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How to Select a Cam

Step 2 - Cam Parameters

From a manufacturing perspective there are many subtle facets of the cam’s design that need to be optimized, none of which I’m going to cover. When selecting a cam for your 911 from a number of designs, the task is somewhat simpler and you really only need to worry about 4 things.
1. Duration: In general the intake duration of the camshaft will determine where in the rev range the peak torque will occur. Unfortunately camshafts often have their duration listed based on different measurement methods. Porsche’s factory camshafts were specified based on 0.1 mm (.0039 inches) valve clearance. A reasonable rule of thumb for a 911 engine (based on the lifts being measured with 0.1 mm of valve clearance) is that the peak torque engine speed will be a function of the following equation:
a. Peak torque engine speed = -3151+(Duration * 32.53)
b. In the case of 911 engines, the following rule of thumb (once again based on a .1 mm valve clearance) can give you an indication of how the exhaust duration will affect a potential engine’s peak HP engine speed. While hardly exact, it can help you to understand the magnitude of the impact that exhaust duration can have.
a. Peak HP Engine Speed = (exhaust duration degrees * 66.62) - 9083
2. Lift: Lift has a more subtle influence on an engine’s performance and is closely tied with the intake porting and all of the trade-offs involved in that subject. In general the greater the lift, the easier it will be for mixture to flow into the cylinders – limited by the flow in the rest of the induction system. So in general if your cam has excess lift, it won’t create any more torque and HP then a cam with the ideal lift for your engine. It will on the other hand generate higher valve accelerations (see below). If on the other hand your camshaft’s lift is insufficient for the engine’s requirements, it will limit the high RPM horsepower as the torque will drop off faster then if you had used an “ideal” camshaft. The best way to determine how much lift you should spec for your camshaft is to have your heads flowed. Below is an example of some flow data for some sampled early 911 heads.
Note that the CIS 2.4TK heads don’t flow more then 150 CFM at .4 inches of lift. In general a camshaft that provides more then .4 inches of lift will not perform much better then a camshaft with .4 inches of lift in a TK head. The 2.2 S head on the other hand (also used in the 2.7RS) keeps flowing more air all the way up to .5 inches. If you were to use an E cam which only lifts to about .4 inches with an S head, you won’t even be using the last 25 CFM of flow that the cam and heads can provide. A chart of your heads’ flows such as this is very useful for comparing the valve lifts defined by the camshaft. Ideally you want a situation where the cam has opened far enough to allow maximum flow when the pistons are undergoing their maximum acceleration. Depending on the rod-stroke ratio, this generally occurs around 75-80 degrees of crank shaft angle. Now compare the head flows with the valve lift graph for the 911 S camshaft below.
Note that the valve has opened to a point that allows maximum flow by the time that the crankshaft has reached maximum acceleration. In some cases in order to achieve this condition of full flow at maximum piston accelerations, the cam designer does have to “over-lift” the valve past the head’s peak flows just to manage the valve accelerations.
3. Overlap: Overlap can be great for extracting those last few ponies from a well tuned engine, but it’s murder if you need to pass any sort of emissions testing. The reason is that when an engine is on-cam, overlap allows a well-tuned exhaust to draw the new charge into the cylinder actually making an engine more efficient then its capacity would suggest. The downside is that when the engine is off-cam, unburned fuel can go out the exhaust causing high emissions and poor drivability and mileage. With a lot of overlap it’s also possible to have the exhaust push back into the cylinder and in extreme situations back up the intake and cause reversion. In general overlap is desirable for high RPM track and race engines, but not desirable for smooth low rev’ing street and autocross engine

4. Valve acceleration: The valve accelerations designed into the cam will weigh heavily into the design of your valve train. In general Porsche’s factory cams had very moderate valve accelerations. As a result it is not unheard of for some of the smaller engines using the full-race 906 cam to achieve 8000 RPM reliably using stock valve springs and retainers. More modern cams have been designed to open the valves faster thus allowing the cam to act like it has a longer duration, while still keeping overlap to a reasonable level. The best of both worlds! But there are some hidden downsides. As the valves got larger and rev’s increase, the inertia involved with the faster accelerations goes up significantly. The result can be increased wear on the opening ramp and valve float. Valve float can be addressed by fitting stiffer springs and lightening the valve retainers. The lighter valve retainers also will help to reduce the wear on the opening ramps. In general, peak negative valve accelerations of less then .000280 inches per degree per degree can be controlled with stock valve springs. The 906 camshaft for example has peak nose acceleration of -.000261. On a more modern cam it is not uncommon to see negative accelerations of .000320 (almost 23% higher), which would necessitate the use of competition valve springs as well as potentially lighter retainers. This is especially true if you plan on spinning the engine faster then 6500 RPM.

Go back to Step1 - Define the Performance Parameters

Selecting the Appropriate Cam for Your Engine.
Cam Shaft Selection Home


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