Quote:
Originally Posted by DGthe3
Why are pushrods considered anchient while OHC's are new? I don't know. They are only about 20 years older than OHC designs.
Pushrod engines have a single cam mounted in the block which activates pushrods which open the valves.
+ inexpensive
+ simple
+ compact
+ lighter than OHC
+ fatter torque band
- exceeding 2 valves/cylinder is difficult (more valves=better breathing)
- difficult to achieve high revs (7500+)
SOHC engines have 1 cam per cylinder bank (inlines have 1 cam, V's have 2). These cams are located above the cylinders and are linked with a long timing chain or belt to keep them in sync. The cams activate the valves directly.
+ improved fuel efficiency
+ It is relatively easy to have a 3 or 4 valve design.
+ freer (sp?) to rev
+ better control over valve timing
- larger
- heavier
- more expensive
DOHC engines are very similar to SOHC but they have 2 cams per cylinder bank. As such, their problems and benefits are the same as those from SOHC but more pronounced.
Also, a OHV can be made to be high reving (302cu Z28's, Nascar) and an OHC can be made to produce lots of torque (5.7L Toyota). Likewise, an OHV powered vehicles are not always less efficient than equivalent OHC's (Ram vs Tundra). These mostly exceptions though.
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More DOHC: Adding a second cam lets you change intake and exhaust cam timing independently so as to allow higher overlap at high RPM without destroying low RPM power or emissions. The high end DOHC rigs also offer variable lift for the same race cam when you want it, little cam when you don’t.
A big cam sounds great, but it can make parallel parking a real pain.