[JLerum]

Duration is the time that a valve opens to the time that a valve closes. On a cam card you get duration on the intake and the exhaust valve. These measurements are usually at certain standard lift values so that you can compare cams. The most important thing to know is when the intake valve closes (use a degree wheel) as this determines how much air/fuel mixture is caught in the cylinder. The later the valve closes the more static compression you need to help the dynamic compression of the motor. The term dynamic compression is the functional compression an engine understands when it is in motion (running). Because of the intake valve closing point being so important, you must build a motor so that the intake valve closes just right for the compression.

In the case of running in a class like Pure Stock Muscle Car Drags it is a little different. When running the (stock) cam you must pay attention to your compression. Design the motor so its compression ratio fits the cam by decking the block, ccing the combustion chambers, and or selecting head gasket thickness. The stock cam is defined by the rules so you must make the compression fit the cam. Do not think that more compression is always better because you can have too much compression for a cam. A good example would be the Buick Stage 1. The PSMDR allows you to blue print the motor to NHRA specs. The specs submitted to AMA would allow this motor to push almost 13.0 to 1 compression. The stock cam valve duration would keep you from using all this compression advantage and would be less than competitive.

To fine tuning a solid lifter cam motor you can change the intake valve closure 2 ways. The first would be on how you degree the cam during setup with the timing chain (advanced or retarded). This is the most effective way to move intake valve closure. When the cam is advanced the intake valve is closed sooner, increasing cylinder pressure effecting low-end torque. Retarded the cam closes the intake valve later and helps top end horsepower. Again this is because better high RPM cylinder filling.

The second would be adjusting lash. (IÃm finally answering your question) The looser the lash the more slack the camshaft lobe must take up before opening the valve. The lifter will have traveled farther up the lobe delaying the valve opening and closing the valve sooner on the back side of the camshaft lobe. In effect you have removed some duration. The other unfortunate result is you have lost lift. Extra slack in the valve train keeps the lobe from displacing its full lift to the valve. This is bad because we know that BB Chevys love lift.

If you are really able to sit on a dyno for an extra day, a lash loop should be tried. Isolate the intake valves and increase/decrease their lash in .004 increments to see how the engine responds. Then try the same thing with the exhaust. This will tell you how well youÃve chosen your compression or camshaft.

If you are running a rapid lift solid lifter camshaft you must be careful how you adjust your lash. Most of these rapid lift cams run .012 to .014 lash(very tight). You can not allow much more lash than the manufacturer has on the cam spec card. If you do the valve train will accelerate so fast that you will get broken valve train parts. Surprisingly you can run considerably less lash. Hope this is informative and gives some insight.



Jim Lerum




[Edited by JLerum (09-24-2001 at 07:11 AM).]

[Edited by JLerum (09-24-2001 at 07:12 AM).]