Driving Impressions and Tech Talk

The Venerable Pushrod Engine
 Are OHC 4-Valve engines really better?
David Cipolloni
April 18, 2003

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Nowadays it seems everyone is sporting the latest engine technology as it applies to variable valve timing, 4-valve cylinder heads, friction reducing materials and all aluminum block and head construction. Just open the hood on a new Mustang Cobra and the massive cylinder heads look like they came from a 426 Hemi of yesteryear. Do we need all of this valve technology to get the job done? Admittedly variable valve timing is essential to producing more power as piston speed increases, but where is the rest of this technology leading us?

Recently Ford had to recall all of the Mustang Cobra’s since the horsepower rating was well overstated. In fact, the horsepower output on the Cobra was more in line with that of the base two valve engine in the Mustang GT. A close look at the Cobra motor revealed the latest in high tech materials and engineering design. These hand built engines sported German made camshafts and crankshaft, blocks and heads cast in Italy, special sintered steel connecting rods, friction reducing piston skirts, electronically controlled ported intake system, etc. etc. etc. This massive engine was a true work of art. Only one problem though, it didn’t make much power. Obviously cylinder filling was coming up a little short on this valve happy induction design. Ford has now corrected the problem though, the addition of a nice big supercharger has corrected the airflow dilemma.



The inexpensive Chevrolet Corvette, with its 2-valve pushrod engine, made quick work of the BMW M3 (SMG II), Porsche 911, and Nissan 350Z.

In a recent shootout performed by Road & Track magazine the Chevrolet Corvette made quick work of the BMW M3 (SMG II), Porsche 911, and Nissan 350Z. The BMW, Porsche and Nissan all incorporate the latest in variable valve timing 4 valve DOHC induction systems. The Corvette on the other hand simply uses the old 2 valve cylinder heads with a single camshaft located down in the engine block. Even the dastardly pushrods are utilized to actuate the valves.

Sure the Corvette does benefit from a larger displacement engine, so remember to yell that over to the Corvette driver as he/she is promptly blowing past your shiny new sportsracer. Now, I’m sure many of you are ready to send me nasty e-mails telling me I’m nuts, or perhaps you are already unlocking the cabinet and taking out the old Browning, so hang on a second and I’ll get to my point.

Regardless of an engines displacement, or induction system design, we need to look at how efficient the engine is. We look at the volumetric efficiency, thermal efficiency and mechanical efficiency of the engine. Let’s define each of these and make a few general comments about each (see illustration).

  • Volumetric Efficiency – volumetric efficiency is the relationship between atmospheric air pressure, and how much air/fuel actually ends up in the cylinder on the intake stroke of the piston. If the cylinder fills, and equalizes at atmospheric pressure, we consider the engine to be 100% volumetrically efficient. Normally aspirated engines can have a VE as low as 80%. Supercharged or turbocharged engines easily exceed 100% VE, making them very attractive, and in my opinion more commonplace in the future. Pressurizing the induction system can do wonders for small displacement engines, just take a look at the new Dodge SRT-4 (Neon). This little econoracer is nipping at the heels of the big Dodge Viper.

  • Thermal Efficiency – thermal efficiency is the relationship between how much heat energy is released during combustion and how much horsepower is generated from the expansion of that energy. There are many thing that effect TE, such as compression ratio, ignition timing, fuel quality and combustion chamber design. Manufacturers are making some inroads into TE these days by increasing the compression ratio and octane requirements on some of their engines. In the early seventies many manufacturers lowered compression ratios in an attempt to comply with newly enacted emissions mandates. The result was the production of some poor performing automobiles. Lots of decals and big chrome bumpers took the place of some slick muscle cars of the sixties.

  • Mechanical Efficiency – Mechanical efficiency is the relationship between how much power the engine produces and how much power is takes to run the engine. Doesn’t that sound a little silly? Unlike a rocket engine where the fuel is turned directly into thrust, a piston engine has many internal moving parts that eat up energy. Overcoming the friction of these moving parts can take a bite out of the ME. Running that big reciprocating mass down in the lower end of the engine, and all the valve train parts in the top end of the engine, can take a bite out potential power output. Oh, and don’t forget, as the engines speed increase so does the friction and the power needed to overcome it.

Drawing in lots of air/fuel, converting it into heat energy, and reducing friction in the engine are the keys to making horsepower. When looking at normally aspirated engines it becomes obvious that large displacement, high compression, low rpm engines can muscle their way to the top of the hill. How does the Corvette trounce the competition? The same way the Dodge Viper does, stick a great big engine under the hood and don’t fuss over the details.


Turbocharger

Want more power from a smaller displacement engine? Put a supercharger or turbocharger on it and hope the guys with the big displacement engines don’t get the same idea. Will multiple valves and double overhead camshafts increase horsepower? Probably, but why split hairs and deal with atmospheric pressure? For street driven automobiles it may not be worth the effort, the simple addition of a forced induction system may be the economical choice for many vehicle manufacturers.

New materials and vastly improved synthetic lubricants make the use of a turbocharger much more attractive to many vehicle manufacturers. Saab has done wonders with the use of smaller displacement turbocharged engines, many of their vehicles perform admirably while delivering in excess of 30 mpg. We have found an excellent blend of power and economy in the Saab that often gets overlooked by horsepower crazed reporters. Saab has used the name ”econopower” to identify some of their engines, I believe this is a realistic branding for the powerplant.

Since automobiles are not taxed on engine displacement in the US, and fuel is relatively inexpensive, we can simply make larger displacement engines when we want more power. I feel it is far more economical in many respects to boost smaller displacement engines and make them do the work of their larger brethren. So much money spent on valve-happy engines that don’t always deliver what could easily be achieved with larger displacement or turbo/supercharging. Does anyone remember when Buick came out with the Grand National? For its day the car was exceptionally powerful, an instant drag racers dream. Just a lowly Buick 3.8 Liter, 6 cylinder, pushrod actuated, 2 valve motor………………….with a nice big turbocharger.

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