Hello, I don't know a hell of a lot about the fine points of engines
beyond gas mileage and 0-60 speed, so I was wondering what practical
differences there are between overhead valve vs. overhead cam
engines. Secondly, assuming an engine has overhead valves
(pushrods?), is there still a timing belt or chain to replace
periodically? My Hondas had a scheduled time for timing belt
replacement whereas my new Malibu (3.9L) engine manual makes no such
mention. Thanks for your input...Ted
First question: less parts, for one. Second: Yes. And this is a sore
spot I have for domestics, altho' that's all we've ever owned/driven for
personal cars. Guess domestics feel their products will fail somewhere else
Well - sorta "Yes" on the second question/answer. Domestics typically have
a timing chain which for all intents and purposes is maintenance free. This
is especially true when contrasted to the use of a timing belt which is
normally a 60,000 mile replacement item.
Replying to my own post - which is sorta like talking to yourself...
One does not escape the timing chain with OHC engines either. The issue is
not whether there is a timing chain in the engine, it's whether the engine
uses a chain or a belt.
Well in an OHC engine you have a lower parts count usually. Simpler
design for the block. Heads can have better valve placement because you
don't have the rocker towers in the way. Less friction in the valve
train because of poor lubrication. And fewer friction points.
OHV engines are a long proven and tested design.
OHC use from 1 to 5 belts or chains to drive the cam(s). They need
regular replacement to prevent engine damage.
OHV engines use gears or 1-2 chains to drive the cam. Because of the
design most are very well lubed reducing wear.
Most imports use an OHC design which require service to change the
belt(s). Your Honda is one of them.
The Malibu uses an OHV design which should last the life of the vehicle.
The Nissan VQ35 is a very proven DOHC design that uses a timing chain.
Lasts the life of the vehicle, this motor has been listed as a top ten
motor for the last 15 years or so.
Typically OHC are higher RPM and horsepower vs the OHV (higher torque)
of the same displacement.
It is a fairly new design, so the track record is unclear. It's
ancestors included the 3.1 and 3.4l GM V-6s which were OK engines that
suffered an unreasonably high intake manifold gasket failure rate, which
failure allowed coolant into the motor oil.
This is hopefully fixed on the 3.9.
The devil is in the details. In theory an overhead cam engine can be
made to produce more power per unit of displacement than can a
cam-in-block "overhead valve" engine. In truth both types use overhead
valves, but the overhead valve designation going along with a
cam-in-block design dates back to when engines switched from
valve-in-block, aka flathead, designs to having the valves in the head
above the pistons. Pushrods are the parts used to translate the motion
from the camshaft up to the rocker arms which then in turn activate the
valves. Overhead cam designs excel at allowing higher RPM operation
since the valve opening mechanism is much more direct.
GM's current practice is to use the pushrod design in most of their
engines while saving overhead cams for "high feature" engine designs
like the Northstar and the Colorado straight-6.
> Secondly, assuming an engine has overhead valves
AFAIK, all pushrod motors currently made use an internal lubricated
timing chain to drive the cambshaft. Overhead cam engines, on the
other hand, are made either with a chain drive or with a belt drive
depending on the individual engine design. The belt drive style is
cheaper to build and generally quieter in operation as well as being
surprisingly precise. However, the rubber belt is an item which
requires periodic replacement. Timing chains and their gears don't
normally have a specified replacement interval, yet they do indeed go
bad. My old '65 Cadillac motor died suddenly one day when the plastic
teeth of the upper timing drive cogs let go. This was once a common
sort of failure. I'm not sure what the failure rates and mileages are
for modern timing chains, but generally they will hold together for as
long as the engine still makes good compression and has good bearings.
Currently Honda's 4 cylinder overhead cam engines use chain drive while
their V-6s use belts. Volvo is all belts AFAIK. I think GM is all
chains, but I might be wrong.
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