Jeep Engines - Oljeep

- n ri4.0L POWER TECH IN-LINE 6 (CAMSHAFT AND VALVE GEAR) 181Looking at Figure 4-23, we can see the layout (plot) of atypical camshaft with the intake and exhaust lobes shown.You can tell it's a fairly mild camshaft because it has justunder ,300" lift (.450" at the valve). This plot shows theoverlap and centerline relationships to TDC, but we'reconcerned with duration.In simple form, duration is the width of the "hump" of thebell-shaped curve at a given lift point. The further up(height) you go from the "zero" lift line, the narrower thecurve gets. To explain, draw a straight line from the .100"lift mark on the left. Where this line intersects the curvegives us the timing events. (Refer to Figure 4-24.)On the exhaust lobe, these points are E.O. = 30 BBDC andE.C. = 30 BTDC. Applying a little math we find that thislobe's duration at .100" is 180 degrees. Drawing a similarline from the .0S0" lift mark yields these events: E.O. = 40BBDC and E.C. = 20 BTDC. Therefore, duration at 50(.OSOll) = 200.This comparison shows the many aspects of duration. It canbe measured at any lift that you'd like. It could be .020",.050", . 100", or any other number. The duration at .020" islarger than the duration at ,050". The duration at .050" islarger than the duration at .100".Advertised duration is a little more complicated. It isgenerally measured between zero lift and .025" lift.However, it is not a constant. Different manufacturers usedifferent numbers. Generally, this number is determined bythe computer software that generates the curve (i.e., lifttable). Since most aftermarket and OE manufacturers usesomewhat different computer software programs, theadvertised duration number comes out different even if theactual profiles are the same. Generally, the advertisedduration is larger than the duration at .050".Going back to our earlier discussion, intake and exhaustopening and closing event numbers aren't the samebetween advertised and .050" duration numbers. However,most aftermarket manufacturers are consistent with theirnumbers; for example, 1.0. at 30 degrees and .050". Ifyou're not sure, simply add the numbers for the events andcompare to the two duration numbers.We hope this helps you to understand duration and whythere are so many duration numbers. However, always keepin mind that duration is not as good as lift for comparing orselecting camshafts.I1EXHAUSTFigure 4 - 23EXHAUSTCamshaft CenterlineFigure 4 - 24INTAKEINTAKEThere are three ways of checking a camshaft for correctinstallation. A common method is to "line-up-the-dots.''This is acceptable for standard vehicles, but is notrecommended for performance camshafts. A secondmethod, recommended by many shops and some camshaftgrinders, is to check the intake and exhaust valve openingand closing events against TDC (top dead center). This isbetter than the "dots" system, but we don't feel it's asaccurate as the "centerline" method. The centerline methodis also much easier to understand.