Pushrod vs. Overhead Cam

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
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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 first!? s
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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.
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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.
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Ted wrote:

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.
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Steve W.
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Steve W. wrote:

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.
my .02
b
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Thanks all, you've cleared up a lot of questions for me. My final question is this: Do any of you have experience with the 3.9L engine? Does it have a good track record?
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Ted wrote:

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.
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Hey guys thanks for all your input. At least I have a good place to look for problems if they occur (hopefully not) in the future...Ted
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on Tuesday 28 August 2007 08:09 am, someone posing as Ted took a rock and etched into the cave:

Here you go...
http://auto.howstuffworks.com/camshaft.htm
...enjoy!
HTH!
HAND.

Maybe.
My Maxima did have a chain. My Kia had a belt.

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Ted wrote:

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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