Jeep Engines - Oljeep

172 MOPAR PERFORMANCE PARTSWhere flow benches become very useful is in makingmodifications to cylinder heads. How big a valve to install?What shape to make the port? Valve seat angles? All portsflowing equally? Obviously, these are questions theprofessional cylinder head service needs answered. Once heknows the answers, the racer is paying for modifications tothe head, not the flow chart. As a racer, don’t end up racingflow benches!As a general rule, don’t flow your cylinder head unless theshop is making a change or modification to the head andyou want to see the resulting gain. Also, don’t get intoabsolute number comparisons. A flow bench is only a tool.As a tool it’s helpful in making modifications, but it’s hardto turn a flow bench into a race vehicle!Evaluating Cylinder Head Air FlowCylinder head air flow is a popular topic because it isgenerally believed that more air flow leads to more power.Although this is true in some cases, there are manyexceptions. The purpose of this discussion is to give yousome tips that will help you evaluate cylinder head air flow.First, let’s look at the current situation. All racers want toknow how much air their heads flow, and then they will tellyou their heads flow “X” cfm. That’s it! All racers seem touse this “one number system” to discuss their heads.Although this seems quite simple, it can lead to muchconfusion. So let’s clarify a few things before we move on.The one number system assumes that the number is the“peak number” for the intake port of the head. Although nopressure drop is typically mentioned, you can probablyassume that it is 28 inches. (Although many people actuallyflow at pressure drops of 3, 10, or 25 inches, 28 gives youthe biggest number, so that has become the “standard.”)There is a chart that allows you to use a correction factor tocorrect flow data from one pressure drop to another. Sincedifferent flow benches DO NOT give the same flownumbers for the same head, we recommend you not get toohung-up on absolute numbers and try to do all your flowtesting on the same flow bench. This tip really relates to theflow bench itself. For now we will assume that you have aflow curve comparison and want to figure out which curveis best. Let’s look at some typical flow curves.In Figure 4- 19 you see the typical flow curve of an intake port.In 1 his curve, we are comparing cylinder head “A” to cylinderhead “B.” This is somewhat typical of a standard productionhead being compared to a race head. Head “A” is obviouslybetter. It has more “area under the curve.” It outflows head“B” at both high lift and low lift. This one is too easy!Figure 4-20 is perhaps more common. In this example, head“B” flows 290 cfm (peak) and head “A” flows 285 cfm(peak). In this case, head “B” has the higher number or betterpeak flow. However, head “A” has more area under thecurve. Area under the curve is the area formed by the X-axis(or valve lift line) and the flow curve line for the head. In thiscase, head “B” only exceeds head “A” at one point (.700“lift), while head “A’ wins at .300”, .400”, and S00” lifts.To evaluate area under the curve properly, you have to dosome calculating. However, the performance press hasn’tyet come up with a common, acceptable-to-all formula forcalculating area under the curve, and there are no readilyavailable numbers for what this “area” is supposed to be.No one gives “area” numbers. However, area under thecurve is what you really what to know! In our example,head “A” will win. It will make the vehicle quicker at thedrag strip and it will make more power on the dyno.Now let’s back up. Let’s assume you have been using head“A” on your current race engine and you bought head “B”because it flowed more air. Figure 4-20 shows “B” with 290cfm at peak, so it does flow more air. You install this newhead and your vehicle goes slower. Now you are probablyupset! Statements like “high flow heads don’t work,” “bigports go slow,” etc., are made. But high flow heads dowork - if they are done like head “A”.