regarding engine width:
from: Car & Driver
A pushrod is basically a metal tube designed to transfer the reciprocating motion of a valve lifter, riding on the camshaft, to the valve.
I found the pushrod fascinating 20 years ago, back when I was a high-school car geek devouring car magazines. It was about the time the Japanese car invasion began and Detroit started to worry about loss of market share here [in the USA]. In the mid-'80s, Detroit stuck by the pushrod, and nearly every available engine used it. Domestic cars were for the most part, however, mediocre contraptions that allowed foreign automakers to make inroads into the U.S. market. Back then our 10 Best Cars competition had five spots for domestics and five for imports. In 1983, the pickings were so slim that the Chevrolet Caprice Classic made the list.
When asked what made Japanese cars better, critics gave a variety of answers, but one stuck in my mind; While the Americans produced crude, underperforming pushrod engines, the Japanese were turning out sophisticated four-cylinder, single- and double-overhead-cam engines with twice the number of valves per cylinder.
The idea of locating the camshafts in the cylinder head was not new, but to a populace still unaware that a 1929 Duesenberg Model J, for example, had double overhead cams and four valves per cylinder, the overhead-cam engine sounded advanced, exotic.
The pushrod was the scapegoat for all that was wrong with Detroit. "No doubt about it. In the '80s, the pushrod was a hated component, a symbol of the uncompetitiveness of the domestic industry," says Sam Winegarden, GM's chief engineer for small-block V-8s.
I called Winegarden because I've always been fascinated by GM's decision to stick with the pushrod when it introduced a new small-block V-8 in 1996.
By the '90s, many domestic four- and six-cylinder pushrod engines had been scuttled in favor of overhead-cam designs. In 1995, Ford axed its venerable pushrod 16-valve V-8 for a V-8 that still had just 16 valves but now ran with a pair of overhead cams.
Keeping the pushrod seemed like yet another example of the General's hanging on to its glory years. But since then, I've come to see that, for a mass-produced V-8 engine, the pushrod layout has more advantages than disadvantages.
"Power is a function of airflow, pure and simple," says Winegarden. "If you're not going to fill the cylinder with valves, why have an overhead cam?"
Although you could design a V-8 engine that uses pushrods to operate four valves per cylinder, the layout lends itself to using just two. Four valves generally have greater total valve area than two, thus creating greater airflow and more power. In the case of the Chevrolet Corvette Z06 and its pushrod two-valves-per-cylinder 405-hp, 5.7-liter V-8, Winegarden says, "We've been able to meet the performance requirements by using more displacement. Two more valves per cylinder would get us another 10 percent in total valve area."
A modern engine is a dizzying array of compromises as designers strive to meet power, cost, reliability, and emissions demands. The Vette V-8 would ultimately have more power if it had four valves per cylinder, but it would lose ground in cost, complexity, and physical size.
"On the Vette, performance density is only beaten by some of the big Ferrari or heavily boosted engines," Winegarden continues. One of the huge advantages of the pushrod V-8 layout is its clever use of space. The camshaft and the pushrods lie in the unused area between the cylinder banks. A DOHC or SOHC V-8 is much wider and a little taller because the camshafts are on top of the cylinder heads.
Enthusiasts often talk about how much power an engine produces as a function of the combined volume of the cylinders (power per liter). But what's often overlooked is the power produced compared with the outside dimensions of a motor.
The small-block is a compact engine. I did some rough measurements of the Z06's V-8, the DOHC V-8 in the Porsche Cayenne S (335 hp, 4.5 liters), and the Northstar V-8 in the Cadillac SRX (320 hp, 4.6 liters). The small-block was about six inches narrower than the Porsche V-8, two inches shorter than the Caddy's V-8, and close in length.
That may not sound like much, but it gives designers more freedom to position the engine for good weight distribution and leaves ample room for suspension and steering components.
Don't forget about the weight advantages. The Vette's engine has one camshaft and a small drive chain that's connected to the crank. A DOHC V-8 has four camshafts and drive gears, beefy heads to hold the shafts, and two long chains. "Our V-8 is a simple and elegant design. It's pretty easy to put together, which helps the reliability, and costs," says Winegarden. He estimates a $400 saving over a DOHC motor. And, of course, with fewer parts comes less weight. (Winegarden says the aluminum small-block is 44 pounds lighter than the Northstar.)
GM also used this architecture for its truck engines. Spreading the tooling and design costs over a large number of vehicles reduced the per-unit engine cost. That's one reason the $52,385 Z06 can compete with sports cars costing twice as much.
So if the pushrod design makes such a good V-8, why does GM make a DOHC V-8 Northstar? "I'm not going to touch that one," laughs Winegarden. GM's party line is that some customers want what it calls "high-feature engines." Winegarden does admit there are some refinement benefits to the DOHC layout, but personally, I don't find the Vette's engine to be a bit unruly.
GM is no longer the pushrod's sole champion. Dodge has reintroduced the pushrod Hemi V-8. And this year, Toyota is running in the NASCAR Craftsman Truck Series with a newly developed pushrod V-8. Of course, that's a motor used only for racing, but is it so far-fetched to wonder if Toyota would make a production pushrod V-8?
Such an engine would be cheaper to build than the complex DOHC V-8 that's now in the Tundra pickup, and Toyota would also realize some marketing benefits from the huge racing investment. The irony is almost too sweet to imagine.
http://www.caranddriver.com/article.asp ... e_number=1