Jeep Engines - Oljeep

4.0L POWER TECH IN-LINE 6 (CAMSHAFT AND VALVE GEAR) 185Second, the base circle is measured as a "height" ordistance from the center of camshaft. On a typical V-8camshaft (for example) this height might be ,750". If thiscamshaft had a lobe lift of .333" (valve lift of SOO"), thenthe nose height would be 1.083'' (.750 + 333). This heightwould also be measured from the center of the camshaft.The important thing to remember is that the nose heightbasically doesn't change for any given engine. The basecircle has to change from one lift profile to another.This oddity of the nose height staying constant and the basecircle changing is caused by a special physical condition inoverhead valve engines. The nose of the camshaft must fit inthrough the camshaft bearings in which the camshaft journalsride. This essentially defines the maximum height of the noseof the camshaft. If the nose height were any higher, itcouldn't be installed through the camshaft bearings. (This isnot a problem on overhead camshaft engines.)Question: If the nose of the camshaft can't be increased,then how are higher lift profiles made? Answer: Bygrinding down the base circle. There really isn't any otherchoice. This may sound bad for high lift camshafts, buteven a ,750" lift, all-out race hydraulic camshaft onlyrequires the base circle to be actually "cut-down" .160"from a SOO" lift.There is one small error in this discussion that can lead toconfusion. We converted our 300 degree camshaft(advertised) to 150 camshaft degrees and compared it to the210 degree base circle. A 300 degree camshaft is typical ofhigh performance hydraulic or a mild mechanical camshaft.However, we left something out to simplify the discussion.We left off the ramps!Although not the same, hydraulic and mechanicalcamshafts both have ramps. Ramps are defined anddesigned differently by different manufacturers and fordifferent applications. They could run 20 to 30 degrees perside which adds to the cam's physical duration andincreases it to 170-180 degrees. This makes the base circleand the "lift" profile itself almost equal in duration. Thissounds like a small detail, but it should not be passed overtoo quickly. Remember, our example is basically a mild,high performance hydraulic design, perhaps 235-240"duration at .050".Many people in the field like to check the cam's lift outsideof the engine by using a dial vernier (or micrometer) andmeasuring across the nose and across the flanks (sides).Actually, they measure from the base circle to the nose ofthe camshaft. In our example, this measurement would be1.833" (1.083 + .750). In theory, the steps of measuringacross the sides would measure from base circle to basecircle. In our example, this would yield 1.500" (.750 +.750). If we subtracted these two numbers, we'd get thecam's lobe lift (1.833 - 1.500 = ,333"). Numbers cun bemagic! Nice theory, and it actually works well on stockcamshafts. However, in high performance designs, we havea small "built-in" error.In the second measurement (across-the-sides), we actuallymeasure from ramp to ramp. By nature, the ramp moves orlifts slowly so we may only be picking up .010" per side forthe ramp lift. In more radical camshafts, this number getsmuch higher. In our mild example, the .010" ramp errorwould mean that we measured from .760" (.750 + .010) to,760 or a total measurement of 1.520". Now this doesn'tseem like much of an error, so why the big fuss?The problem comes about relative to the camshaft liftcalculation. In our example, we'd now incorrectly calculatethat the cam's lift is ,313" (1.833" - 1.520"). Still not surewhy so small an error is important? The whole reason formaking these measurements is to identify the camshaft. Inmany cases, we don't know what the actual lift is. We aretrying to use the actual, measured lift to identify thecamshaft. Therefore, we'd multiply the measured camshaftlift by 1.5, and find that the camshaft has .470" (.313 x 1.5)of valve lift. Comparing this to a camshaft lift table, we'dpickhdentify the wrong camshaft.Note: Always measure a performance or race camshaftwith a micrometer to ensure accuracy.VALVE GEARThis section is designed to tie all the valve gear informationfor the 4.0L Power Tech engine together in one place. Thevalve gear - especially camshaft and valve spring selectionand their effect upon the remaining components -is one ofthe most important areas in the performance of your enginenext to the induction system. The valve train area has beenexpanding rapidly in recent years which has led to a lot ofconfusion. Deciding what piece to use, where and withwhat other parts has become very complicated. Add to thisthe similar parts available from Mopar Performance Parts,Crane, Competition Cams, Isky, etc., and it becomes almosthopeless to sort it all out.In this chapter we will concentrate on the 4.0L Power Techengine which is a wedge. We will stay strictly in the valvetrain area. Related subjects and areas such as engineblueprinting and individual engine part numbers andspecifics are covered in separate sections.Some of the information that will be covered has beenstated in other sections of this book. Valves, cylinder heads,and camshafts are closely related areas, and much of theinformation overlaps. We will put all this currentlypublished information together in this section. We'll alsoadd some new parts, part numbers and information thathave become available, but things are changing quickly.Please keep in mind that the information gathered in thissection contains the latest recommendations and that other(older) sources may differ.T-1 n