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Can anybody tell me about the merits and demerits of OHV (overhead valve) vs OHC (overhead camshaft)?

Some experts say that an OHC engine will run faster than OHV engine.

Why is this?

  • When you say run faster, do you mean a higher redline, or just be faster in terms of speed? – George Apr 5 '16 at 11:43
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OHC engines tend to be able to rev higher mostly due to the significantly lower valve train weight which makes for greatly improved valve control and higher potential RPM before float occurs. OHV engines have pushrods and relatively heavy rockers as opposed to OHC which does away with that. The inertia on the valve is lower and therefore controlling it becomes easier. This means that a light spring pressure can be used for a given lift and duration before float is encountered.

Of course it goes far deeper than simply that, a few other comparisons between the two types reveals the following:

  1. With a DOHC setup you have 4 (or 5 sometimes) valves which does three things to improve efficiency: the total volume of two smaller ports is greater than one big port, smaller ports have greater airflow velocity and can fill cylinder more effectively and coming back to valve train weight; two small valves are lighter and require again lighter springs and hardware.
  2. OHV engines can have long and poorly angled pushrods which can cause valve control problems and even catastrophic failure at high RPMs if not addressed. OHC engines run the camshaft directly on followers which are attached straight to the valve - no possibility of energy loss due to pushrod flex or misalignment.
  3. DOHC specifically can have lower per valve max lift than an OHV engine but still have more total lift since you have 2 or more valves (5mm x 2 for instance) again this means a lighter valve spring that need not be compressed as much as single spring. You also have more airflow for a given duration since you have two ports working at the same time. Since you have two separate camshafts, true variable valve timing is possible. You can control opening and closing events of both intake and exhaust as well as lobe separation angle and more depending on manufacturer.

All this being said, this does not explicitly mean that DOCH/SOHC is always better than OHV. If you look at a modern Nascar or Pro Stock OHV motor you will see 10 000 - 12 000 RPM and at the same time see some modern production car DOHC/SOHC motors that can barely muster 5000RPM. As with all things, it depends completely on application, parts used, cost to produce etc.

Generally however it is cheaper to build a high revving OHC engine as opposed to OHV as you do not need special tapered/thick wall pushrods, special rocker shafts, exotic material valves and springs in order to cope with extreme valve spring pressures and valve inertia - if you go the exotic route on OHC engines you get the likes of 14 000RPM race engines and even 20 000RPM F1 engines.

At the end of the day a given motor will not run faster simply because the way the valves operate changes from OHV to OHC - all factors need to be considered for there to be a true comparison.

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OHC vs OHC - Pro's and Con's

Overhead valves and a single overhead cam can have very similar results. If you add in DOHC you will get into another world of comparison.

Here is an excellent post regarding the delta between DOHC and OHC.

Notice the OHC has rocker arms and the DOHC has the cam acting directly on a 'bucket'. The OHC rocker arm adds weight to the overall reciprocating mass whereas the DOHC is very simply with very little weight. Overall, the objective for higher RPM is directly tied to reciprocating mass to prevent valve float.

This is a great post related to various valve and tappet designs that is good foundation information.

As well as this post.

And a final piece regarding the Chevy OHC engine and why it is desirable.

Pro's and Con's OHV

  • OHV Con - In order to keep RPM's higher the valve train from the cam in the crankcase through the pushrods to lifter must be lightened in order to obtain greater efficiency seen at higher RPM's.

  • OHV Con - More moving parts = more points of failure.

  • OHV Pro - Simpler design

  • OHV Pro - Weight distribution and a lower CG. Since the cam is in the crankcase and many of the components to hold it into place OHV typically has a lower CG

Pro's and Con's OHC

  • OHC Con - More reciprocating weight than DOHC

  • OHC Con - Taller engine, more space is required to accommodate for the cam and valvetrain.

  • OHC Con - Higher CG. The taller heads make for more weight higher in the engine effecting the overall center of gravity of the vehicle.

  • OHC Con - Increased production costs due to a more complex cylinder head.

  • OHC Pro - Less moving parts

  • OHC Pro - Less reciprocating weight

  • OHC PRO - Higher RPM's due to less reciprocating weight of the valve train.

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One advantage of OHC is that the valves can work much faster therefore you can get higher RPM's.

Think harley, that has a redline of 5,000 rpms, vs a ducati V-twin that revs over 10,000 RPMS.

Both are v-twins but the limiting factor is how fast the valves can open and close.

BTW I think racing harleys can redline a little higher maybe 7500 or so.

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