Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian

       Timing of the valve opening and closing events is critical for air/fuel intake and exhaust evacuation. (Photo Courtesy Crower Cams and Equipment)

      Overlap

      The goal with camshaft timing is to maximize cylinder-fill by using an earlier intake valve opening during the intake stroke, in combination with a later exhaust valve opening in order to maximize the benefit of the combustion process. As engine speed increases, this scavenging effect increases.

      Valve overlap is measured in degrees, from the intake-valve opening event to the exhaust-valve closing event (at the end of the exhaust stroke and the start of the intake stroke). In other words, this is the period where both valves are open at the same time. This event helps the exhaust gas velocity pull more of the intake air/fuel charge into the cylinders. Most high-performance street engines benefit from valve overlap in the 50- to 75-degree range, while a high-output drag racing engine may use overlap in the 100-degree range. In very general terms, the higher the engine’s power output, the more it requires increased valve overlap. You can think of overlap as a scavenging effect that aids the exhaust gas to draw a greater charge into the cylinder.

      Referring to the camshaft specifications in a manufacturer’s catalog or the camshaft’s cam card, you can calculate valve overlap by adding the intake-opening event in degrees before top dead center (BTDC) to the exhaust valve closing in degrees after top dead center (ATDC). For example, if the intake opens at 27 degrees BTDC and the exhaust valve closes at 25 degrees ATDC, you have 52 degrees valve overlap (27 + 25). Always refer to the camshaft’s advertised duration timing numbers, not at .050-inch duration.

      Overlap is affected by lift, duration, and LSA. An increase in lift or duration increases overlap. If LSA is decreased, overlap is increased. Increasing valve overlap tends to increase top-end power, but can reduce low-end power and can degrade idle quality.

      Exhaust Valve Lift

      Lift is the maximum amount of open valve lift that occurs when the peak of the lobe contacts the lifter. On your camshaft specification card, lobe lift and valve lift are listed. Lobe lift refers to the distance between the camshaft base circle and the lobe peak. Actual valve lift is magnified by the rocker arm ratio. Here’s the formula for determining total valve lift:

      Valve Lift = camshaft lobe lift × rocker arm ratio

      For example, if the camshaft features a lobe lift of .367 inch and the rocker arm features a ratio of 1.7:1, the formula works out to .6239 inch lift (.367 × 1.7).

      If you move to a 1.8:1 rocker arm ratio, the same camshaft achieves a total valve lift of .6606 inch (.367 × 1.8).

      A camshaft’s exhaust valve lift should accommodates your planned exhaust system. As you increase valve lift, there’s more room for the exhaust gases to leave the engine. This in turn means that you have more area to fill via the intake system and past the intake valve. Initially, you can assume that the more air you move, the more power you make. However, if valve lift is excessive (too high), the exhaust valves are open during the combustion process, which reduces power.

      While we’re discussing the exhaust valve lift, consider the exhaust valve head diameter. Anyone who has looked at an assembled cylinder head has noticed that exhaust valve diameters are smaller than the head’s intake valve diameters. Because an engine has an easier time pushing air out of the engine than drawing air into the engine, the exhaust valve area doesn’t need to be as large as the intake valve. Typically, the exhaust valve diameter is about 57 percent of the intake-valve diameter.

      Duration

      A camshaft’s duration represents how long the valves stay open in relation to degrees of crankshaft rotation. Longer-duration camshafts allow the engine to breathe better at higher RPM. Shorter durations, along with shorter valve lift, speed up the intake and exhaust flow. With a cylinder head that features a somewhat restrictive exhaust side, this generally requires greater exhaust duration to help pull the intake charge into the combustion chamber and cylinder.

      This is the basic theory of variable valve timing, since a variable valve timing system maximizes the intake and exhaust valve events. The duration doesn’t change, but the timing of the valve overlap events change. This helps explain why higher lift, combined with larger diameter valves and longer duration, is selected for engines that are to make maximum power at higher engine speeds.

      Duration is recorded in two ways: advertised duration and duration at .050 inch. Advertised duration represents the angle of crankshaft degrees relative to a position of the crankshaft, but different manufacturers may use different points of reference. For this reason, it’s best to compare camshaft profile durations based on reference to .050-inch lifter rise.

      By increasing camshaft duration, the valve remains open for a longer period, which promotes peak power at a higher engine speed range. The intake valve begins to open at a point BTDC and closes at a point after bottom dead center (ABDC). The exhaust valve begins to open at a point before bottom dead center (BBDC) and closes at a point ATDC. Remember that the distance, or number of degrees, between TDC and BDC is 180 degrees. Here’s the formula for finding total duration:

      Duration = opening duration at BTDC + closing duration at ATDC + 180

      Using a specific camshaft as an example, let’s say that the cam causes the intake valve to begin to open at 17.5 degrees BTDC and closes the intake valve at 58.5 degrees ABDC. Using the formula, you get an intake duration of 256 degrees (17.5 + 58.5 + 180).

      If the same camshaft begins to open the exhaust valve at 69.5 degrees BBDC and closes the exhaust valve at 14.5 degrees ATDC, the exhaust duration is 264 degrees (69.5 + 14.5 + 180).

      Therefore, in this example, the camshaft features a duration of 256 degrees intake and 264 degrees exhaust. Such a split-duration camshaft (where intake and exhaust duration are different) is generally best used with a cylinder head that features a more-restrictive exhaust side.

      With a cylinder head that has a more restrictive exhaust side, you can increase camshaft exhaust duration and increase the exhaust primary tube diameter to effectively increase the flow of the exhaust port in the cylinder head. Simply put, adding a bit more duration helps in dealing with a less-than-ideal cylinder head design.

      Camshaft Centerline

      Camshaft centerline refers to the point halfway between the intake and exhaust valve centerlines. The intake centerline refers to the position of the peak of the intake lobe in crankshaft degrees ATDC. The exhaust centerline refers to the peak of the exhaust lobe in crankshaft degrees BTDC.

      Camshaft for Forced-Induction Applications

      As mentioned earlier, camshaft overlap is used to help the negative exhaust pressure wave to aid in pulling the air/fuel charge into the cylinder. With a forced-induction setup, the turbocharger or supercharger creates a boost to the air/fuel charge. With a supercharger, too much overlap can be detrimental, forcing exhaust gas out excessively, which reduces or eliminates the scavenging effect.

      For a turbocharged system, a smaller camshaft is preferred, compared to a cam designed for a naturally aspirated engine. The turbocharger is already packing in a higher-density air charge, so less duration is needed. The exhaust drives the turbo in order to pack in more intake charge, so exhaust “pull”