[Tone]

Car and Driver did a road test of the new Chevy Equinox and were pretty impressed by it.

The article concludes that "The Equinox suggests a whole new attitude at Chevrolet."

See - http://www.caranddriver.com/article.asp?se...article_id=7878

I'm not much of a fan of SUVs, but I have to admit, this one looks very good. Especially considering how most of these vehicles are actually used.

 

[Tone]

While I agree with the sentiment ChevyExuinox expresses regarding the ohc vs. ohv issue, the statement isn't exactly accurate.

All kinds of engines were produced during the first couple decades of the automobile. Some early engines used atmospheric valves with no cam at all - the low pressuure in the cylinder opened the intake valve when needed, for example.

Overhead cam engines were common in race cars very early in the century. But pushrods were common as well. In fact, there were a number of kits for the Model T Ford to convert its four cylinder engine to overhead valve (common) or overhead cam (more exotic and less common).

Supercharging also existing, both in production and the aftermarket. Point being that as far as configurations go, there is very little that is new. What is different today is the electronics side - the computing power and various sensors that make it possible to have much tighter control of what goes on in an engine. That, and huge advances in material sciences mean that today's engine with electronic controls is a fairly exotic piece compared to production engines of the 1960s - whether they are ohc OR ohv.

 

[Tone]

Good points ... and they few times I've driven a rental with the 3.4 engine, I've not been overly impressed with it from a NVH point of view.

That said, I do think that OHV engines have a couple of potential advantages you haven't gone into. They can be made to package better in V configured engines. GM's 60 degree family of sixes are surprisingly compact, enabling them to fit into a fairly small engine compartments. If a vehicle is designed around this engine, it provides options for more passenger room or some advantages from a crashworthiness perspective that might be attractive.

This allows GM to squeeze a relatively large engine into a given space. This provides the next advantage - a fair amount of torque and the ability to operate happily at low rpm. Low revs equates with lower frictional losses and (hopefully, if it is pulling against tall gears) lower pumping losses. This means an optimized, simple OHV engine can be quite efficient in terms of fuel consumption, while still feeling responsive to the driver.

The NVH issues have very little - if anything - to do with the valve configuration. Japanese automakers like Honda do a whole lot of work on engine NVH, spending special attention to anything that might resonate and doing a lot of work on bracket mounts and engine mounts to isolate the engines as much as possible - very important when you have a four cylinder engine that likes lots of revs. GM could stand to spend a little more attention to their "bread and butter" engines in this regard.

As an enthusiast, I like the idea that GM has an engineering philosphy of its own that says pushrods can be appropriate and can offer some advantages in the right application. It's a refreshing change from the typical "me too" approach. I do think they've proven their point with their V8 engine family, but work seems to remain on their more "modest" engines.

 

[Tone]

Actually there are two components to variable valve timing. One is advancing and retarding carm timing in relation to the crank. The other is using a second, different cam lobe for high and low speed operation (i.e. Honda).

My understanding is that GM did some research and discovered it could gain most of the advantages of variable valve timing by adjusting cam timing alone. Thus, all will be required is an adjustable cam timing gear and related sensors and electronic controlers.