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Originally Posted by SaabJohan
Chains are used in both push rod and OHC engines (BMW and Saab are examples on the latter). Chains tend to make more noise, especially when they become worn. (basic engineering)
Many high revving motorcycles use chains for the cams, but at high performance applications gears are common. This introduces however more noise.
The service interval on belts have become a lot better lately, 60k is very low. But the cost of a belt is low and many people can do it theirself if they want (not that much more complex than to change waterpump/generator/servopump/AC belt). A chain lasts longer, but not until the end of engine life (unless you have a low quality engine), and it's more expensive to change.
Both chains and belts are stretched somewhat during their life.
Both push rod and OHC engines need oil to the top of the motor, escecially with high performance engines when valve spring cooling is important.
The difference in size of a push rod and DOHC engine is very small, the increase in performance more than compensate for this.
The largest valvetrain problem is probably hydraulic lifters, which can cause wear of too high pressure against the cam or start to leak and the cam to lifter clearance till be off.
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eh, sorta....OHV motors really only need a smidgen of oil to lube the ball/rocker assembly, pushrod end and cool the springs.
since the cams in an xOHC motor are held in aluminum, in most cases without benefit of a bearing shell, forming a hydrodynamic wedge is critical - and maintaining same across the cam. On a DOHC V, there are 4 steel cams that can wipe out the aluminum bosses in MINUTES, not miles. In the old days, OHV engine had a simple 'x lbs per y rpm' oil pressure requirement...today, the xOHC motors tend to require much more oil rpessure at much lower rpms to ensure all the cams are oiled at idle.
Im not sure what you mean by size, do you mean physical size or displacement? displacement wise, you can have equal displacements with either method of valve actuation, however in terms of physical size - use the example I gave you, the 4.6L SOHC ford pickup motor is LARGER in physical dimensions than the 7.3L diesel!!! and note, a DOHC motor makes it worse.
As for your valvetrain comments....you're all wet. Hydraulic cam followers are what EVERY manu has went to. Now note, save geometry, the parts and use of are the same - with a solid setup, you must set the clearance correctly - just having said clearance leads to a LOT of noise (just ask anyone with an older honda ;car or bike; or toyota) and it accelerates wear. with hydraulic lash, the tension on the plunger is measured in inch-lbs, whereas the seat pressure on the valve is measured in ft-lbs. In solid designs, the lobe must whack the follower which deforms the iron or in some cases steel slowly but surely. In hydraulic designs, the cams are offset ground (in OHV applicatins) to rotate the follower/lifter - wear is negligible. whereas solid deisgns require readjustment every 15Kish miles due to wear, hydraulic designs last forever. but please note, the loss of oil pressure on either will cause negative results. I have seen cam lobes wiped clean, but the vast majority as any professional will tell you will be incorrectly set solid designs - once the clearance is gone, the cam lobe is ALWAYS against seat pressure, (as opposed to plunger pressure in a hydraulic setup) Do hydraulic lifters collapse? yep, but it is so rare as to not require mention.
Of note is the use of roller followers today, I and many others have taken roller lifters out of 200K mile motors and reused them after inspection on a fresh cam. they just dont wear out.