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What are the main differences in OHC and OHV engine designs, and which is faster or more powerful? Also, which is more reliable?

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OHC refers to overhead camshaft and OHV refers to overhead valve, two different configurations in engine design.

The key performance differences are described briefly in this Wikipedia article

The fundamental reason for the OHC valvetrain is that it offers an increase in the engines' ability to exchange induction and exhaust gasses. (This exchange is sometimes known as 'engine breathing'.) Another performance advantage is gained as a result of the better optimised port configurations made possible with overhead camshaft designs. With no intrusive pushrods, the overhead camshaft cylinder head design can use straighter ports of more advantageous crossection and length. The OHC design allows for higher engine speeds, which in turn will increase power output for a given torque.

So OHC can provide greater power. This does not necessarily relate directly to speed, but a reasonable assumption is that yes, you would expect it to be faster.

DOHC, or dual overhead camshaft, allows even freer gas flow so can provide even higher power output.

OHC is easier to maintain, as more components tend to be external to the engine block, so they can be adjusted and maintained, as well as checked for problems.

These days there are very few OHV engines anyway.

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Also note that pushrod engines have more reciprocating mass, that would be detrimental at high RPM. As I’ve read, manufacturers of motorcycles, for instance, use pushrod system on cruisers not only because it is a “classic” design, but also because the way they made, they produce a lot of low-RPM torque. However, this would not work for a high-RPM, high-powered sport-bike motor. –  theUg Jun 12 '13 at 16:50
    
From what I have seen OHV engines can be built a little bit more compact so size constraints might be a factor as-well. –  Mike Saull Jun 12 '13 at 17:11
    
@MikeSaull, that is also true, since there are no camshaft on top of the cylinder. Issue of space is even more so important for aforementioned cruiser big twins, because they tend to be a long stroke motors, while a lot of OHC engines sometimes have bore larger than the stroke, especially in big thumpers (600‒800 cc single cylinders such as Suzuki DR650 or smaller BMW dual-sports). –  theUg Jun 12 '13 at 19:47
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