Valvetrain Dynamics

As we mentioned earlier, the greatest differencebetween an overhead cam engine and its cam-in-block pushrod cousin ishow the valvetrain is put together. Whether one system is more difficultto build and maintain is something we'll leave for others to argue, butone thing is for certain: If you are accustomed to buildingpushrod-based engines, building a cylinder head for a racing overheadcam engine is an entirely different animal. It can be quite difficultbecause one change--such as the depth of the valve seat--can affectclearances in several other areas of the head.

For that reason, we traveled to KT Engine Development as cylinder headspecialist Kevin Troutman rebuilt an overhead cam head for a Ford 2.3Lengine that races in a Mini-Stock class. One of the biggestdifficulties, Troutman says, is pushing the valve lift limits set by therule book while still keeping everything else correct to make sure theengine will live.

After cleaning and pressure-checking the head, Troutman performs all thenecessary machining steps to bring it back to spec. New valve seats are,of course, part of the package when rebuilding a head, but they arecritical in an overhead cam engine. Troutman must make sure they notonly remain as shallow as possible (to reduce shrouding and minimizecombustion chamber volume), but he must also try to keep the seats allat the same height relative to each other.

Unfortunately, when it comes to making sure the geometry is correct inan overhead cam cylinder head, the best way is simply to test-fiteverything. Even if it has the same specs as the previous cam, if youare installing a new camshaft you have to recheck everything because thesize of the base circle affects so much. Troutman begins by installingthe intake, exhaust valves, and springs into the first chamber andsliding the new camshaft into place in the head's cam towers. Afterinstalling the 1.66:1 ratio rocker arms between the solid lifters andthe valves, he sets the lash to 0.010 by adjusting the height of thelifter.

Troutman already knows that the springs (which are 100 pounds on theseat and 250 pounds over the nose) have more than the necessary 0.060 to0.070 clearance before going into coil bind at maximum valve lift--forthis head, it's limited to 0.450 inch by the rules for both the exhaustand intake valves. With everything installed, his biggest concern now isto make sure the camshaft provides the appropriate amount of lift to thevalves while staying centered on the hardened rocker pads.

Adjusting lift is all a matter of raising or lowering the lifter inrelation to the top of the valve stem. Of course, doing this changes thelash also. To bring the lash back into spec, you have to adjust theheight of the top of the valve stem (which affects the height of thevalve side of the rocker arm). If that all sounds confusing, that'sbecause it is. We'll try to simplify things by walking through anexample.

Looking at the cylinder head from the front, the first valve for thenumber-one cylinder is the exhaust. The lifter is on the right side, thevalve is on the left, and the camshaft rotates clockwise. The locationsfor the lifter and valve for the intake for that cylinder are reversed.For our example, let's say that the maximum valve lift exceeds themaximum 0.450 allowed by the rules. Increasing the stem height allowsyou to shorten the solid lifter, which will keep the lash the same andreduce lift. This can be done either by installing new valves withlonger stems, or, if you need to make only a small change, by grindingthe seat area of the valve itself. This allows the valve to sit deeperin the seat and raises the tip of the stem on the other side of thehead. If you need to increase lift, you can shorten the stem by grindingthe tip of the stem. This allows you to raise the lifter while keepingthe lash the same. Just be careful you don't grind so much that itcauses clearance problems between the rocker and spring retainer.

You can check valve lift by using a dial indicator on a metallic base.Set the indicator so that it rides on the spring retainer parallel withthe motion of the valve. Rotate the camshaft 360 degrees and see howmuch the retainer moves. Once you have the intake and exhaust correct,if the seat heights in the cylinder head are all same, you should beable to make the same changes to the rest of the valves. Regardless,make sure to always check every valve after final assembly. That's theonly way to know everything on your cylinder head is working properly.

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