Jeep Engines - Oljeep

--r n174 MOPAR PERFORMANCE PARTSIf they all point in the same direction, you have a winner! Ifthey don’t (which is more likely), you have to makechoices. Pick the best intake based on area and the bestexhaust based on area or mid-lift flow. Multiply the intake’speak flow by .80 and compare to that head’s exhaust flow.For example, in Figure 4-20, head “A” was selected as thebetter head and it had a peak flow of 285. The 80 percentnumber would be 228 for the exhaust side on this head. The70 percent number would be 200 cfm. You would want theactual exhaust flow to be between 200-228 cfm.I Air Flow - cimTYPICAL AIR FLOWIntake PovtIf you had two heads that both flowed 285 cfm on the intakeside, then the head that flows better up to the 80 percentnumber on the exhaust side is the best choice. This soundslike it’s easy to get to 80 percent on the exhaust side.Nothing could be further from the truth. Remember, theexhaust side gets the smaller valve. The typical productionvalve area comparison between intake and exhaust valvediameters gives the exhaust valve 65-70 percent of the areaof the intake. To get 80 percent flow out of a valve area thatis 30-35 percent smaller means the exhaust port has toactually be a better flowing port than the intake.Putting in bigger exhaust valves will limit the size of valvewe can put on the intake side. That is why the exhaust sideis so important. You want to get the flow out of the portitself, not out of a bigger valve.L50’Air Flow - cfrnI I I I I1W 200 300 400 500 600 700Valve LiftFigure 4 - 20TYPICAL AIR FLOWIntake PortLir Flow - cfmTYPICAL AIR FLOWIntake Port50 -’I I I I I I100 200 300 400 500 600 700Valve LiftI I I I 1 I I100 200 300 400 500 600 701Valve LiftFigure 4 - 19Figure 4 - 21Oversize Valve Seats and GuidesIn some cases, cylinder head porting has to be done tochange some design characteristic of the head. In othercases, it must be done because heads are worn-out or havebecome damaged due to a bent valve, etc. In a cast ironhead, the seats and guides are machined directly into thecast iron. There is no seat or guide insert. (There areinstances where inserts are used in cast iron heads, but thistends to be the exception.)