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Old 02-17-2003, 12:38 AM
bobbyv bobbyv is offline
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Join Date: Aug 1999
Location: ajax, ontario, canada
Posts: 773
the issue is the Reciprocating Mass of the valvetrain.

a pushrod design has high reciprocating mass, with the pushrods themselves responsible for a large percentage of this. This means that with a given valve spring rate/cam profile (among other factors), the redline (as defined by the point at which valve float occurs), will be low.

an overhead cam (OHC) design has a lower valvetrain reciprocating mass than a pushrod design. Compared with a pushrod engine, its redline will be higher for a given valve spring rate/cam profile.

most engines nowadays are of the "cross-flow" design, with the intake and exhaust valves splayed at an angle (done for a number of reasons I am not sure of). A double-overhead cam (DOHC) design has the potential for an even lower reciprocating mass than an SOHC (single overhead cam) design in this situation because the valve followers can be made even smaller and lighter than in an SOHC design. As such, one can easily design a DOHC engine to have a higher redline than an SOHC one.

for a pushrod engine to have a higher redline, one would normally increase the valve spring rates, but this would result in higher frictional losses. As such, pushrod engines typically have low redlines and are typically tuned for torque down low in the rpm range.

sporty engines will typically be tuned for high-end power, and as such, will benefit from the higher redlines allowed by the overhead cam designs.

multivalve engines (e.g. those with 4valve heads) provide better breathing at higher RPM, and to maximize this, one needs a high redline. The V-arrangement of the valves also makes the DOHC designs a natural application for this.

DOHC designs also allow independent variation of the intake and exhaust cam characteristics, in variable-camshaft offset applications (which are typically used to enhance the torque characteristics of an engine).

also, DOHC engines look sexier than OHC or pushrod engines in the engine bay (especially inline engines).
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