A very good example of a factory engineered high static compression ratio combination was Chevrolet's ZL1 engine of 1969. This particular engine was fitted with large domed pistons capable of producing static compression ratios in excess of 12.25:1. The camshaft fitted to the engine made use of a very long duration, high lift design that in most instances could be considered to be only marginally streetable. In practice, the cars fitted with this combination were rough idling, but still maintained exceptional throttle response and of course, outstanding horsepower. How was this accomplished? Through the use of carefully selected and matched components. The high static compression ratio offset the increased duration and overlap figures found in the camshaft.

Because of what seemed to be an almost incredible amount of horsepower potential from the Chevrolet "180" grind camshaft (ZL1 cam), a number of enthusiasts were quick to jump on the power bandwagon (yours truly included). Unfortunately, many of these backyard heroes soon found out that the ZL1 grind was not streetable in their engine combination. The reason? The final combination of components -- specifically the true static compression ratio, camshaft design and cylinder pressure weren't matched. The Chevrolet "180" camshaft was not suited to operation in a low static compression

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ratio environment. As a result, the cylinder pressure went away completely. Throttle response turned out to be terrible and in many cases, the vehicles could not pull themselves away from a stoplight. If these same engine combinations had taken the static compression ratio of the ZL1 into account, horsepower (and considerable amounts of it) would have been restored.

How do you solve this cylinder pressure dilemma in your combination? Select a cam grind that is suited to the application. A low compression ratio engine will typically respond favorably to a camshaft that features relatively short duration figures, wide lobe centers, rapid valve opening rates and a high (comparatively) lift number. In many cases, in the actual design of the camshaft, incorporating some of the above concepts can produce favorable cylinder pressures and other attributes. On the flip side, an engine that features a high static compression ratio can easily use more duration and tighter lobe centers.

How Long Should The Valves Remain Open?

A very large concern when selecting a camshaft for a vehicle is duration -- both advertised and the more important .050-inch lift number. Massive advertised duration numbers may sound impressive to your pals and can make for a neat bench-racing topic, but if the figures are accurate, can you live with them in a dual duty, street-strip application? In the vast majority of cases with long duration camshafts, the overlap numbers are increased and with this increased overlap, the intake and exhaust valves are opened simultaneously. The engine idle then gets out of hand and you are forced to increase idle speed dramatically just to make the engine run. The reason for this is rather simple: When overlap is increased, manifold vacuum decreases. With low intake manifold vacuum figures, tuning becomes increasingly difficult. Carburetor idle mixture screws become insensitive and throttle response is very difficult, if not impossible to obtain. Fixes for this problem include increasing initial spark timing (and removing an equal amount of spark from the mechanical advance curve), drilling carburetor primary throttle plates, increasing the diameter or size of the carburetor idle feed restrictors and other band-aid fixes.

Aside from tuning aggravation, a lack of intake manifold vacuum can wreak havoc with an automatic transmission-equipped vehicle (even those that are fitted with a small diameter, high stall torque converter). If the manifold vacuum is low, dropping the transmission lever into gear almost always results in stalling or stumbling. Attempting to obtain part throttle response under load is also difficult (again, if not impossible). The key in this area is to select the camshaft carefully. A long duration camshaft designed for use with a stick shift transmission will not work well in an automatic transmission application.