Valve Lift - The distance that the lobe of the cam lifts the tappet multiplied by the rocker arm ratio determines the valve lift.
Duration - The time in which the valve is off the seat during tappet lift, measured in CRANKSHAFT degrees. As there has to be some point in which you begin to measure the lift of a cam there are usually two figures given on a spec card. The Advertised Duration and the Duration at some arbitrarily chosen point (Usually .050" lift) Some manufacturers use a different amount of lift and this can cause confusion. Most Cam manufacturers use the .050" figure, but it is wise to be sure when comparing different grinds. When checking a cam you should always check it at the tappet rather than the valve because of minor variations due to lash, and rocker arm ratio.
Centerline - This is where people begin to get confused. The Centerline of a Cam is the actual position or phasing of the cam in relation to the Crankshaft. To wit: The position of the center line of the #1 INTAKE Lobe of the cam in relation to the position of the #1 Piston measured in Crankshaft degrees of rotation AFTER TDC. This is the figure that is used when we talk about 'Degreeing " a cam.
Lobe Separation - This is the PHYSICAL configuration of the cam in relation to the actual spacing of the intake and exhaust lobes from each other. Lobe separation is ground into the camshaft. You CANNOT change it (Unless you reground the cam). You CAN change the Centerline by degreeing. These two terms are often confused with each other.
Overlap and Compression- A very common idea, although for the most part incorrect, is that overlap bleeds off compression. Overlap, by itself, does not bleed off compression. Overlap is the angle between the exhaust closing and intake opening and is used to tune the exhaust's ability draw in additional intake charge as well as tuning idle vacuum and controlling power band width. Cylinder pressure is generated during the compression cycle, after the intake valve has closed and before the exhaust opens. Within practical limits, an early intake closing and late exhaust opening will maintain the highest cylinder pressure. By narrowing the Lobe Seperation Angle 'LSA' for a given lobe duration, the overlap increases, but the cylinder pressure can be increased as well. Thus cylinder pressure/compression can actually increase in this scenario, by the earlier intake closing and later exhaust opening. By increasing duration for a given LSA, the overlap will increase, the intake closing will be delayed, and the exhaust opening will occur earlier. This will decrease cylinder pressure, but the decrease/bleed-off of compression is not due to the overlap, it is due to the intake closing and exhaust opening events.
Base Circle - This is the lowest part of the cam lobe also referred to sometimes as the "Heel" of the cam. This is the fully closed position of the valve. This is also where you should make your valve adjustments. If you adjust your valve lash at any other point on the cam, you will have problems. We will discuss the proper way to adjust your valves later on. Some Stroker motors require the use of a "Reduced Base Circle cam" in order to clear the rods. These cams are ground with a smaller base diameter and require some specialized components such as longer pushrods.
Symmetrical - A cam that is Symmetrical has both sides of the cam lobe exactly the same. In other words, the intake ramp of the cam lobe that accelerates the lifter to actuate the valve has the same shape as the portion of the ramp on the downside of the lobe that lowers the lifter. These designs are very easy on the valve train as it is a smooth transition from open to closed.
Asymmetrical - An Asymmetrical cam has opening and closing ramps that are unlike and unequal. This profiles usually found on high performance cams and offers a high velocity opening and a lower velocity closing ramp in order to snap the valve open quickly and then set it back down more gently.
Dual Pattern - Again, a grind that is usually found in a high performance cam. The Intake lobe configuration is different from the exhaust lobe. Usually the exhaust lobe is ground with slightly more duration that the intake lobe. Small block Chevy engines really like more duration on the exhaust in most cases. A single pattern cam (Both lobes giving the same amount of duration) work well in street engines.
Cam Walk - A phenomenon that occurs with Roller cams due to slight inaccuracies in the lifter bore spacing. Most Roller cams use a cam button to control the tendency of the cam to unscrew itself from the block. Bushing the lifter bores can control this problem but is very expensive. Pontiacs don't have this problem because of the retaing plate that holds the cam in.
Aside from the obvious such as proper assembly of the engine, and proper degreeing of the cam etc. Setting the valve lash is THE MOST IMPORTANT aspect of your engine. By experimenting with various lash settings you can actually see how different grinds will behave in your engine. You can stagger lash values and find out what your engine likes. the main problem that most people have is getting the valves to stay set once you have the lash setting you want.
No matter if you use regular poly locks or a shaft mounted setup such as a Jesel system the method is exactly the same. The outer nut on a poly lock system is used to adjust the lash and the set screw is used to bottom out on the top of the stud and lock the setting. The use of a stud girdle sometimes requires a slight compensation in the lash setting . You will have to check this out for yourself to find the optimal setting.
The method of setting your valve lash properly is so simple that to do it any other way makes no sense to begin with.
The lash must be adjusted when the tappet is on the heel of the cam lobe. remember that this is the lowest part of the lobe, hence the tappet is as far down in the bore as it will go. Being as you can't see this when the heads are on, the way that you determine when the tappet is in the proper spot to adjust is as follows;
Adjust the Exhaust valve when the INTAKE valve has just closed . Your setting will not change if you do your valves this way.
Home