarimanAsgharian - If you are aware of everything I've stated here, then you'd understand what you're asking is not a concern for the engineers. Also, if you review the first sentence I typed in the answer, it says the same. Jan 22 at 16:42
  • Reminds me of when I was a boy and had a mechanical cable to my Mallory dual distributer and could adjust timing as I drove. Jan 22 at 17:42
  • @Paulster2 - You're right but maybe I have problem communicating what I mean. The fact that in some cars, cams stay put even without locking pins is like a good SAFETY MEASURE because if a mechanic removes a pin by mistake, cams don't rotate and there is no risk of valves hitting the pistons. I think this subject is important enough to be considered by engineers. In some cars, this phenomenon happens by chance because timing holes are exatly in a position where forces are equalized but this is not the case in other cars. This has nothing to do with how timing affects low or high RPM torque. Jan 22 at 19:40
  • You can stop an engine at ANY point, lock both cams and crankshaft at that point and then remove the belt or chain and reinstall the belt or chain without affecting engine timing. So, when this is possible, why don't we choose a position in which cams are stationary as an extra safety measure? Jan 22 at 19:48

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