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Old 02-16-2003, 09:53 PM
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What are the disadvantages of an OHC engine?

Are there any DISADVANTAGES in a OHC design engine vs. pushrod type?

I own both types--a 4.5 SOHC Mercedes, and a 455 Trans Am.

I always assumed the OHC design was a better design in every respect. So I was suprised to hear someone say the pushrod type offers the advantage of a flatter HP/tourque curve over the full RPM range. Is this true? Are there any other comparisons that favor either motor?

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Old 02-16-2003, 10:01 PM
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I prefer OHC designs, specifically DOHC designs but pushrod motors are generally more compact, simpler (less to go wrong) and if made of the same materials, lighter.
Glen Tokuhara
Beauty & the Beast and the wagon that could!
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Old 02-16-2003, 11:00 PM
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Pushrod engines still use some sort of cam drive system, generally gears and a chain. Given their short path, they tend to last a long time, but still need periodic replacement. My small block hot-rod engine had a gear drive for reliability and timing precision.

Pushrod engines cannot rev as high thanks to the rocker arm and pushrod design. Not that you can't build a high revving OHV engine, but it requires some serious bits.

The real advantage of DOHC designs are the ability to have independant intake and exhaust cams. With the cams acting directly on the rockers, the valve train is light and high revs need light valve trains to keep from floating the valves. The four or five valve per cylinder designs also allow a greater intake valve area which means higher flow and greater efficiency.

OHV designs tend to be of higher displacement and square or undersquare configurations. The higher displacement is required as they have less efficiency, but the lower revving design means a flatter torque curve.

OHV designs do not inherently make more torque. It's the nature of the compromise that the OHV engine forces makers into. You could build a low revving square/undersquare torquey V-8 with a DOHC valve train, al-la Corvette C5, but most European designers wanted to shrink displacements while keeping power, and this requires higher revs. It depends on cam profiles, piston speeds, and gearing to really define the character of the engine/driveline.
John Shellenberg
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Old 02-17-2003, 12:38 AM
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Location: ajax, ontario, canada
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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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Old 02-17-2003, 06:16 AM
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Location: Epsom Downs, England
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What are the disadvantages of OHC?

The highest point-loading (where the oil is most likely to break down) in an engine is the cam/cam follower interface.

The snag with OHC is this is the furthest distance from the oil.

FORD found this out the hard way with the Kent engine which used to wreck camshafts and followers at very low mileages, but didn't show up in pre-production testing.

As a manufacturer, you have to be very careful in applying accelerated-life testing to real-world usage.

Other disadavntages:

Makes engine taller/larger.
Chains are noisy, belts have unpredictable life.
More gubbins to remove for a head-job.
Probably more............

Overhead Valve engines can be made very efficient using modern tecniques. A perfect example is the TVR Cerbrera, this has a Overhaead Valve 4.2 litre engine (single cam)

Despite this apparent stone-age design this car has the following figures:

0-60: 4.0 sec
Top whack: 185mph
Consumption: 19 to 25 mpg

These figures are not too shabby, who needs more cams?
Paul Gibbons
'93 320CE
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike
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